Sunday, 29 September 2013

Plants And Light


Energy Input Into Ecosystems

Green plants are the primary source for all of the biotic energy requirements of an ecosystem.

Photosynthesis vs respiration

In green plants both photosynthesis and respiration occur. In relatively bright light photosynthesis is the dominant process (meaning that the plant produces more food than it uses during respiration). At night, or in the absence of light, photosynthesis essentially ceases, and respiration is the dominant process; the plant consumes food (for growth and other metabolic processes).
Photosynthesis and respiration
The two processes are shown in the simplified equation above. Photosynthesis absorbs energy (from sunlight) whereas aerobic respiration yields energy (as a result of the oxidation of glucose, the carbohydrate molecule shown here).
Note that these are essentially "competing" processes, one producing glucose (photosynthesis) and the other consuming glucose (respiration).

Factors Affecting The Rate Of Photosynthesis

Compensation point for light

Compensation point for light
One simple way to get an estimate of the level of phototsynthetic activity in a green plant is to place the plant in a sealed container and measure the rate at which oxygen is produced.
When such an experiment is actually performed it is found that increasing the brightness (intensity) of the light increases the rate of photosynthesis, but only up to a certain point, beyond which increasing the brightenss of the light has little or no effect on the rate of photosynthesis.
Conversely, reducing the brightness of the light causes a decrease in photosynthetic activity.
The light intensity at which the net amount of oxygen produced is exactly zero, is called the compensation point for light. At this point the consumption of oxygen by the plant due to cellular respiration is equal to the rate at which oxygen is produced by photosynthesis.
The compensation point for light intensity varies according to the type of plant, but it is typically 40 to 60 W/m2 for sunlight. The compensation point for light can be reduced (somewhat) by increasing the amount of carbon dioxide available to the plant, allowing the plant to grow under conditions of lower illumination.

Compensation point for carbon dioxide

Compensation point for carbon dioxide
Under conditions of constant and uniform illumination the rate of photosynthesis can be increased by simply increasing the amount of carbon dioxide (i.e. increasing the atmospheric partial pressure) available to plants.
As before, one can measure the rate of photosysthesis as a function of carbon dioxide pressure by placing a green plant in a sealed container and measuring the rate at which oxygen is produced.
As the partial pressure of carbon dioxide increases there is an almost linear increase in the rate of oxygen production, which implies an identical increase in the rate of phtotosythesis.
This increase eventually levels off, and further increases in the concentration of carbon dioxide have no further effect.
Conversely, reducing the carbon dioxide concentration reduces the rate of photosynthetic activity. The level at which the oxygen production rate drops to zero is called the compensation point for carbon dioxide.

A Day In The Life Of A Plant

Compensation Point for Light (of photosynthetic plants) is the intensity of light at which the rate of carbon dioxide uptake (photosynthesis) is exactly balanced by the rate of carbon dioxide production (respiration) or equivalently, the light intensity at which the rate of oxygen production is exactly balanced by the rate of oxygen consumption.
Since it is primarily food production we are interested in, we will consider the third equivalency, the rate at which the food produced (carbohydrates) is exactly balanced by the rate at which the food is consumed.
In the figure to the left the red line shows the rate of carbohydrate productiondue to plant photosynthesis.
The green line shows the rate of carbohydrate consumption due to respiration.
The shape of the photosynthesis curve is due to increased sunlight during the day and the shape of the respiration curve is due to increasedtemperature during the day.
Seeds hard shell

Since photosynthesis produces carbohydrates, the rate at which the amount the carbohydrates change is positive for photosynthesis, that is, the amount increases.
On the other hand, respiration consumes carbohydrates, hence the rate at which carbohydrates change is negative for respiration, that is, the amount decreases.
This is shown in the graph to the left.
The area in yellow represents the total amount of carbohydrate produced in a 24h period (due to photosynthesis). The area in green represents the total amount of carbohydrate consumed due to respiration.
For a green plant to survive, grow, and produce mature fruit, area (a) (yellow), must exceed area (b) (green).
Embryonic plant forms within seed

The area (a), that is the total amount of carbohydrate production due to photosynthesis, can be increased in two ways:

1. Increase the intensity (brightness) of the light.

The danger is that if the light is too intense the heat it produces can damage the delicate plant cells, as well as increasing the transpiration rate, causing the leaves to wilt.
Of course, there is a limit beyond which increasing the light intensity has no significant effect on the rate of photosynthesis. This occurs for most plants at a light intensity of about 40% full daytime sunlight.
Seed begins to germinate

2. Increasing the duration of the light which illuminates the plant leaves.

In the case of natural sunlight it is generally not possible to increase the time during which the plants receive light beyond the length of natural daylight hours.
To increase the length of time during which photosynthesis occurs requires the use of artificial lights.
Seed begins to germinate
If there is enough electrical energy available both the duration and intensity of the light can be controlled to provide optimum growing conditions for green plants.
The problem is that using artificial light to grow plants is an extremely inefficient use of energy.
Seed begins to germinate

Things that we know about the relationship between plants and light

  • All green plants need some light.
  • Too little light is bad for green plants (below the compensation point).
  • Too much light is bad for all plants.
  • Increasing the carbon dioxide concentration increases the rate of photosynthesis (over a small range of carbon dioxide enhancement).

Things we would like to know about tomato plants

  • What is the absolute minimum light intensity needed for tomato plants to survive?
  • To what extent can the duration of light exposure compensate for low light intensity?
  • How does low light exposure affect a tomato plant's ability to produce fruit?
Although the maximum intensity (brightness) of sunlight on Mars is much less than on the Earth, the seasons are twice as long as on Earth. It is assumed that in the beginning all Mars habitation will occur near the Martian equator where seasonal changes are less noticeable.

shailesh kr shukla
directoratace@gmail.com


Deoxyribonucleic Acid (DNA)


What is DNA?

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is calledmitochondrial DNA or mtDNA).
The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.
An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.
DNA is a double helix formed by base pairs attached to a sugar-phosphate backbone.

Who discovered DNA?

The German biochemist Frederich Miescher first observed DNA in the late 1800s. But nearly a century passed from that discovery until researchers unraveled the structure of the DNA molecule and realized its central importance to biology.
For many years, scientists debated which molecule carried life's biological instructions. Most thought that DNA was too simple a molecule to play such a critical role. Instead, they argued that proteins were more likely to carry out this vital function because of their greater complexity and wider variety of forms.

The importance of DNA became clear in 1953 thanks to the work of James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin. By studying X-ray diffraction patterns and building models, the scientists figured out the double helix structure of DNA - a structure that enables it to carry biological information from one generation to the next.

Where is DNA found?

DNA is found inside a special area of the cell called the nucleus. Because the cell is very small, and because organisms have many DNA molecules per cell, each DNA molecule must be tightly packaged. This packaged form of the DNA is called a chromosome.
During DNA replication, DNA unwinds so it can be copied. At other times in the cell cycle, DNA also unwinds so that its instructions can be used to make proteins and for other biological processes. But during cell division, DNA is in its compact chromosome form to enable transfer to new cells.
Researchers refer to DNA found in the cell's nucleus as nuclear DNA. An organism's complete set of nuclear DNA is called its genome.
Besides the DNA located in the nucleus, humans and other complex organisms also have a small amount of DNA in cell structures known as mitochondria. Mitochondria generate the energy the cell needs to function properly.
In sexual reproduction, organisms inherit half of their nuclear DNA from the male parent and half from the female parent. However, organisms inherit all of their mitochondrial DNA from the female parent. This occurs because only egg cells, and not sperm cells, keep their mitochondria during fertilization.

What is DNA made of?

DNA is made of chemical building blocks called nucleotides. These building blocks are made of three parts: a phosphate group, a sugar group and one of four types of nitrogen bases. To form a strand of DNA, nucleotides are linked into chains, with the phosphate and sugar groups alternating.
The four types of nitrogen bases found in nucleotides are: adenine (A), , thymine (T), guanine (G) and cytosine (C). The order, or sequence, of these bases determines what biological instructions are contained in a strand of DNA. For example, the sequence ATCGTT might instruct for blue eyes, while ATCGCT might instruct for brown.
Each DNA sequence that contains instructions to make a protein is known as a gene. The size of a gene may vary greatly, ranging from about 1,000 bases to 1 million bases in humans.
The complete DNA instruction book, or genome, for a human contains about 3 billion bases and about 20,000 genes on 23 pairs of chromosomes.

What does DNA do?

DNA contains the instructions needed for an organism to develop, survive and reproduce. To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies.


DNA structureDNA is usually a double-helix and has two strands running in opposite directions. (There are some examples of viral DNA which are single-stranded). Each chain is a polymer of subunits called nucleotides (hence the namepolynucleotide).
Each strand has a backbone made up of (deoxy-ribose) sugar molecules linked together by phosphate groups. The 3' C of a sugar molecule is connected through a phosphate group to the 5' C of the next sugar. This linkage is also called 3'-5' phosphodiester linkage. All DNA strands are read from the 5' to the 3' end where the 5' end terminates in a phosphate group and the 3' end terminates in a sugar molecule.

Each sugar molecule is covalently linked to one of 4 possible bases (Adenine, Guanine, Cytosine and Thymine). A and G are double-ringed larger molecules (called purines); C and T are single-ringed smaller molecules (called pyrimidines).
In the double-stranded DNA, the two strands run in opposite directions and the bases pair up such that A always pairs with T and G always pairs with C. The A-T base-pair has 2 hydrogen bonds and the G-C base-pair has 3 hydrogen bonds. The G-C interaction is therefore stronger (by about 30%) than A-T, and A-T rich regions of DNA are more prone to thermal fluctuations.

How are DNA sequences used to make proteins?

DNA's instructions are used to make proteins in a two-step process. First, enzymes read the information in a DNA molecule and transcribe it into an intermediary molecule called messenger ribonucleic acid, or mRNA.
Next, the information contained in the mRNA molecule is translated into the "language" of amino acids, which are the building blocks of proteins. This language tells the cell's protein-making machinery the precise order in which to link the amino acids to produce a specific protein. This is a major task because there are 20 types of amino acids, which can be placed in many different orders to form a wide variety of proteins.

What is the DNA double helix?

Scientist use the term "double helix" to describe DNA's winding, two-stranded chemical structure. This shape - which looks much like a twisted ladder - gives DNA the power to pass along biological instructions with great precision.
To understand DNA's double helix from a chemical standpoint, picture the sides of the ladder as strands of alternating sugar and phosphate groups - strands that run in opposite directions. Each "rung" of the ladder is made up of two nitrogen bases, paired together by hydrogen bonds. Because of the highly specific nature of this type of chemical pairing, base A always pairs with base T, and likewise C with G. So, if you know the sequence of the bases on one strand of a DNA double helix, it is a simple matter to figure out the sequence of bases on the other strand.
DNA's unique structure enables the molecule to copy itself during cell division. When a cell prepares to divide, the DNA helix splits down the middle and becomes two single strands. These single strands serve as templates for building two new, double-stranded DNA molecules - each a replica of the original DNA molecule. In this process, an A base is added wherever there is a T, a C where there is a G, and so on until all of the bases once again have partners.

shailesh kr shukla 
directoratace@gmail.com

Saturday, 28 September 2013

SUN: Ultimate Source Of Earth's Energy 



    10 Need-to-Know Things About the Sun:

    1. The sun is a star. A star does not have a solid surface, but is a ball of gas (92.1 percent hydrogen (H2) and 7.8 percent helium (He)) held together by its own gravity.
      Illustration showing Earth's tiny size compared to the sun.
      Earth compared to the sun.
    2. The sun is the center of our solar system and makes up 99.8% of the mass of the entire solar system.
    3. If the sun were as tall as a typical front door, Earth would be about the size of a nickel.
    4. Since the sun is not a solid body, different parts of the sun rotate at different rates. At the equator, the sun spins once about every 25 days, but at its poles the sun rotates once on its axis every 36 Earth days.
    5. The solar atmosphere (a thin layer of gases) is where we see features such as sunspots and solar flares on the sun.
    6. The sun is orbited by eight planets, at least five dwarf planets, tens of thousands of asteroids, and hundreds of thousands to three trillion comets and icy bodies.
    7. The sun does not have any rings.
    8. Spacecraft are constantly increasing our understanding of the sun -- from Genesis (which collected samples of the solar wind and returned the particles to Earth) to SOHO, STEREO, THEMIS, and many more, which are examining the sun's features, its interior and how it interacts with our planet. .
    9. Without the sun's intense energy there would be no life on Earth.
    10. The temperature at the sun's core is about 15 million degrees Celsius (27 million degrees Fahrenheit).

    BASIC GOVERNING PHYSICAL PROPERTY OF ALL STARS

    The lifecycle of a star is based entirely on its mass and the law of Gravity that operates on that value. Since gravity works to force all mass toward a center, its is concluded that within a mass of a star, the center of mass is the center of the star, and the place where that mass is condensed to the smallest possible space. Unusual properties of Physics govern the form of material in a stellar core, and we will look into those unusual properties later. For now, the simplest rule is that where gravitational pressure is greatest and thus the packing of material the most dense, the temperature will be the greatest. A star with much mass will have a greater gravitational force operating on its mass, generating greater internal pressures and thus higher temperatures. Higher temperatures means greater kinetic energy of the molecules and increased collision frequency, resulting in a greater release of energy. A star with less mass will have less gravitational force operating on it, resulting in lower internal pressures and lower temperatures. The low mass star will have interior particles at lower energy levels, reducing collision frequency and yielding a lower release of energy. To put it more simply, high mass stars burn hot and energetically, and low mass stars burn cool and less energetically. It may seem that high mass stars ought to live longer owing to their greater amount of material, but it is the low mass stars that live longest because they burn what little material they have more slowly.
    Our Sun is an average star, and of spectral class G2V. No one on Earth can life long enough to watch the entire lifecycle of our Sun, so we turn to the stars in space to see those of similar mass and at different stages of their lifecycle to make a theoretical picture of what our Sun's life may have been like and will be. We turn to the HR Diagram for this help.

    The Sun's power (about 386 billion billion mega Watts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.
    The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.

    A small region known as the chromosphere lies above the photosphere.
    The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during a total solar eclipse (left). Temperatures in the corona are over 1,000,000 K.

    It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth a major journey.
    It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth a major journey.
    The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Its magnetosphere (also known as the heliosphere) extends well beyond Pluto.

    In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis.
    Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, than it does at lower latitudes. The composition of the solar wind also appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed.
    The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula).

    Shailesh kr shukla
    directoratace@gmail.com
    The discovery of new elements   
      
    How are new elements discovered?
    There are probably nearly as many answers to this question as there are elements. Many elements were found more or less by accident. Others were discovered as a result of research into a particular compound or mineral. Others were predicted to exist – on the basis of Mendeleev’s Periodic Table, for example – so the discoverer knew what he or she was looking for. However, from time to time a new chemical technique is developed or discovered that leads to the discovery of several new elements in a short time. You can use the interactive Periodic Table to investigate this idea.
    Activity
    The following activities are based around the interactive Periodic Table.
    a) Run the Animate function and watch as the elements are displayed in the order in which they were discovered over time. Do they seem to appear at a steady rate?
    b) Use the Histogram function to plot the number of elements discovered in each century. Does this confirm your impression?
    You should see that there are several periods in which many elements were discovered with periods of time in between when none were discovered. Some of the bursts of discovery were caused by the development of new chemical techniques.
    It is a great achievement to discover a single element out of the 111 or so that are now known. So you may be surprised to find that there are several people who have discovered more than one element, in one case as many as 11. Here are some brief descriptions of the work of some chemists who discovered more than one element. In many cases they took advantage of a new technique.
    ·         Humphry Davy – electrolysis
    ·         Robert Bunsen – flame colours

    Jons Jacob Berzelius – reaction with carbon
    Jons Jacob Berzelius (see box) obtained four elements (thorium, cerium, selenium and impure silicon) mainly by reduction with carbon.
    Activity
    a) Ytterby has the unique distinction of having four elements named after it. Look at the Periodic Table and suggest what these elements might be. You could try to confirm your suggestions by doing a search on the internet or research in a library.
    b) Use an internet search engine to help you find out what sort of symbols were used by chemists to represent elements before the letter symbols introduced by Berzelius in the early 1800s.
    Jons Jacob Berzelius
    Jons Jacob Berzelius (1779 - 1848) is probably Sweden's most famous chemist. As well as his own discoveries, some of his colleagues also discovered elements. Lithium was discovered by Johann Arfvedson, whom he had trained, and Berzelius’ former pupil in Stockholm (Carl Mosander) discovered lanthanum, erbium and terbium. Mosander did not use a new technique but was fortunate to work near Ytterby - a village near Vaxhol, Stockholm in Sweden  with a quarry where compounds containing many of the elements of atomic numbers between 58 and 71 (called the lanthanides) were found. Berzelius was also the first to use letter symbols for elements like the ones we use today.

    Jons Jacob Berzelius. Reproduced courtesy of the Library and Information Centre, The Royal Society of Chemistry.
    Question
    Q 1.     Berzelius used the technique of reaction with carbon to isolate elements from their compounds. One common compound of silicon is silicon dioxide (SiO2), which is found in sand.
                (a) Suggest word and symbol equations for the reaction of carbon with silicon dioxide to form silicon.
                (b) What other compound would be produced?

    Humphry Davy - electrolysis
    Humphry Davy (see box) took advantage of new technology to discover six elements. He used the then-newly invented voltaic pile (we would call it a battery), to pass electricity through molten salts of alkali metals. This produced highly reactive metals at the negative electrode. He obtained sodium and potassium, then magnesium, calcium, barium and strontium.
    Sir Humphry Davy
    Davy (1778 - 1829) was born Cornwall but worked in Bristol and then at the Royal Institution in London which is still a prestigious centre of scientific research. Like many scientists at the time Davy worked in many fields. He investigated the anaesthetic properties of dinitrogen oxide (‘laughing gas’) on himself, and his name will always be associated with the miners’ safety lamp which he invented to prevent explosions in mines caused by the use of naked flames. He was well-known as a lecturer and was succeeded by his assistant, Michael Faraday, who became equally famous as a scientist for developing the electric motor and generator.

    One of Davy’s lectures at The Royal Institution. Reproduced courtesy of the Library and Information Centre, The Royal Society of Chemistry.


    Questions
    Q 2.     (a) Why could Davy not isolate, say, sodium from sodium oxide by reducing the oxide with carbon as Berzelius had done with silicon, cerium, thorium and selenium?
                (b) Why did Davy electrolyse molten salts rather than solids?
                (c) Why did Davy electrolyse molten salts rather than solutions of salts in water?

    Robert Bunsen - flame colours
    Robert Bunsen (1811 - 1899) and Gustav Kirchoff (1824 - 1887) (see box) were early users of the technique of examining the light given out by heated compounds to recognise new elements. Have you noticed that a wire dipped into sodium chloride solution gives an intense yellow flame colour or that when a pan of salted water boils over it colours the gas cooker flame yellow? This colour is characteristic of sodium and is also seen in street lamps that are filled with sodium vapour.
    A development of this method is still used today to measure the amounts of certain elements in mixtures.
    Robert Bunsen and Gustav Kirchoff
    In 1861 Bunsen and Kirchoff jointly discovered caesium (which gave a blue flame) and rubidium (which gave a red flame). Bunsen (who devised, or at least developed, the Bunsen burner) discovered only two elements himself, along with Kirchoff, but his technique was used to discover several more.
    Paul Emile Lecoq de Boisbaudran (1838 - 1912) used flame colours (called emission spectra) to search for more elements. He discovered gallium (1875), samarium and dysprosium. Gallium was the first element to be found whose properties matched elements predicted in detail by Mendeleev in 1870, dramatic proof of his ideas about the Periodic Table.
     

    Kirchoff (left) and Bunsen. Reproduced courtesy of the Library and Information Centre, The Royal Society of Chemistry.

    Activity
    a) The flame colours of some metals are clear enough to be used to identify them by eye – the orange-yellow colour of sodium is an example. Make a list of flame colours of metals that you know, then use an internet search to check your answers and fill in some you are not familiar with.
    b) The element helium was discovered from its spectrum. Find out using an internet search what was unusual about this discovery.

    William Ramsay - the distillation of liquid air
    William Ramsay (see box) investigated the observation that nitrogen made by removal of other gases from air had a slightly different density to nitrogen made by chemical decomposition. First he discovered argon and then predicted a complete family of elements between Groups 7 and 1 of the Periodic Table. We now call these the noble gases. By fractional distillation of liquefied air, he and Morris Travers then discovered neon, krypton and xenon. Ramsay won the 1904 Nobel prize for chemistry.
    When Ramsay discovered a gas of relative mass approximately 40, chemists were at first reluctant to believe that a whole new group of elements remained to be discovered. At first, Mendeleev was a disbeliever because he felt that the discovery undermined his Periodic Table. Later, however, he came to see that it was actually a confirmation of the basic idea behind the Table.
    Some tried to explain the new gas as an allotrope of nitrogen, N3. (Allotropes are forms of the same element which differ in the arrangement of their atoms.) This seemed possible because oxygen has an allotrope O3, usually called ozone. N3 would have a relative molecular mass of 42, close to the measured value for argon of 40. However, N3 has, up to now, never been found or made.
    Sir William Ramsay
    Ramsay (1852 - 1916) was Glasgow-born but his main research was at London University. He discovered argon by taking a sample of air and first removing all the oxygen. He then passed the remaining gas (mostly nitrogen) over hot magnesium. Magnesium is reactive enough to combine with nitrogen to leave a solid called magnesium nitride. After doing this repeatedly, he was still left with some gas whose relative mass was 40. This gas didn’t seem to fit into Mendeleev’s Periodic Table!
    Later, using newly-discovered techniques for cooling and liquefying gases, he was able to separate other gases from air – neon, krypton, xenon and radon – and it was realised that he had discovered a whole group of elements, none of which had been known to Mendeleev. The final member of the group, helium, had been discovered a few years earlier – in the Sun. Pierre Jules C├ęsar Janssen had noticed some lines in the spectrum of sunlight that didn’t belong to any known element and suggested a new element, which he called helium, existed in the Sun. Ramsay was the first to recognise helium on Earth so he discovered all of the noble gases (almost).

    Sir William Ramsay. Reproduced courtesy of the Library and information Centre, The Royal Society of Chemistry.
    Questions
    Q 3.     (a) One possible way of removing oxygen from the air is to pass it over heated copper. Write a word equation for the chemical reaction that happens.
                (b) Why will copper react with oxygen but will not react with nitrogen?
                (c) Why will magnesium react with nitrogen? Write a word equation for the chemical reaction that happens.
                (d) What other gases would Ramsay have had to remove from air before he could start his experiments? Suggest a way of removing each of these gases.

    Marie Curie - radioactive elements
    Marie Curie, along with her husband, Pierre (see box), investigated radioactive elements, eventually extracting less than a gram of a new element, radium, from over eight tonnes of the ore pitchblende.
    Marie Curie
    Marie Curie (1867 - 1934) discovered two elements as she investigated what became known as radioactivity. First she identified polonium, which she named after her native Poland, then, with her husband Pierre, she found the more intensely radioactive element radium. She won the 1911 Nobel prize for chemistry for discovering the two elements after she had shared the 1903 physics prize with Pierre, and Henri Becquerel. She is unique in being a double Nobel prize winner, having a chemical element (curium) and a unit (the curie, which measures radioactivity) named after her.

    Marie Curie and husband Pierre. Reproduced Courtesy of the Library and Information Centre,
    The Royal Society of Chemistry.

    Activity
    a) Search the internet to find lists of Nobel prize winners in science (there are three science categories – Chemistry, Physics, and Physiology and Medicine) to find out what else is unique about the Curie family and the Nobel prize.
    b) Search the internet to find which other elements were discovered by women.

    Glenn T Seaborg - making new elements with sub-atomic particles
    Glenn T Seaborg (see box), with his co-workers identified 11 elements. These all have atomic number greater than 92 (uranium) and are man-made rather than occurring naturally. The discovery that uranium atoms could be bombarded with neutrons to create new elements led to an extension of the Periodic Table beyond uranium.
    The elements found by chemists before Seaborg were discovered – they existed on Earth already, combined with other elements to form compounds in most cases. Chemists had to extract them and show that they really were new elements. The elements found by Seaborg and his colleagues were actually made – they are elements that do not exist naturally on Earth. The heaviest element that does exist on Earth is uranium which has 92 protons.
    Protons are positively charged and tend to repel one another (they are held in the nucleus against this repulsion by a force called the strong nuclear force). Atoms whose nuclei have more than 92 protons tend to break apart because of this repulsion. This is why they are not found on Earth. Scientists found that when they fired neutrons at uranium atoms, one would occasionally stick to a uranium nucleus. This increased the relative atomic mass of the atom by one but kept the atomic number the same. Sometimes this neutron then ‘spat out’ an electron and turned into a proton. This meant that the nucleus now had 93 protons and was a new element, of atomic number 93, which was christened neptunium, Np. This was actually done by Edwin McMillan and Philip Abelson. Seaborg then took over working as the leader of a group of scientists. Bombardment with other sub-atomic particles allowed them to make elements numbers 94 - 103 in the same sort of way.
    Glenn T Seaborg
    Seaborg (1912 – 1999) won the 1951 Nobel prize for chemistry. After some argument between the USA and the rest of the world, element 106 was named seaborgium shortly before he died. This was a matter of some controversy because the International Union of Pure and Applied Chemistry, IUPAC, the body that deals with naming in chemistry, had previously ruled that elements should not be named after living people.

    Glenn T Seaborg (left) with United States president John F Kennedy. Reproduced Courtesy of the Ernest Orlando Lawrence Berkeley National Laboratory.
    The elements of atomic number greater than 92 are all radioactive; their nuclei break up changing them into different elements. In some cases, this can happen only fractions of a second after they have been made, making it difficult to be sure that they have actually been made. We say they have very short half lives. There are only three research centres that are capable of making these so-called transuranic elements, one in Berkeley, California in the USA, one in Dubna in Russia and the third in Darmstadt in the former East Germany. Often, two centres claimed to have made a particular element first and therefore also claimed the right to name it. A situation arose where several elements had more than one name. So, to avoid confusion, IUPAC made a temporary ruling that the elements from atomic number 104 onwards should be named unnilquadium, unnilpentium etc from the Latin for 104, 105 and so on.
    Later IUPAC examined the evidence and decided which research team had actually discovered which element and allowed the discoverers to name them. More controversy followed and IUPAC changed its ruling which led to further changes. So the same name was sometimes used for two different elements at different times! The situation is summarised in the Table below.
    Atomic number
    104
    105
    106
    107
    108
    109
    Initially suggested name
    Rutherfordium, Rf or Kurchatovium, Ku
    Hahnium, Hn or Nielsbohrium, Ns
    Seaborgium, Sg
    Nielsbohrium, Ns
    Hassium, Hs
    Meitnerium, Mt
    IUPAC temporary name
    Unnilquadium, Unq
    Unnilpentium, Unp
    Unnilhexium, Unh
    Unnilseptium, Uns
    Unniloctium, Uno
    Unnilennium, Unn
    Agreed name 1995
    Dubnium, Db
    Joliotium, Jl
    Rutherfordium, Rf
    Bohrium, Bh
    Hahnium, Hn
    Meitnerium, Mt
    Agreed name 1996
    Rutherfordium, Rf
    Dubnium, Db
    Seaborgium, Sg
    Bohrium, Bh
    Hassium, Hs
    Meitnerium, Mt

    Activity
    Most, but not all, of the names of elements 104 - 109 honour scientists such as Seaborg. Hassium is named after Hesse, the province where the German research centre is located and Dubnium after the location of the Russian research centre. Use an internet search to find out who the other elements are named after and what their scientific achievements were. Which scientist would you suggest is worthy of having a new element named after him or her?

    Answers to questions
    Q 1.     (a) silicon dioxide + carbon → silicon + carbon dioxide
                     SiO2(s) + C(s) → Si(s) + CO2(g)
                (b) The new compound is carbon dioxide
    Q 2.      (a) Carbon is not reactive enough to remove oxygen from sodium oxide.
                (b) Solid salts do not conduct electricity, while molten ones do (because the ions are free to move in liquids but not in solids).
                (c) Any sodium produced would immediately react with water / the products of the electrolysis of water would also be formed.
    Q 3.     (a) copper + oxygen → copper oxide
                (b) Nitrogen is much less reactive than oxygen.
                (c) Magnesium is one of the more reactive elements.
                     magnesium + nitrogen → magnesium nitride
                Note the name of the product is magnesium nitride, not magnesium nitrate. The ending ‑ide means that there are only two elements in the compound. The ending ‑ate means that there are at least three elements, one of which is oxygen.
                (d) Carbon dioxide and water vapour. Carbon dioxide (an acidic gas) could be removed by passing air over an alkali and water by passing it over a drying agent such as anhydrous copper sulfate.



    shailesh kr shukla
    directoratace@gmail.com