1887 – USA
‘The aim of the experiment was to measure the effect of the Earth’s motion on the speed of light’
This celebrated experiment found no evidence of there being an effect.
1931 – USA
‘A framework for understanding the electronic and geometric structure of molecules and crystals’
An important aspect of this framework is the concept of hybridisation: in order to create stronger bonds, atoms change the shape of their orbitals (the space around a nucleus in which an electron is most likely to be found) into petal shapes, which allow more effective overlapping of orbitals.
A chemical bond is a strong force of attraction linking atoms in a molecule or crystal. BOHR had already shown that electrons inhabit fixed orbits around the nucleus of the atom. Atoms strive to have a full outer shell (allowed orbit), which gives a stable structure. They may share, give away or receive extra electrons to achieve stability. The way atoms will form bonds with others, and the ease with which they will do it, is determined by the configuration of electrons.
Earlier in the century, Gilbert Lewis (1875-1946) had offered many of the basic explanations for the structural bonding between elements, including the sharing of a pair of electrons between atoms and the tendency of elements to combine with others to fill their electron shells according to rigidly defined orbits (with two electrons in the closest orbit to the nucleus, eight in the second orbit, eight in the third and so on).
Pauling was the first to enunciate an understanding of a physical interpretation of the bonds between molecules from a chemical perspective, and of the nature of crystals.
In a covalent bond, one or more electrons are shared between two atoms. So two hydrogen atoms form the hydrogen molecule, H2, by each sharing their single electron. The two atoms are bound together by the shared electrons. This was proposed by Lewis and Irving Langmuir in 1916.
In an ionic bond, one atom gives away one or more electrons to another atom. So in common salt, sodium chloride, sodium gives away its spare electron to chlorine. As the electron is not shared, the sodium and chlorine atoms are not bound together in a molecule. However, by losing an electron, sodium acquires a positive charge and chlorine, by gaining an electron, acquires a negative charge. The resulting sodium and chlorine ions are held in a crystalline structure. Until Pauling’s explanation it was thought that they were held in place only by electrical charges, the negative and positive ions being drawn to each other.
Pauling’s work provided a value for the energy involved in the small, weak hydrogen bond.
When a hydrogen atom forms a bond with an atom which strongly attracts its single electron, little negative charge is left on the opposite side of the hydrogen atom. As there are no other electrons orbiting the hydrogen nucleus, the other side of the atom has a noticeable positive charge – from the proton in the nucleus. This attracts nearby atoms with a negative charge. The attraction – the hydrogen bond – is about a tenth of the strength of a covalent bond.
In water, attraction between the hydrogen atoms in one water molecule and the oxygen atoms in other water molecules makes water molecules ‘sticky’. It gives ice a regular crystalline structure it would not have otherwise. It makes water liquid at room temperature, when other compounds with similarly small molecules are gases at room temperature.
One aspect of the revolution he brought to chemistry was to insist on considering structures in terms of their three-dimensional space. Pauling showed that the shape of a protein is a long chain twisted into a helix or spiral. The structure is held in shape by hydrogen bonds.
He also explained the beta-sheet, a pleated sheet arrangement given strength by a line of hydrogen bonds.
He devised the electronegativity scale, which ranks elements in order of their electronegativity – a measure of the attraction an atom has for the electrons involved in bonding (0.7 for caesium and francium to 4.0 for fluorine). The electronegativity scale lets us say how covalent or ionic a bond is.
Pauling’s application of quantum theory to structural chemistry helped to establish the subject. He took from quantum mechanics the idea of an electron having both wave-like and particle-like properties and applied it to hydrogen bonds. Instead of there being just an electrical attraction between water molecules, Pauling suggested that wave properties of the particles involved in hydrogen bonding and those involved in covalent bonding overlap. This gives the hydrogen bonds some properties of covalent bonds.
1922 – while investigating why atoms in metals arrange themselves into regular patterns, Pauling used X-ray diffraction at CalTech to determine the structure of molybdenum.
When X-rays are directed at a crystal, some are knocked off course by striking atoms, while others pass straight through as if there are no atoms in their path. The result is a diffraction pattern – a pattern of dark and light lines that reveal the positions of the atoms in the crystal.
Pauling used X-ray and electron diffraction, magnetic effects and measurements of the heat of chemical reactions to calculate the distances and angles between atoms forming bonds. In 1928 he published his findings as a set of rules for working out probable crystalline structures from the X-ray diffraction patterns.
1939 – ‘The Nature of the Chemical Bond and the Structure of Molecules’
Pauling suggests that in order to create stronger bonds, atoms change the shapes of their waves into petal shapes; this was the ‘hydridisation of orbitals’.
Describing hybridisation, he showed that the labels ‘ionic’ and ‘covalent’ are little more than a convenience to group bonds that really lie on a continuous spectrum from wholly ionic to wholly co-valent.
Pauling developed six key rules to explain and predict chemical structure. Three of them are mathematical rules relating to the way electrons behave within bonds, and three relate to the orientation of the orbitals in which the electrons move and the relative position of the atomic nuclei.
As carbon has four filled and four unfilled electron shells it can form bonds in many different ways, making possible the myriad organic compounds found in plants and animals. The concept of hybridisation proved useful in explaining the way carbon bonds often fall between recognised states, which opened the door to the realm of organic chemistry.
X-ray diffraction alone is not very useful for determining the structure of complex organic molecules, but it can show the general shape of the molecule. Pauling’s work showed that physical chemistry at the molecular level could be used to solve problems in biology and medicine.
A problem that needed resolving was the distance between particular atoms when they joined together. Carbon has four bonds, for instance, while oxygen can form two.It would seem that in a molecule of carbon dioxide, which is made of one carbon and two oxygen atoms, two of carbon’s bonds will be devoted to each oxygen.
Well-established calculations gave the distance between the carbon and oxygen atoms as 1.22 × 10-10m. Analysis gave the size of the bond as 1.16 Angstroms. The bond is stronger, and hence shorter. Pauling’s quantum .3-2. explanation was that the bonds within carbon dioxide are constantly resonating between two alternatives. In one position, carbon makes three bonds with one of the oxygen molecules and has only one bond with the other, and then the situation is reversed.
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1963 – USA
‘The behaviour of a dynamic system depends on its small initial conditions’
While working at the Massachusetts Institute of Technology, Lorenz developed a simple computer model to forecast changes in weather at a number of places. In one of his equations he used a rounded number (for example 0.156 127 became 0.156); his model now predicted quite different conditions.
He suggested that even a small initial unpredictable condition such as a flapping butterfly could produce a larger global change in weather.
The butterfly effect is one aspect of chaos theory that describes disorderly systems. The behaviour of a chaotic system is difficult to predict because there are so many variable or unknown factors in the system.
1909 – USA
‘The charge on the electron’
Millikan measured the charge on the electron.
His experiment showed that the electron is the fundamental unit of electricity; that is, electricity is the flow of electrons. From the experiment Millikan calculated the basic charge on an electron to be 1.6 × 10-19 coulomb. This charge cannot be subdivided – by convention this charge is called unit negative, -1, charge.
Millikan also determined that the electron has only about 1/1837 the mass of a proton, or 9.1 × 10-31 kilogram.
1965 – USA
‘The number of transistors on a computer doubles every 18 months or so’
In 1965, one of the founders of chipmaker Intel observed the exponential growth in the number of transistors per silicon chip and made his prediction which is now generally referred to as Moore’s law.
In 1971 the first Intel chip, 4004, had 2300 transistors. In 1982 the number of transistors increased to 120,000 in the 286, in 1993 to 3.1 million in the Pentium and in 2000 to 42 million in the Pentium 4.
Heat production is now the limiting factor in the production of silicon chips with millions of transistors.
1929 – USA
‘Galaxies are moving away from each other and us at an ever-increasing rate. The more distant the galaxy, the faster it is moving away’
This means that the universe is expanding like a balloon. The principle of an expanding cosmos is at the heart of astronomical theory.
Before 1930, astronomers believed that the Milky Way was the only galaxy in the universe. The discovery of Cepheid variables, which brightened and dimmed in a regular rhythm gave a clue as to the true size of the universe.
In 1923, Hubble spotted a Cepheid variable in the Andromeda Nebula, previously supposed to be clouds of gas. This led to the conclusion that Andromeda was nearly a million light years away, far beyond the limits of the Milky Way and clearly a galaxy in its own right. Hubble went on to discover Cepheids in other nebula and proved that galaxies existed beyond our own.
He began to develop a classification system, sorting galaxies by size, content, distance, shape and brightness. He divided galaxies into elliptical, spiral, barred spiral and irregular. These are subdivided into categories, a, b and c according to the size of the central mass of stars within the galaxy and the tightness of any spiraling arms.
The Earth’s atmosphere alters light rays from outer space; the Hubble Space Telescope, being above the atmosphere, receives images with far greater clarity and detail than any Earth-based optical instrument and its camera can achieve a resolution ten times greater than the largest Earth based telescope.
Construction began on the HST in 1977 and it was launched by the space shuttle Discovery on 25 April 1990. The instruments can detect not only visible light but also infra-red and ultra-violet.
Hubble noticed that the galaxies appeared to be moving away from the region of space in which the Earth is located. It appeared that the further away a galaxy was, the faster it was receding. The conclusion was that the universe, which had previously been considered static is in fact expanding.
In 1915, EINSTEIN’s theory of relativity had suggested that owing to the effects of gravity, the universe was either expanding or contracting. Einstein knew little about astronomy and had introduced an anti-gravity force into his equations, the cosmological constant. Hubble’s discoveries proved Einstein had been right after all and Einstein later described the introduction of the gravitational constant as ‘the biggest blunder of my life’.
Hubble’s discovery that the universe is expanding led to the development of the ‘big-bang’ model of the universe.
1752 – The New World
‘If you would not be forgotten when you are dead and rotten, either write things worth reading, or do things worth writing about’
Curious about how just about everything works, from governments to lightning rods, Franklin’s legacy, in addition to the many inventions such as lightning conductors, bifocal lenses and street lamps, was one of learning. He established one of the first public libraries as well as one of the first universities in America, Pennsylvania. He established the Democratic Party. Franklin was one of the five signatories of the Declaration of Independence from Great Britain in 1776 and was a later participant in the drafting of the American Constitution.
‘Benjamin Franklin’s choice for the signs of electric charges leads to electric current being positive, even though the charge carriers themselves are negative — thereby cursing electrical engineers with confusing minus signs ever since.
The sign of the charge carriers could not be determined with the technology of Franklin’s time, so this isn’t his fault. It’s just bad luck.’
Franklin was a pioneer in understanding the properties and potential of electricity. He undertook studies involving electric charge and introduced the terms ‘positive’ and ‘negative’ in explaining the way substances could be attracted to or repelled by each other according to the nature of their charge. He believed these charges ultimately cancelled each other out so that if something lost electrical charge, another substance would instantly gain the same amount.
His work on electricity climaxed with his kite flying experiment of 1752. In order to prove lightning to be a form of electricity, Franklin launched a kite into a thunderstorm on a long piece of conducting string. Tying the string to a capacitor, which became charged when struck by lightning, vindicated his theories.