ROBERT MILLIKAN (1868-1953)

1909 – USA

The charge on the electron’

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ROBERT ANDREWS MILLIKAN

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 his 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.

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LEO BAEKELAND (1863-1944)

1909 – USA

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LEO BAEKELAND

‘Synthetic bakelite, the first plastic’

Bringing formaldehyde and phenol together under high temperature and pressure produced the world’s first thermosetting plastic.

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ROBERT GODDARD (1882-1945)

1915 – USA

‘Demonstrates that rocket engines can produce thrust in a vacuum’

‘Robert Goddard stands as the epitome of the early American desire to conquer space’

It was generally believed that it would be impossible for a rocket to move outside of the Earth’s atmosphere, as there was nothing for it to push against in order to gain propulsion. Goddard had already gone a long way to revoking this assumption by 1907 in completing calculations to show that a rocket could thrust in a vacuum, and had backed up this concept with physical experiment in 1915.

His booklet “A Method of Reaching Extreme Altitudes” described the multi-stage principle and presented advanced ideas on how to improve the performance of solid-fuel rockets.

‘I have read very attentively your remarkable book A Method for Reaching Extreme Altitudes edited in 1919 and I have found in it quite all the ideas which the German Professor H.Oberth published in 1924′ (in a letter from Soviet engineer & author Nikolai Alexsevitch Rynin)

In 1926 he launched the world’s first liquid-fuelled rocket using gasoline and liquid oxygen; the 2.5 second, 41 feet flight proved that liquid-fuel propellants could be used to power a rocket instead of exploding in a catastrophic detonation.
Over the next decade, Goddard filed patents for guidance, control and fuel pump mechanisms.

In spite of his success (by 1935 he had launched a rocket at Roswell, New Mexico which traveled faster than the speed of sound and another which achieved an altitude of 1.7 miles – a record at that time) the US Government largely ignored his efforts until the space race gathered momentum in the 1940s and 1950s.
The government was eventually forced to pay one million dollars to Goddard’s widow for patent infringement in acknowledgement of the use they had made of his designs as a basis from which to begin development.

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EDWIN HUBBLE (1889-1953)

1929 – USA

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EDWIN HUBBLE

‘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.
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. Its camera can achieve a resolution ten times greater than the largest Earth based telescope.

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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’.

This detailed picture of the Helix Nebula shows a fine web of filaments, like the spokes of a bicycle, embedded in the colorful red and blue gas ring around this dying star. The Helix Nebula is one of the nearest planetary nebulae to Earth, only 650 light years away.This "double cluster," NGC 1850, is located in the Large Magellanic Cloud. It consists of a large cluster of stars, located near a smaller cluster (below and to the right). The large cluster is 50 million years old; the other only 4 million years old. The cluster is surrounded by gas believed to be created by the explosion of massive stars.This youngest-known supernova remnant in our galaxy lies 10,000 light years away in the constellation Cassiopeia. The light from this exploding star first reached Earth in the 1600s.

Hubble’s discovery that the universe is expanding led to the development of the ‘big-bang’ model of the universe.


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LINUS PAULING (1901- 94)

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. The two atoms are bound together by the shared electrons. This was proposed by Lewis and Irving Langmuir in 1916. Two hydrogen atoms form the hydrogen molecule, H2, by each sharing their single electron.

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.Water_animation

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.

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, now known as an alpha-helix. 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.

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.

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.

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’.

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.

      

      

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.

1951 – published his findings one year after WILLIAM LAWRENCE BRAGG’s team at the Cavendish Laboratory.

CARBON BONDING
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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SUBRAHMANYAN CHANDRASEKHAR (1910- 95)

1931 – USA

‘The maximum possible mass of a white dwarf star is 1.4 times the Sun’s mass’

Photograph of SUBRAHMANYAN CHANDRASEKHAR (1910-95) Using Albert Einstein’s special theory of relativity and the principles of quantum physics, Chandrasekhar formulated the idea in 1930 that it is impossible for a white dwarf star, which is supported solely by a degenerate gas of electrons, to be stable if its mass is greater than 1.44 times the mass of the Sun.

SUBRAHMANYAN CHANDRASEKHAR

The Chandrasekhar limit is a physical constant. It is the greatest mass a white dwarf star can have before it goes supernova, approximately 1.44 solar masses.
Chandrasekhar showed that it is impossible for a white dwarf star, which is supported solely by electron degeneracy pressure, to be stable if its mass is greater than 1.44 times the mass of the Sun. If such a star does not completely exhaust its thermonuclear fuel, then the limiting mass may be slightly larger.
Above this limit a star has too much mass to become a white dwarf after gravitational collapse. A star of greater mass will become a neutron star or a black hole.

The radius of a black hole is the radius of the event horizon surrounding it. This is the Schwarzschild radius, after the German astronomer Karl Schwarzschild (1873-1916) who in 1916 predicted the existence of black holes.
The Schwarzschild radius is roughly equal to three times the weight of the black hole in solar masses. A black hole weighing as much as the Sun would have a radius of 3 kilometres, one with the mass of the Earth would have a radius of only 4.5 millimetres.
A black hole’s effects occur within ten Schwarzschild radii of its centre.

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THOMAS HUNT MORGAN (1866-1945)

1933 – USA

‘The Mechanism of Mendelian Heredity’ (1915), ‘The Theory of the Gene’ (1926)’

Morgan laid the foundation for understanding MENDEL’s observations and helped to provide the science required to reinforce CHARLES DARWIN’s conclusions.

Starting with Mendel’s laws of segregation and independent assortment, Morgan investigated why there are far fewer chromosomes – the long thread-like structures present in the nucleus of every living cell, which grow and divide during cell splitting, – than there are ‘units of heredity’. Morgan could not see how these few chromosomes could account for all the changes that occur from one generation to the next.

Mendel’s ‘factors of heredity’ had been renamed ‘genes’ in 1909 by the Dane Wilhelm Johannsen.

When the organism forms its reproductive cells (gametes), the genes segregate and pass to different gametes.
Since it had been separately established that chromosomes play an important part in inheritance, then groups of genes had to be present on a single chromosome.
If all the genes were arranged along chromosomes, and all chromosomes were transmitted intact from one generation to the next, then many characteristics would be inherited together. This implicitly invalidates Mendel’s law of independent assortment, which dictated that hereditary traits caused by genes would occur in all possible mathematical combinations in a series of descendants, independent of each other.

Experimental evidence often seemed to back-up the mathematical forecasts for characteristics present in descendants that Mendel had suggested; Morgan felt that the law of independent assortment could not accurately model the process of arriving at the end result.

He began his experiments with the fruit fly, which has just four pairs of chromosomes, in 1908.
He observed a mutant white-eyed male fly, which he extracted for breeding with ordinary red-eyed females. Over subsequent generations of interbred offspring, the white-eyed trait returned in some descendants, all of which turned out to be males. Clearly, certain genetic traits were not occurring independently of each other but were being passed on in groups.
Rather than invalidating Mendel’s law of independent assortment, a simple adjustment was required to unite it with Hunt’s belief in chromosomes to produce his thesis.
He suggested that the law of independent assortment did apply – but only to genes found on different chromosomes. For those on the same chromosome, linked traits would be passed on; usually a sex-related factor with other specific features (such as, the male sex and the white-eyed characteristic).

The results of his work convinced Morgan that genes were arranged on chromosomes in a linear manner and could be mapped. Further testing showed that, as chromosomes actually break apart and re-form during the production of sperm and egg cells, linked traits could occasionally be broken during the exchange of genes (recombination) that occurred between pairs of chromosomes during the process of cell division. He hypothesised that the nearer on the chromosome the genes were located to each other, the less likely the linkages were to be broken. Thus by measuring the occurrence of breakages he could work out the position of the genes along the chromosome.
In 1911 he produced the first chromosome map showing the position of five genes linked to gender characteristics.

In 1933 Hunt Morgan received the Nobel Prize for Physiology.

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