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Democritus
All matter (through space and time) is made up of atoms (small, invisible, indestructible and physically indivisible particles which are infinite in number).
A void (empty space) exists between atoms.
Characteristics of an object are determined by the shape of its atoms.
Atoms are homogeneous, solid and differ in size, shape, mass, position and arrangement and will always be in motion.
Democritus contributed to our understanding of the atom as he suggested the actual existence of it. -
Isaac Newton - England
- Theorised a mechanical universe with small solid masses in motion.
- Defined the laws of gravity and motion Newton's law of gravity concludes that all objects attract everything other object in the universe with gravitational forces. Atoms are solid, massey, hard, impenetrable, moveable particles From his laws of gravity and motion, it could be concluded that atoms are held together by opposing forces. Newton's theories and laws were used as a basis for subsequent research in atomic theory.
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Dalton - England
Experimented with the combination of atoms in gases. Created idea of the table of elements
Matter is made of indivisible spherical atoms
Atoms in an element are identical. Atoms of different elements have different masses, which, varies the characteristics.Compounds are made of atoms combined in definite proportions..Dalton’s atomic theory established that atoms and their weight determines the composition of matter and that new matter can be created by combining elements -
George Johnstone Stoney - Ireland
Determined that electricity was made of negative particles. In 1891, he named these negative particles “electrons” although; his idea of “an absolute unit of electricity” was formed in 1874
Estimated the number of molecules in a cubic millimetre of gas at room temperature and pressure from the theory of gases
G.J.Stoney introduced the term “electron” to describe the fundabmental unit of electricity. His contributions had laid foundation for the discovery of the electron as a subatomic structure -
Joseph John Thomson - England
Thompson discovered that atoms contains negative particles known as electrons inside a cloud of positive charge.This proved Stoney’s hypothesis correct In 1897, Thomson experimented with cathode rays passing from a negative electrode towards a positive electrode from which he concluded that cathode rays are streams of electrons
In order to show how electrons were situated in the atom, Thomson proposed the “plum pudding” model which aroused questions and further research. -
Max Planck - Germany
He is the founder of the quantum theory.
Proposed the idea of quantization to explain how a hot, glowing object emitted light. In 1897, Planck realised that energy can only be emitted in separate amounts.
Max Planck developed the cornerstone of modern physics. Quantum theory can be used to explain why atoms are stable. -
Hantaro Nagaoka - Japan
Nagaoka developed a “Saturnian Model” of the atom. From the model, it could be concluded that:
-There is a large nucleus containing a positive charge
-Electrons revolving around the nucleus are bound by electrostatic forces. Nagaoka thought that the electrons orbiting the nucleus would lose energy and collide with the nucleus.
Although the model was unstable and incorrect, the idea that electrons revolve around the nucleus provided a general idea of the positions of the nucleus and electrons. -
Robert Milikan - America
His experiment measured the force on tiny charged droplets of oil suspended against gravity between two metal electrodes. Knowing the electric field, the charge on the droplet could be determined. HIs results found 1.592 × 10−19 coulomb to be the charge on a single electron. This quantity was a constant for all electrons. Milikan accurately determined the electrical charge of a single electron. His research continued to support the existence of subatomic particles. -
Ernest Rutherford - New Zealand
Ernest Rutherford’s model described the atom as having a small, dense, positive nucleus which is orbited by negatively charged electrons in the void outside it.
He named the nucleus and proton. By using alpha radiation to split an atom’s nucleus, Rutherford changed nitrogen into oxygen.
-probed the atoms in a piece of thin gold foil suggesting that an atom is not as solid as previously believed.
-predicted the existence of the neutron.
His model rejected J.J. Thompson’s ‘plum pudding" model. -
Henry G.J. Moseley - England
Moseley used X-ray diffraction to show that each element has a different atomic number. He found that the atomic number of an element is equal to the number of protons (Positive charges) in the nucleus. With this, he was able to predict elements for the six remaining gaps in the periodic table and to determine identities of rare earths elements. H.G.J. Moseley’s work was used to reorganize the periodic table based upon atomic number instead of atomic mass. -
Niels Bohr - Denmark
- Postulated that electrons moved in fixed orbital shells around the atom’s nucleus and that the outer, higher-energy orbits determined the chemical properties of the different elements
- the outer orbit of an atom could hold more electrons than the inner orbit.
- suggesting that as electrons jumped from higher to lower orbits, Bohr's work helped solve the problems classical physics could not explain about the nuclear model of the atom. Refined the nuclear model.
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James Chadwick - England
Early experiments established that atomic number is determined by the number of protons in an atom His experiments on the bombardment of light elements with alpha particles led to the discovery of the neutron (1932).
Also had worked on the development of the atomic bomb.Proved Rutherford’s proposal of the existence of the neutron to be correct and finalised the substructure of the atom. -
Lise Meitner,Hahn, Fritz Strassmann - Sweden
December 1938, reported they had detected the element barium after bombarding uranium with neutrons. Results interpreted as nuclear fission. She was also Max Planck's assistant and often attended his lectures.
They had discovered nuclear fission.. -
Glenn Seaborg - Sweden
Discovering the element plutonium and 9 other transuranium elements. Suggested a change in the layout of the periodic table. Also Identified Isotopes.
Added to the periodic table of elements. -
Enrico Fermi - Italy
Created the first man-made nuclear reactor.
Demonstrated the existence of new radioactive elements produced by neutron irradiation, discovered how nuclear reactions are brought about by slow neutrons
