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Jan 1, 1000
Democritus, Greece
460 - 370 B.C.
Ancient Greece
Democritus developed a theory which outlined that the universe was made of empty space, and an almost infinite number of indivisible, tiny particles, called atoms. These atoms were different in form, position and arrangement. Natural phenomena's occurred because of the interaction and/or movement of atoms.
Democritus formed the beginning of the atom. His theory set up the research and discoveries that would follow. -
Isaac Newton, England
Newton believed creation was not pure chance as the Ancient Greeks had believed, but the work of a Divine being, i.e. God. However his laws on gravitation (every object in the universe attracts every other object with a force, gravitational force) enhanced the development of atomic theory. -
John Dalton, England
Dalton built on a scientist named Lavoisier's observations, stating that each element was a unique substance and all the atoms forming a particular element were similar and identical in weight.
Dalton said:
- all matter is made of minute particles called atoms
-atoms cannot be broken down into smaller particles
-atoms of the same element are alike
-atoms join together in different atoms
Dalton suggested a simple rule: if two elements form one compound, assume the compound has only one atom -
G.J. Stoney, Ireland
Stoney introduced the electron as the fundamental unit of electricity. He said that the electron was a twentieth (1o-20) of a quantity of electricity otherwise known as an ampere.
Stoney introduced the term 'electron' in 1891 but introduced the concept as early as 1874. -
J.J. Thomson, Britain
Thomson proposed that atoms were divisible and consisted of electrons. He knew that atoms had no charge thus the conclusion that atoms were made of negatively charged particles would be incorrect. Thomson then proposed that the electrons were distributed throughout a sea of positively charged particles like plums were distributed throughout a plum pudding (This was his Plum Pudding Model). -
Nagaoka, Japan
Nagoaka developed an analogy for the layout of an atom. He proposed the Saturnian Model which showed that the movement of electrons around the positively charged nucleus was due to a huge nucleus - Saturn's rings are stable because they are orbiting such a massive planet. Also the electrons moving around the nucleus were bound by electrostatic forces- like gravitational pull keeping the rings of Saturn revolving. -
Millikan, USA
Millikan conducted an oil drop experiment many times and found the charge of the electron to be 1.592 × 10−19 coulomb. His answer is lower than the modern value probably because he measured air viscosity wrong,
The knowledge of it's charge meant knowledge of its mass.
So he found that :
a. Charge is quantised
b. The Electron is a sub atomic particle -
E. Rutherford, America
Rutherford made a model of the subatomic particles in 1911. In this model there was a central charge surrounding by orbiting particles. His model portrayed the nucleus as very small, but he said it made up most of the weight of the atom. This analogous to the solar system. -
H.G.J. Moseley, England
Moseley's law outlines that the ''square root of the frequencies of lines in atomic x-ray spectra depends linearly on the atomic number of the emitting atom.'' -
Bohr, Germany
Bohr introduced 'shells' in atoms. He produced this as a reason why electrons are stable in an atom. Bohr proposed that electrons moved in orbits of one size and energy.
Each shell can only have a set number of electrons and when full is moved up to the next level. -
James Chadwick, England
Chadwick proved the existence of the neutron- it's mass was 0.1% more than the proton's and it had a neutral charge.
