| John Dalton
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- Five main points of Dalton’s Atomic Theory
- All matter is composed of extremely small particles called atoms.
- All atoms of a given element are identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from those of any other element.
- Atoms cannot be created, divided into smaller particles, or destroyed.
- Different atoms combine in simple whole-number ratios to form compounds.
- In a chemical reaction, atoms are separated, combined, or rearranged.
Weakness
- Atoms consist of even smaller particles called electrons, protons and neutrons.
- Atoms can be created and destroyed in the nuclear reactions such as nuclear fusion and nuclear fission..
- Atoms of the same element can have different physical properties, for example, isotopes of hydrogen.
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J.J. Thomson  |
The electrons were positioned uniformly throughout the atom. |
| Ernest Rutherford
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- the atom is mostly empty space,
- most of the atom’s mass concentrated in a tiny center, the nucleus and electrons being held in orbit around it by electrostatic attraction.
- The nucleus was around 10-15 meters in diameter, in the centre of a 10-10 metre diameter atom.
- Those alpha particles that had come into close proximity with the nucleus had been strongly deflected whereas the majority had passed at a relatively great distance to it.
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Neils Bohr
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- Electrons in an atom of an element are not randomly distributed around the atomic nucleus.
- Electrons move around the nucleus in fixed orbits.
- Each orbit forms a circle and has a fixed distance from the nucleus.
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James Chadwick
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- Chadwick discovered the presence of neutrons in the nucleus.
- He concluded that the nucleus contains another tiny particle known as a neutron that has no charge.
- The neutron mass is almost similar to the proton mass.
- All nuclei contain protons and neutrons, except for the hydrogen which contains protons. only
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