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John Dalton
Suggested that matter is made of small, hard spheres called atoms that are different for different elements.- All matter is made of small particles called atoms.
- Atoms cannot be created, destroyed or divided into smaller particles.
- All atoms of the same element are identical in mass and size, but they are different in mass and size from the atoms of other elements.
- Compounds are created when atoms of different elements link together in definite proportions.
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JJ Thomson
Discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube. Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of bodies much smaller than atoms. This lead to his plum pudding model (electrons are embedded in a sphere of positive electrical charge) -
Ernest Rutherford
Rutherford designed an experiment to probe the atom by using alpha particles as atomic bullets. A beam of alpha particles was shot at a thin sheet of gold foil He concluded an atom is made of:
A nucleus: a tiny core that is very small in volume, dense and intensely positive
An electron cloud: an “envelope” very large in volume, light and negatively charged -
Neils Bohr
Bohr proposed that electrons surround the nucleus in specific “energy levels” or “shells.” In the Bohr model, the electron moves in a circular orbit at a fixed distance from the nucleus. -
Quantum Mechanical Model
According to quantum theory, it’s impossible to know the exact position and momentum of an electron at the same time. -