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400 BCE
Democritus
Democritus theorised that the tiniest parts of matter could eventually not be divided.
Other Greek philosophers including Aristotle believed that matter was made up of only 4 elements; earth, fire, water and air. -
John Dalton
John Dalton was pioneer of modern atomic theory. Dalton made many observations about particles; the first part states that all masses is made of atoms which are indivisible.
The second part says that all atoms of a given element are identical in mass and properties.
The third part says that says that compounds are combinations of two or more different types of atoms.
The fourth part of a theory states that a chemical reaction is a rearrangement of atoms. -
JJ Thompson
JJ Thompson experimented with electric charges and cathode ray tubes and suggested the movement of negatively charged particles were part of the atom. He later proposed the model of atom, like 'sultanas' in a 'plum pudding'. -
Ernest Rutherford
Ernest Rutherford did experiments firing positive charges at gold atoms and found, some charges passed right through, while others bounced back.
He proposed a model where the atom was made of mostly empty space with all the positive charge in the centre which he called the nucleus. He thought the electrons moved around the nucleus in fixed orbits like planets move around the Sun. -
Niels Bohr
Bohr could see that Rutherford's model was unstable and suggested a more complicated one that was consistent with observations by scientists.
Bohr believed that most of the atom was empty space with most of the mass in the central nucleus where the positive charges were located.
He proposed that the electrons changed orbit forming an electron cloud around the nucleus. -
James Chadwick
James Chadwick and Ernest Rutherford noticed that the overall mass of the nucleus was more than could be accounted for by protons alone.
In 1932 Chadwick found proof of another type of particle the same size as the proton with no charge. This was the neutron.
