In 400 B.C., Democritus (Early Greek Philosopher) hypothesised that all matter (plus space and time) is composed of tiny indestructible units, called atoms. He thought the characteristics of an object are determined by the shape of its atoms. So, for example, sweet things are made of smooth atoms.
In the 1800s, John Dalton developed the atomic theory. As he experimented he worked out the atomic weights and invented symbols for atoms and molecules. His experiment concluded that: - All matter is made of atoms - Atoms can’t be broken down - Atoms are rearranged in chemical reactions - Compounds are made when 2 or more different atoms join together
In 1897, J.J. Thomson determined the charge to the mass ratio of electrons. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He noticed that an atom can be divided. He noticed atoms are made of positive cores and negatively charged particles, within it. He developed the Plum Pudding Model before the atomic nucleus was discovered.
In 1911 Ernest Rutherford showed that an atom needs to contain a central nucleus which was further evidence that atoms contain smaller pieces.
In 1913, Niels Bohr further developed Rutherford’s atom model by using experimental evidence to support the idea that electrons occupy particular orbits or shells around the nucleus.
In 1932, James Chadwick played a vital role in the atomic theory, as he discovered the Neutron in atoms. Neutrons are located in the center of an atom, in the nucleus along with the protons. They have neither a positive nor negative charge.