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400 BCE
400 BC - Democritus
That all matter is alternatively formed of different tiny, solid and unbreakable particles. Additionally, he found that the properties of these particles also determined the properties of matter. He was the first to use the term ‘atomos’ coming from the word ‘atom’. -
1807 - John Dalton
That each element consists of its own brand of invisible atoms, which are very unique. He also assigned atomic weights to the atoms, only of the 20 he knew. -
1903 - JJ Thompson
He had discovered that atoms were also made up of smaller components. He referred to the components as "corpuscles”, which are now known as electrons. -
1911 - Ernest Rutherford
Development of the nuclear model of an atom. This model showed that most of the mass is contained in a small positive nucleus, surrounded by negative electrons. -
1913 - Niels Bohr
Electrons can only travel around the nucleus on certain pathways, or orbits, similar to planets. Because of this, it was often called the planetary model. -
1932 - James Chadwick
James Chadwick discovered neutrons. This proved that in the nucleus, there weren’t just positively charged molecules, but negative as well.