Summary
Highlights
The earliest significant theory on matter proposed that everything is made of tiny, indivisible particles called atoms, separated by empty space. This idea was first suggested by Democritus around 500 BC. John Dalton later built upon this in the 1800s, describing atoms as solid spheres and suggesting that different types of spheres made up different elements.
In 1897, J.J. Thomson's experiments showed that atoms were not solid spheres but contained negatively charged particles, now known as electrons. He proposed the 'plum pudding model,' where the atom was a positively charged sphere with discrete electrons embedded within it.
In 1909, Ernest Rutherford and his students conducted an experiment where they fired positively charged alpha particles at a thin gold sheet. Contrary to the plum pudding model's prediction, some alpha particles were deflected, and a few even bounced back, indicating that the positive charge in an atom was concentrated in a small, dense nucleus, with electrons orbiting around it in a 'nuclear model'.
Rutherford's nuclear model faced a challenge because it didn't explain why negatively charged electrons wouldn't collapse into the positively charged nucleus. In 1913, Niels Bohr proposed that electrons orbit the nucleus in specific shells, similar to planets orbiting the sun. This model prevented the atom from collapsing and is a fundamental part of our current understanding.
Further experiments by Rutherford revealed that the positive charge in the nucleus is made up of smaller discrete particles called protons. Shortly after, James Chadwick provided evidence for neutral particles in the nucleus, which were named neutrons. These discoveries completed the modern atomic model, which remains largely consistent with our current understanding.