The Discovery of Which Particle Proved that the Atom is Not Indivisible?

The atomic theory of matter, which states that all matter is composed of atoms, has been around for centuries. However, until the early 20th century, scientists believed that atoms were indivisible, meaning that they could not be broken down into smaller particles. This all changed with the discovery of subatomic particles. But which discovery proved that the atom is not indivisible? Was it protons, electrons, or neutrons?

The answer is: protons.

In 1917, Ernest Rutherford, a British physicist, conducted an experiment that involved firing alpha particles (heavy, positively charged particles) at a thin sheet of gold foil. Rutherford expected the alpha particles to pass straight through the foil, as he believed that atoms were mostly empty space. However, to his surprise, some of the alpha particles were deflected at large angles or even bounced back towards the source.

Rutherford realized that the only explanation for the alpha particles' behavior was that they were being repelled by a dense, positively charged region in the atom's center. This region, which Rutherford called the nucleus, was made up of protons.

Before Rutherford's discovery, the prevailing model of the atom was known as the Thompson model (named after J.J. Thompson), which proposed that atoms were composed of a uniform positive charge with negatively charged electrons scattered throughout. The discovery of the nucleus and its positively charged protons suggested a new model of the atom, in which electrons orbit the nucleus like planets around the sun.

However, there was still one missing piece in the puzzle: the neutron. Neutrons, which have no charge, were not discovered until 1932 by James Chadwick. With the discovery of neutrons, scientists were finally able to understand how atoms of different elements could have different masses (since each element has a different number of protons and neutrons in their nucleus).

In conclusion, it was the discovery of protons in the nucleus that proved the atom was not indivisible. This discovery was a significant breakthrough in our understanding of the nature of matter and paved the way for many subsequent discoveries in the field of particle physics.