Nuclear physics showed that half or more of the mass of most nuclei is due to uncharged particles fractionally heavier than protons, called neutrons.

The Atomic Number of an element = number of protons.
The Atomic Weight of an element = number of protons + neutrons.

Since atoms are neutral, the number of protons = the number of electrons.

The chemical properties of an atom are largely determined by the configuration of the electrons associated with it.

Bubble Chamber