Atomic structure At the turn of the 18th century, the generally held view of the atom was that put forward by John Dalton. His atomic theory had as its basis that atoms were indestructible. It was not until around the turn of the 19th century that this was shown to be incorrect. Atoms are actually composed of subatomic particles known as neutrons, protons, and electrons. The bulk of the mass of the atom is made up of the mass of the neutrons and protons, each given an atomic mass of 1, the mass of the electron being negligible. The neutron has no charge whereas the proton has a single positive charge and the electron a single negative charge.