The term 'atom' comes from the Greek word atomos, meaning 'indivisible.' Earlier, it was considered that the atoms were the smallest unit of a substance and could not be split further. Today, we know that atoms can be further split into smaller particles. An atom is made up on the atomic nucleus, which consists of positively charged protons and an equal number of neutrons. and atomic shells. These are occupied by negatively charged electrons, which move at high speeds around the atomic nucleus - like planets around the sun.