[Science Form 4] Numbers In Symbols

An atom, which is the smallest unit of all matter, consists of a nucleus and an outer region.

The nucleus at the centre of an atom contains nucleons (protons and neutrons), and is surrounded by electrons orbiting in the outer region.

A neutral or stable atom has an equal number of protons and electrons.

The number of protons, also called the atomic number, is written as a subscript to the left of the element symbol.

In contrast, the total number of protons and neutrons, also called the nucleon number or mass number, is written as a superscript to the left of the element symbol.