1.1.5: Atomic Nomenclature

An atom is typically described using the following notation.

\begin{align} ^A_Z\text{X} \end{align}

Atomic Number (Z)

The atomic number, frequently written as Z, is the number of protons in the atom. It is written at the bottom left of the atomic symbol. The atomic number determines the element of the atom.

Atomic Weight (A)

The atomic weight is the number of protons and neutrons in the atom, typically written as A.


Isotopes have the same atomic number (Z) but differing atomic weights (A). Isotopes may be stable or unstable. Unstable isotopes are radioactive.


An isomer occurs when the nucleus is in an energy state above baseline, known as a metastable state. This is denoted by a small letter 'm' after the atomic number, eg:

\begin{align} ^{99m}_{43}\text{Te} \end{align}

Isomers tend to give off their excess energy as electromagnetic energy (photons). This is known as gamma radiation.