1.1.3—Terms: atomic and proton number, mass

The atomic number, Z, is the number of protons in the nucleus. It identifies the element because every atom of that element has the same proton number.

The mass number, A, is the total number of protons and neutrons in the nucleus. Therefore, neutron number = A − Z. Electrons are not included in the mass number because their mass is negligible relative to nuclear particles.

A nuclide can be written as ^A_ZX: A is the mass (nucleon) number at the upper left, Z is the atomic (proton) number at the lower left, and X is the element symbol. Read both numbers before identifying the particle counts.

Changing Z changes the element; changing A while keeping Z fixed changes the isotope. Do not confuse mass number for one nuclide with relative atomic mass, which is an average based on isotope abundances.