Mass number
The total number of protons and neutrons in the nucleus of an atom, denoted by A. Mass number does not include electrons. The number of neutrons in an atom is calculated as A − Z (mass number minus atomic number). Different isotopes of the same element have the same Z but different A values.
Formula
A = Z + N
Real World
Carbon dating relies on distinguishing carbon-14 (A = 14) from carbon-12 (A = 12); archaeologists at the British Museum used this to date the Lindow Man bog body to around 2 BC.
Exam Focus
Always show N = A − Z working explicitly when calculating neutron number — one mark is often awarded for the method.
How well did you know this?