Mass Number – IGCSE Chemistry Definition and Key Facts

Mass number is one of the first atomic structure concepts in the Cambridge 0620 syllabus. It links directly to isotopes, relative atomic mass, and subatomic particle calculations that appear on every Paper 1 and Paper 2.

The 0620 definition

The mass number (symbol A) is the total number of protons and neutrons in the nucleus of an atom.

Mass number = number of protons + number of neutrons

It is also called the nucleon number, since protons and neutrons are both nucleons (particles in the nucleus).

Notation

The mass number is written as a superscript to the upper left of the element symbol. The proton number (Z) is written as a subscript below it.

For example, carbon-12 is written as ¹²₆C, meaning:

• Mass number (A) = 12 (top)
• Proton number (Z) = 6 (bottom)
• Number of neutrons = 12 - 6 = 6

Calculating subatomic particles

Given the mass number and proton number, you can find all three subatomic particles:

Particle	How to find it
Protons	Equal to the proton number (Z), found on the periodic table
Neutrons	Mass number - proton number (A - Z)
Electrons	Equal to the proton number in a neutral atom; different in ions

Examples

Atom/Ion	Mass number	Proton number	Protons	Neutrons	Electrons
²³₁₁Na	23	11	11	12	11
³⁵₁₇Cl	35	17	17	18	17
⁵⁶₂₆Fe	56	26	26	30	26
²³₁₁Na⁺	23	11	11	12	10
³⁵₁₇Cl⁻	35	17	17	18	18

Note: ions have different numbers of electrons. A positive ion has lost electrons; a negative ion has gained electrons. The mass number and number of protons and neutrons remain the same.

Mass number and isotopes

Isotopes are atoms of the same element with the same proton number but different mass numbers. This means they have different numbers of neutrons.

Example: chlorine has two isotopes:

• Chlorine-35 (³⁵₁₇Cl): 17 protons, 18 neutrons
• Chlorine-37 (³⁷₁₇Cl): 17 protons, 20 neutrons

Both are chlorine (same proton number = 17), but they have different masses because of different neutron counts.

Mass number vs relative atomic mass

Feature	Mass number	Relative atomic mass
Definition	Protons + neutrons in one atom	Weighted average mass of all isotopes
Always a whole number?	Yes	No (often a decimal)
Example for Cl	35 or 37	35.5
Found on periodic table?	No (specific to each isotope)	Yes (the number shown)

The relative atomic mass of chlorine is 35.5 because it is a weighted average of 75% chlorine-35 and 25% chlorine-37.

Worked exam question

An atom has 12 protons and 12 neutrons. (a) State the mass number. [1] (b) Identify the element. [1] (c) Another atom of the same element has a mass number of 26. State the number of neutrons in this atom. [1]

(a) 24 (12 + 12) [1]

(b) Magnesium (Mg) — identified by the 12 protons / proton number 12 [1]

(c) 26 - 12 = 14 neutrons [1]

Common exam mistakes

1. Confusing mass number with relative atomic mass. Mass number is a whole number for a specific isotope; relative atomic mass is an average and may be a decimal.
2. Thinking the mass number includes electrons. Electrons have negligible mass — the mass number counts only protons and neutrons.
3. Forgetting to subtract proton number when finding neutrons. Neutrons = A - Z, not A alone.
4. Changing the mass number when forming an ion. Gaining or losing electrons changes the electron count but does not affect the mass number.