Mass Number (Nucleon Number)
Summary: The mass number (A) is the total number of protons and neutrons in the nucleus of an atom, used to identify specific isotopes and calculate the number of neutrons. Tags: igcse chemistry Created: 2026-07-18
The mass number (also called the nucleon number) is the total number of protons and neutrons (collectively called nucleons) in the nucleus of an atom. It is given the symbol A and is always a whole number. Unlike the atomic number (Z), which is fixed for a given Element, the mass number can vary between different isotopes of the same element — for example, carbon-12 has A = 12 while carbon-14 has A = 14, yet both have Z = 6. The mass number is written as a superscript to the left of the element symbol in nuclide notation (e.g. ^23_11Na). From the mass number and atomic number, the number of neutrons can be calculated using the simple relationship neutrons = A - Z. The mass number should not be confused with relative atomic mass (Ar), which is a weighted average across all isotopes and is often not a whole number.
Definition and Notation
The mass number (A) is defined as:
A = number of protons + number of neutrons
It is also called the nucleon number because protons and neutrons are both nucleons (particles found in the nucleus).
In nuclide notation, the mass number appears above the atomic number:
Mass Number → A
X ← Element symbol
Atomic Number → Z
Examples Table
| Nuclide Notation | Element | Atomic Number (Z) | Mass Number (A) | Protons | Neutrons (A - Z) | Electrons (neutral) |
|---|---|---|---|---|---|---|
| ^1_1H | Hydrogen | 1 | 1 | 1 | 0 | 1 |
| ^4_2He | Helium | 2 | 4 | 2 | 2 | 2 |
| ^12_6C | Carbon | 6 | 12 | 6 | 6 | 6 |
| ^16_8O | Oxygen | 8 | 16 | 8 | 8 | 8 |
| ^23_11Na | Sodium | 11 | 23 | 11 | 12 | 11 |
| ^27_13Al | Aluminium | 13 | 27 | 13 | 14 | 13 |
| ^35_17Cl | Chlorine | 17 | 35 | 17 | 18 | 17 |
| ^37_17Cl | Chlorine | 17 | 37 | 17 | 20 | 17 |
| ^40_20Ca | Calcium | 20 | 40 | 20 | 20 | 20 |
Calculating Neutrons
The most common use of the mass number is to find the number of neutrons:
Number of neutrons = A - Z
For example, a ^56_26Fe atom has A = 56 and Z = 26, so it contains 56 - 26 = 30 neutrons.
Mass Number and Isotopes
Isotopes are atoms of the same element (same Z) with different mass numbers (different A). For instance, chlorine exists as two stable isotopes: ^35_17Cl (A = 35, 18 neutrons) and ^37_17Cl (A = 37, 20 neutrons). Both have Z = 17 (17 protons) and therefore identical chemical properties, but different mass numbers and slightly different physical properties such as density and rate of diffusion.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.2 Core 4, Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Mass number is the same as relative atomic mass” | Mass number is a whole number for a specific isotope (e.g. ^35Cl has A = 35). Relative atomic mass (Ar) is a weighted average of all naturally occurring isotopes (e.g. Cl = 35.5). |
| ”Mass number equals the number of protons” | Mass number equals protons plus neutrons. The number of protons alone is the atomic number (Z). |
| ”If two atoms have the same mass number, they are the same element” | Different elements can have the same mass number (isobars). For example, ^40_18Ar and ^40_20Ca both have A = 40 but are different elements determined by their different Z values. |
| ”The mass number is found on the periodic table” | The periodic table shows relative atomic mass (Ar), not mass number. Mass number applies to specific isotopes and must be given or calculated. |