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Mass Number

Mass number is the total number of protons and neutrons present in the nucleus of an atom.

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Student-friendly explanation

Mass number is represented by A. Since protons and neutrons have nearly one unit mass each and electrons have very small mass, mass number mainly counts the heavy particles in the nucleus. It is a whole number for a particular atom and should not be confused with average atomic mass.

How to write this in exams

  1. 1

    Start with the exact idea

    Mass number is the total number of protons and neutrons present in the nucleus of an atom.

  2. 2

    Then show how to use it

    Take proton count from Z, take neutron count if given, add them for A; if A and Z are given, subtract to find neutrons.

  3. 3

    Add one concrete example

    An atom with 9 protons and 10 neutrons has mass number 19.

  4. 4

    Avoid this incomplete answer

    For 11 protons, 12 neutrons, and 11 electrons, writing mass number 34 is wrong because electrons are not included.

Definition

Mass number is the total number of protons and neutrons present in the nucleus of an atom.

Example

An atom with 9 protons and 10 neutrons has mass number 19.

Rule to remember

A = number of protons + number of neutrons; neutrons = A - Z.

Memory hook

Mass number counts the nucleus team: protons plus neutrons.

Examples and method

Worked example

If Z = 15 and A = 31, protons = 15 and neutrons = 31 - 15 = 16.

Method to apply

Take proton count from Z, take neutron count if given, add them for A; if A and Z are given, subtract to find neutrons.

Diagram support

Use a nucleus diagram with protons and neutrons only highlighted for mass number.

How CBSE asks it

Asked as calculation of A, comparison with atomic number, or neutron finding from A and Z.

Avoid common mistakes

Common confusion

Students often include electrons while calculating mass number.

Common wrong answer

For 11 protons, 12 neutrons, and 11 electrons, writing mass number 34 is wrong because electrons are not included.

Exam tip

For school-level calculation, count only protons and neutrons for mass number.

Quick check

Why are electrons not added while finding mass number?

Electrons are not added while finding mass number because their mass is very small compared with protons and neutrons, so mass number counts protons and neutrons.

Answer writing and exam use

1-mark answer

Mass number is the total number of protons and neutrons present in the nucleus of an atom.

2-mark answer

Mass number is the total number of protons and neutrons present in the nucleus of an atom. A = number of protons + number of neutrons; neutrons = A - Z. An atom with 9 protons and 10 neutrons has mass number 19.

3-mark answer

Mass number is represented by A. Since protons and neutrons have nearly one unit mass each and electrons have very small mass, mass number mainly counts the heavy particles in the nucleus. It is a whole number for a particular atom and should not be confused with average atomic mass. A = number of protons + number of neutrons; neutrons = A - Z. If Z = 15 and A = 31, protons = 15 and neutrons = 31 - 15 = 16. Asked as calculation of A, comparison with atomic number, or neutron finding from A and Z. For 11 protons, 12 neutrons, and 11 electrons, writing mass number 34 is wrong because electrons are not included.
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