Mass number

Definition

Mass number is the total count of protons and neutrons in an atom's nucleus. It determines the specific isotope of an element.

5 Must Know Facts For Your Next Test

1. The mass number is represented by the symbol 'A'.
2. Mass number differs from atomic number, which only counts protons.
3. Isotopes of an element have the same atomic number but different mass numbers.
4. The mass number is used to calculate the average atomic mass of an element.
5. In a neutral atom, the number of electrons equals the number of protons, but it does not affect the mass number.

Review Questions

• What particles are counted in determining an atom’s mass number?
• How does a change in neutron count affect the mass number?
• Why do isotopes have different mass numbers?

Related terms

Atomic Number: The total count of protons in an atom's nucleus.

Isotope: Atoms of the same element with identical proton numbers but differing neutron counts.

Neutron: A subatomic particle found in an atom’s nucleus with no electric charge and a similar mass to a proton.