Mass number

Definition

Mass number is the total number of protons and neutrons in an atomic nucleus. It is denoted by the symbol A.

5 Must Know Facts For Your Next Test

1. Mass number (A) is always a whole number.
2. It determines the isotope of an element, as different isotopes have different mass numbers.
3. Mass number can be calculated using the formula: $A = Z + N$, where Z is the atomic number (number of protons) and N is the number of neutrons.
4. Mass number does not include electrons, as their mass is negligible compared to protons and neutrons.
5. In nuclear reactions, mass numbers are conserved; the sum of mass numbers before and after a reaction remains constant.

Review Questions

• How do you calculate the mass number of an atom?
• Why are electrons not included in the calculation of mass number?
• What role does mass number play in identifying isotopes?

Related terms

Atomic Number: The total number of protons in an atom's nucleus, denoted by Z.

Isotope:

Atoms of the same element that have different numbers of neutrons and hence different mass numbers.

Neutron:

A subatomic particle found in the nucleus with no electric charge and a similar mass to a proton.