Mass number (A)

Definition

Mass number (A) is the total number of protons and neutrons in an atomic nucleus. It determines the mass of an atom.

5 Must Know Facts For Your Next Test

1. Mass number (A) is always a whole number.
2. It can be calculated as $A = Z + N$, where $Z$ is the atomic number (number of protons) and $N$ is the number of neutrons.
3. Isotopes of an element have the same atomic number but different mass numbers due to varying numbers of neutrons.
4. Mass number is not listed on the periodic table; it refers to individual isotopes, not elements as a whole.
5. The mass number helps in identifying specific isotopes, which are denoted in the form ${}^{A}_{Z}X$, where $X$ is the chemical symbol.

Review Questions

Related terms

Atomic Number (Z): The number of protons in an atom's nucleus, which defines the element.

Isotope:

Atoms of the same element with different numbers of neutrons and therefore different mass numbers.

Neutron:

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