Atomic Number & Mass Number

Atomic Number

The atomic number is the number of protons in the nucleus of an atom. It is also called the proton number. This number is unique to each element and defines the identity of the element. For example, every atom with 6 protons is carbon, and every atom with 8 protons is oxygen.

The atomic number is always a whole number and is written as Z in chemical notation.

Because the number of protons determines the element, changing the atomic number means changing the element itself.

Mass Number

The mass number is the total number of protons and neutrons in the nucleus of an atom. It is always a whole number and is written as A.

Since neutrons have roughly the same mass as protons, the mass number gives an approximate mass of the atom6s nucleus in atomic mass units (amu).

The mass number helps distinguish between atoms of the same element that have different numbers of neutrons (called isotopes, which are covered in a separate topic).

Relationship Between Atomic Number and Mass Number

The atomic number is always less than or equal to the mass number because the mass number includes both protons and neutrons:

$$\text{Atomic number} \leq \text{Mass number}$$

The number of neutrons in an atom can be found by subtracting the atomic number from the mass number:

$$\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$$

This relationship is important because it allows you to calculate the number of neutrons if you know the atomic and mass numbers.

For instance, if an atom has an atomic number of 11 and a mass number of 23, the number of neutrons is:

$$23 - 11 = 12$$

So, there are 12 neutrons in this atom.