# Calculate the mass number of Oxygen for all its isotopes.

The complete amount in the protons and neutrons in the nucleus of an atom is the mass number.

## Definition

The number of protons and neutrons combined to give us the mass number of an atom. It is represented using the letter ‘A.’ The combination of both protons and neutrons are called nucleons.

### Isotope

The number of protons remains the same in all atoms of an element, the number of neutrons can vary. Thus, atoms of the same element can have different mass numbers, and these are called isotopes.

## Mass number of Oxygen

Isotopes are named for their mass numbers. Mass number is denoted by sum of atomic number and neutrons.

Atomic number: The number of protons (positive charges) in the nucleus of an atom is given by its atomic number and is denoted by Z

Oxygen has three naturally occurring isotopes.

Atomic number is 8.

• The most common isotope of oxygen has 8 neutrons.

Its atomic number is 8.

A=Z+N

A=8+8=16

Second isotope of oxygen is oxygen-16.

• Second isotope of oxygen has 9 neutrons.
A=8+9=17

This isotope is oxygen-17.

• Third oxygen isotope has 10 neutrons.
A=8+10=18

This oxygen isotope is oxygen-18.