Lesson 1.8 - Relative Atomic Mass

In this lesson:

• Atomic mass of an atom

• Isotopes

• Relative atomic mass

Atomic mass of an atom

As stated in previous lessons, we can calculate the atomic mass of an atom by adding the number of protons and neutrons.

This is the atomic mass of one type of an element.

Isotopes

In order to understand the meaning of relative atomic masses, we need to understand isotopes.

An isotope is a variant of an element. It varies in the number of neutrons it has.

For instance, there are 3 carbon isotopes:

• Carbon-12 (6 neutrons)

• Carbon-13 (7 neutrons)

• Carbon-14 (8 neutrons)

Note: the number of protons in an element is always the same.

Relative atomic mass

The relative atomic mass is the average atomic mass of every atom in the element.

Let's imagine a made-up element. We'll call it... zodior.

There are a total of 50 zodior atoms in the world:

• 42 zodior-10 atoms (mass 10)

• 5 zodior-11 atoms (mass 11)

• 3 zodior-12 atoms (mass 12)

To calculate the relative atomic mass (just find the average):

(10 * 42 + 11 * 5 + 12 * 3) / 50 = 10.22

So the relative atomic mass of zodior is 10.22.

Essentially, the relative atomic mass is the average mass of atoms.