1.1.9 Relative Atomic Mass (Ar)

infoNote

Relative atomic mass (Ar) is a measure of the mass of one atom of an element compared to 1/12th the mass of a carbon-12 atom. It takes into account the abundance of each isotope of the element.

The relative atomic mass is a weighted average of all the isotopes of an element.
It is not the same as the mass number of a single atom, which refers to just one isotope.

To calculate the relative atomic mass, use the formula:

\text{Ar} = \frac{(\text{isotope mass} \times \text{abundance}) + (\text{isotope mass} \times \text{abundance}) + \dots}{\text{total abundance}}

lightbulbExample

Example:

Chlorine has two common isotopes: Chlorine-35 and Chlorine-37.
Approximately 75% of chlorine atoms are Chlorine-35.
Approximately 25% of chlorine atoms are Chlorine-37.
The relative atomic mass of chlorine is calculated considering the masses and abundances of these isotopes:

\text{Ar} = \frac{(75 \times 35) + (25 \times 37)}{100} = 35.5

infoNote

What You Need to Know: Understanding Ar: Know how to calculate the relative atomic mass from the masses and abundances of isotopes.

Periodic Table: The relative atomic masses of elements are listed on the periodic table. These are the numbers you use when doing calculations involving moles, empirical formulas, and molecular formulas.

Isotopes: Understand that different isotopes of an element contribute to its relative atomic mass. Be able to explain why the Ar is not always a whole number.

Calculations: You may be asked to calculate the relative atomic mass of an element if given the necessary data about its isotopes.

You will use relative atomic masses in various calculations, such as determining the relative formula mass (Mr) of compounds, calculating the amount of a substance (in moles), and balancing chemical equations. Convert between mass and moles using the formula: $\text{Moles} = \frac{\text{Mass}}{\text{Ar or Mr}}$

Relative Atomic Mass (Ar) Simplified Revision Notes for GCSE AQA Chemistry

1.1.9 Relative Atomic Mass (Ar)

500K+ Students Use These Powerful Tools to Master Relative Atomic Mass (Ar) For their GCSE Exams.

Other Revision Notes related to Relative Atomic Mass (Ar) you should explore