9701 · 2.1
Relative masses of atoms and molecules — FAQ
Frequently asked questions for 9701 Relative masses of atoms and molecules. Direct answers first, then deeper explanation — then practise with marking.
Is relative atomic mass the same as mass number?
No. Mass number (A) is the total count of protons and neutrons in a single atom's nucleus (an isotope) and is always an integer. Relative atomic mass () is the weighted average mass of all naturally occurring isotopes of an element compared to the carbon-12 standard, and is often a decimal value.
Why do we use 1/12th of a carbon-12 atom as the standard?
Carbon-12 was chosen as a stable, common, and solid reference. Defining its mass as exactly 12 conveniently makes the relative masses of most other elements and isotopes very close to whole numbers, simplifying calculations. It replaced earlier standards based on hydrogen and oxygen to create a single, unified scale for both physicists and chemists.
Do I need to memorise the relative atomic masses for the exam?
No, you will always be provided with a Periodic Table in your exam which contains all the necessary relative atomic masses. However, with practice, you will become familiar with the values for common elements (like C=12.0, H=1.0, O=16.0, N=14.0), which will help you work faster.