Relative Atomic Mass | AQA GCSE Chemistry: Combined Science Revision Notes 2018

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What is relative atomic mass?

Atoms are so tiny that we cannot really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass (A_r) is used
The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element
The relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom
All other elements are measured by comparison to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units
For example, hydrogen has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon

How to calculate relative atomic mass (Ar)

The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element
The equation below is used where the top line of the equation can be extended to include the number of different isotopes of a particular element present

$A_{r} = \frac{(% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B)}{100}$

So, if there were 3 isotopes present then the equation would read:

$\frac{(% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B) + (% of isotope C x mass of isotope C)}{100}$

Worked example

The table shows information about the isotopes in a sample of rubidium

Isotope	Mass Number	Percentage abundance
1	85	72
2	87	28

Use information from the table to calculate the relative atomic mass of this sample of rubidium.

Give your answer to one decimal place.

Answer:

Relative atomic mass = $\frac{(72 \times 85) + (28 \times 87)}{100}$
Relative Atomic Mass = 85.6

Examiner Tip

Isotopes are easy to recognise from their notation as they have the same symbol but different mass numbers. For example, the two stable isotopes of copper are ⁶³Cu and ⁶⁵Cu

Relative Atomic Mass (AQA GCSE Chemistry: Combined Science)

Revision Note

What is relative atomic mass?

How to calculate relative atomic mass (Ar)

Worked example

Examiner Tip

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Author: Stewart