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1. Relative atomic mass

Relative atomic mass is a weighted average mass of all the isotopes of an element. Weighted means that it takes into account the natural abundance of each isotope.

To find the relative atomic mass from given atomic masses of isotopes and their abundances, multiply corresponding atomic masses and their abundances in decimals and add the multiplication products together. If you use abundances as percentages, divide your final result by 100%. 1

Magnesium has three stable isotopes, ²⁴Mg (79%), ²⁵Mg (10%) and ²⁶Mg (11%). What is the relative atomic mass of magnesium? 2

Chlorine has two stable isotopes, ³⁵Cl (75%) and ³⁷Cl (25%). What is the relative atomic mass of chlorine? 3

Copper has two main isotopes, ⁶³Cu and ⁶⁵Cu with their abundances respectively being 70% and 30%. What is the relative atomic mass of copper? The data provided has been adjusted for the question. 4

What is the relative atomic mass of tungsten? Give your answer to 2 decimal places. 5

What is the relative atomic mass of platinum correct to 3 decimal places? The data in the table has been adapted for the question. End of page