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Chemical calculations - both Tiers

Relative atomic mass

Different atoms have different masses. Atoms have such a small massmass: The amount of matter an object contains. Mass is measured in 'kg'. it is more convenient to know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass (Ar) of 12.

Atoms with an Ar of less than this have a smaller mass than a carbon atom. Atoms with an Ar which is more than this have a larger mass than a carbon atom.

The table below shows some Ar values (you do not need to remember them: you will be given them in the exam if you need them to answer a question).

Ar values of elements


These values tell you that magnesium atoms are twice as heavy as carbon atoms, and 24 times heavier than hydrogen atoms; while hydrogen atoms are 12 times lighter than carbon atoms. They also allow you to work out that three oxygen atoms weigh the same as two magnesium atoms.

Chlorine's Ar of 35.5 is an average of the masses of the different isotopesisotopes: Atoms of an element with the same number of protons and electrons but different numbers of neutrons. of chlorine.

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