Cambridge International AS and A Level Chemistry Coursebook

Book Preface

For thousands of years, people have heated rocks anddistilled plant juices to extract materials. Over the past two centuries, chemists have learnt more and more about how to get materials from rocks, from the air and the sea, and from plants. They have also found out the right conditions to allow these materials to react together to make new substances, such as dyes, plastics and medicines. When we make a new substance it is important to mix the reactants in the correct proportions to ensure that none is wasted. In order to do this we need to know about the relative masses of atoms and molecules and how these are used in chemical calculations.

Masses of atoms and molecules Relative atomic mass, Ar
Atoms of different elements have different masses. When we perform chemical calculations, we need to know how heavy one atom is compared with another. The mass of a single atom is so small that it is impossible to weigh it directly. To overcome this problem, we have to weigh a lot of atoms. We then compare this mass with the mass of the same number of ‘standard’ atoms. Scientists have chosen to use the isotope carbon-12 as the standard. This has been given a mass of exactly 12 units. The mass of other atoms is found by comparing their mass with the mass of carbon-12 atoms. This is called the relative atomic mass, The relative atomic mass is the weighted average mass of naturally occurring atoms of an element on a scale where an atom of carbon-12 has a mass of exactly 12 units.

We use the average mass of the atom of a particular element because most elements are mixtures of isotopes. For example, the exact Ar of hydrogen is 1.0079. This is very close to 1 and most periodic tables give the Ar of hydrogen as 1.0. However, some elements in the Periodic Table have values that are not whole numbers. For example, the Ar for chlorine is 35.5. This is because chlorine has two isotopes. In a sample of chlorine, chlorine-35 makes up about three-quarters of the chlorine atoms and chlorine-37 makes up about a quarter.