 Discovering chemistry

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# 4 How small is an atom?

At this point you are going to look again at the basic building blocks of matter introduced in Session 1 – atoms, and using the concept of the mole, and the Avogadro constant, attempt to get an idea of the size of their size..

Let’s use gold as an example.

The relative atomic mass of gold is 197, so, rounding things up a little one mole of gold has a mass of about 200g. Now gold is a dense metal: each cubic centimetre has a mass of about 20g, so one mole of gold occupies a volume of about 10 cm3, which is 1 x 10-5 . This volume, therefore contains 6 x 1023 gold atoms.

So 6 x 1023 gold atoms occupy 1 x 10-5 .

This means 1 gold atom occupies 1 x 10-5 / 6 x 1023 m3 = 1.7 x 10-29 m3.

Let’s think of this tiny volume as a cube.

• What is the length of its side?

• It is the length which when multiplied by itself, and then by itself again, gives the volume of 1.7 x 10-29 m3. This length is 2.6 x 10-10 m.

So using the Avogadro constant you arrive at an atomic size (2.6 x 10-10 m) in line with what was given in Session 1, and if you want to visualise how small this number is; you could set about 30 million gold atoms in a line across your fingernail!

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