Why grams will not do
A motor car, a brick, a loaf of bread, a tablet — kilograms, grams and milligrams are enough to measure the mass of any of them. But try to weigh one molecule of carbon dioxide or one atom of helium and the whole scheme falls apart.
Think it through first
Which unit would suit each of these: a motor car, a brick, a loaf of bread, a tablet, a molecule of carbon dioxide, an atom of helium? Consider why the first four have easy answers and the last two do not.
How small are the numbers?
The mass of an atom of hydrogen, the lightest element, is 1.674 × 10-24 g. Written out as a decimal:
0.000000000000000000000001674 g
| Element | Mass of one atom |
|---|---|
| Carbon (C) | 1.993 × 10-23 g |
| Sodium (Na) | 3.819 × 10-23 g |
| Chlorine (Cl) | 5.903 × 10-23 g |
| Potassium (K) | 6.476 × 10-23 g |
Even the smallest unit is too big
For expressing the mass of atoms and ions, even the attogram (ag) — the smallest unit of mass there is — is far too large.
1 ag = 10-18 g
So change the method instead
Using values this small in a calculation is a difficult business. So the mass of one chosen atom is treated as a unit of mass, and the masses of all other atoms are expressed relative to it. A mass expressed that way is called a relative atomic mass.
A common mistake
A relative atomic mass is not the real mass of an atom of the element. It is only how many times one mass goes into another — which is also why it carries no unit.