Weighing something you cannot see
One atom is far too light for a school balance. So we compare atoms. Put two atoms on the see-saw. The heavier atom goes down.
Loading lab…
The rule
Carbon-12 is the carbon atom with 6 protons and 6 neutrons. u is the unit for atomic mass. One u is one twelfth of a carbon-12 atom.
By agreement, one carbon-12 atom is fixed at 12 u. Every other atom's mass is compared to that one. This comparison is the atom's mass, in u.
Atomic masses
A symbol is the short letters for an element, such as H.
| Element | Symbol | Mass (u) |
|---|---|---|
| Hydrogen | H | 1 |
| Carbon | C | 12 |
| Nitrogen | N | 14 |
| Oxygen | O | 16 |
| Sodium | Na | 23 |
| Magnesium | Mg | 24 |
| Aluminium | Al | 27 |
| Sulfur | S | 32 |
| Chlorine | Cl | 35.5 |
| Potassium | K | 39 |
| Calcium | Ca | 40 |
Oxygen is 16 u. Hydrogen is 1 u. So one oxygen atom is 16 times heavier than one hydrogen atom.
The see-saw shows which atom is heavier. To see how many times heavier, divide their masses. Magnesium is 24 u. Oxygen is 16 u. So 24 ÷ 16 = 1.5. A magnesium atom is 1.5 times heavier than an oxygen atom.
Watch out!
u is not grams. One u is a tiny mass. 16 u means 16 times one u.