Which atom is it
Hydrogen has 1 proton. Helium has 2. The number of protons tells which atom it is.
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The idea
A hydrogen atom has 1 proton. A helium atom has 2 protons. Yet a helium atom is about 4 times as heavy as hydrogen. Two protons alone would make it only 2 times as heavy. So there must be something else in the nucleus.
In 1932, James Chadwick found it: the neutron. It has almost the same mass as a proton. It has no charge. Every nucleus has neutrons, except the common form of hydrogen. That form has 1 proton and 0 neutrons.
| Particle | Symbol | Relative charge |
|---|---|---|
| Electron | e⁻ | −1 |
| Proton | p⁺ | +1 |
| Neutron | n⁰ | 0 |
The atomic number, Z, is the number of protons. It decides which element the atom is. A neutral atom has no overall charge. So it has Z electrons too.
The mass number, A, is protons plus neutrons. Protons and neutrons are called nucleons.
A = protons + neutrons. Z = protons. So neutrons = A − Z.
Beryllium has 4 protons and 5 neutrons. So Z = 4 and A = 4 + 5 = 9. Its symbol is Be. A symbol starts with a capital letter. A second letter is small: Cl, not CL.
Electrons are so light that we ignore their mass here.
Watch out!
The mass number is not the atomic number. Phosphorus has Z = 15 and A = 31. Its neutrons are 31 − 15 = 16.