An atom is made of three subatomic particles. The proton and neutron sit in the central nucleus, while the electrons move around the nucleus in shells.
The subatomic particles
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | 1/1840 (≈0) |
A neutral atom has equal numbers of protons and electrons, so the charges cancel.
Proton number and nucleon number
- Proton number (Z) = number of protons = number of electrons in a neutral atom.
- Nucleon number (A) = number of protons + number of neutrons.
- So number of neutrons = A − Z.
Key idea / 要点
Number of neutrons = nucleon number − proton number = A − Z. In a neutral atom, number of electrons = proton number.
Worked example / 例题
An atom of sodium is written 2311Na.
Proton number Z = 11, so it has 11 protons and, being neutral, 11 electrons.
Nucleon number A = 23, so number of neutrons = 23 − 11 = 12 neutrons.
Electron arrangement: 2, 8, 1 (filling shells 2 then 8 then the rest).
Electron arrangement
Electrons fill shells from the innermost outward. The first shell holds up to 2 electrons, the second up to 8, the third up to 8 (for the first 20 elements). For example, chlorine (Z = 17) has the arrangement 2, 8, 7. The electrons in the outermost shell are called valence electrons, and they largely decide how an atom reacts. An atom that has a full outer shell, like a noble gas, is very stable and unreactive, which is why the electron arrangement of an element is so useful for predicting its chemical behaviour.
Remember / 记住
- Protons and neutrons are in the nucleus; electrons are in shells.
- Neutrons = A − Z.
- Neutral atom: electrons = protons.