Atomic Structure and Subatomic Particles — AQA GCSE Chemistry
Atoms are made of three subatomic particles arranged in a nucleus surrounded by electrons.
Structure of the atom
An atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels). The atomic radius is about 1 × 10⁻¹⁰ m, and the nucleus is around 1/10 000 of that size but contains almost all the mass.
The subatomic particles
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | nucleus |
| Neutron | 0 | 1 | nucleus |
| Electron | −1 | very small (≈1/1840) | shells |
Atomic and mass numbers
- Atomic (proton) number = number of protons — this defines which element it is.
- Mass number = number of protons + neutrons.
- Atoms have no overall charge because the number of protons equals the number of electrons.
To find the number of neutrons: mass number − atomic number.
Worked example
Sodium has atomic number 11 and mass number 23.
- Protons = 11, electrons = 11, neutrons = 23 − 11 = 12.
Exam tips
- Learn the three particles with their relative charge and mass.
- Atomic number = protons; mass number = protons + neutrons.
- Neutrons = mass number − atomic number.
- Almost all the mass is in the nucleus; the atom is mostly empty space.