The structure of the atom
Inside an atom
Atoms are tiny: an atom has a radius of about 1 × 10⁻¹⁰ m. At its centre is the , which has a radius less than 1/10 000 of the atom's. Almost all of the atom's is in the nucleus.
| Particle | Where | Relative charge | Relative mass |
|---|---|---|---|
| Nucleus | +1 | 1 | |
| Nucleus | 0 | 1 | |
| Energy levels around the nucleus | −1 | Very small |
An atom has , because it has the same number of electrons as protons.
Key terms
Energy levels
Electrons are arranged in (shells) at different distances from the nucleus.
Electrons can move between levels:
- If an electron absorbs electromagnetic radiation, it moves to a higher energy level, further from the nucleus.
- If an electron drops back to a lower energy level, closer to the nucleus, it emits electromagnetic radiation.
Key terms
Atomic number, mass number and isotopes
- The is the number of . Every atom of an element has the same number of protons.
- The is the number of .
So neutrons = mass number − atomic number.
Example: sodium-23 has atomic number 11. It has 11 protons, 23 − 11 = 12 neutrons, and 11 electrons.
are atoms of the same element with different numbers of neutrons: the same atomic number, but a different mass number. Carbon-12 and carbon-14 both have 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8.
An atom becomes a if it loses one or more outer electrons.