Electron arrangements

1. The electrons in an atom are not spread randomly around the nucleus, but are arranged at different distances from it.
2. Each allowed distance is called an energy level, or shell.
3. An electron can only exist in one of these energy levels and cannot sit in the gaps between them.
4. Electrons in levels further from the nucleus have more energy than electrons in levels closer to it.
5. The electrons in the outermost level are the outer electrons, and they are the least strongly attracted to the positive nucleus.

Absorbing radiation

1. An electron can absorb energy from electromagnetic radiation.
2. When it absorbs energy, the electron moves up to a higher energy level, further from the nucleus.
3. The electron is then said to be excited, and the atom is in an excited state.
4. An electron can only make this move if the radiation supplies exactly the energy difference between the two levels.
5. Absorption therefore changes the arrangement of the electrons in the atom, even though the number of electrons stays the same.

Emitting radiation

1. An excited electron does not stay in the higher energy level for long.
2. The electron falls back to a lower energy level, closer to the nucleus.
3. The energy it loses is emitted as electromagnetic radiation.
4. The energy carried by the emitted radiation is equal to the difference in energy between the two levels.
5. A larger drop in energy level gives out radiation of higher frequency, while a smaller drop gives out radiation of lower frequency.
6. The light from a sodium street lamp is produced this way, as excited electrons in sodium atoms fall back to lower levels and emit yellow-orange light.

Forming ions

1. If an outer electron absorbs enough energy, it can escape from the atom completely.
2. The atom now has more protons than electrons, so it has an overall positive charge.
3. A charged atom is called an ion, and an atom that loses electrons becomes a positive ion.
4. The loss of an outer electron in this way is called ionisation.
5. Outer electrons are lost most easily because they are furthest from the nucleus and least strongly attracted to it.
6. The charge of the ion equals the number of electrons lost, so an atom that loses two electrons becomes an ion with a charge of $+2$.
7. Radiation that carries enough energy to remove electrons from atoms is called ionising radiation, and it includes alpha, beta and gamma radiation.