Electron energy levels and ionisation

In a nutshell

Electrons occupy the different energy levels of an atom. Each energy level can only have a certain number of electrons occupying it. An electron can be excited to a higher energy level if it absorbs electromagnetic radiation. Ionisation occurs when electrons leave the atom.

Electrons and energy levels

Negatively charged particles called electrons orbit the nucleus at high speeds. Electrons occupy different energy levels which are at different distances from the nucleus. 

Tip: Energy levels can also be referred to as electron shells.

Each energy level can only have a certain amount of electrons occupying it. Only two electrons can occupy the first energy level and the second and third shells can contain up to eight electrons each.

1. Positive nucleus 2. The first (lowest) energy level contains up to two electrons 3. The second energy level contains up to eight electrons 4. The third energy level (the outer shell in this case) can hold up to eight electrons 5. Increasing energy due to further distance from the nucleus

Electrons closer to the nucleus have less energy, while electrons further from the nucleus have more energy. Energy levels closer to the nucleus fill up first since atoms like to be in the lowest energy state possible (where they are most stable). 

An atom with the maximum number of electrons in its outer shell is stable - it is not able to react chemically with other atoms. Its electron shells are full.

If the outer shell has less than the maximum number of electrons, then the atom is chemically unstable - it will react with other atoms to get a full outer shell.

Electrons can move to a higher energy level if they absorb enough electromagnetic radiation. This is called excitation and the atom becomes 'excited'. They can then move back down to lower energy levels by releasing electromagnetic radiation. 

1. Nucleus 2. Electromagnetic radiation 3. Outer shell 4. Inner shell 5. Excited electron

Ionisation

Ions are atoms of the same element with a different number of electrons. 

They are written as $$A^{n\pm}$$ where $$n$$ is the difference between the number of electrons and protons in the ion. An ion can be positively ($$+$$) or negatively ($$-$$) charged.​

Example

Sodium has $$11$$​ electrons in the neutral form of its atom. If it lost two electrons it would only have $$9$$​. The atom would then be a sodium ion and would be written as $$Na^{2+}$$ as it has has two more protons than electrons. 

If an electron on the outer shell (furthest energy level from the nucleus) of an atom absorbed enough electromagnetic radiation, it can leave the atom. This means that the resultant atom would have less negative charge and therefore would be a positively charged ion. This process is known as ionisation.