Electronic structure; electrons occupy energy levels (shells); writing electron configurations for the first 20 elements and linking them to the periodic table.

What this dot point is asking

AQA wants you to describe how electrons occupy energy levels, write the electronic structure of the first 20 elements as numbers or diagrams, and link the electron arrangement to the element's position in the periodic table. The single most useful skill is being able to write a configuration from the number of electrons and then read off the group and period from it.

Filling the shells

Electrons fill the shell nearest the nucleus first because it is the lowest energy level. Only when a shell is full do electrons start filling the next one out. This is why the arrangement is built up from the inside.

The number of electrons in a neutral atom equals its atomic number (the number of protons), so you can always start from the atomic number on the periodic table.

Writing electron configurations

Write the number of electrons in each shell, separated by commas, starting from the inner shell:

• Carbon (6 electrons): $2,4$
• Oxygen (8 electrons): $2,6$
• Sodium (11 electrons): $2,8,1$
• Chlorine (17 electrons): $2,8,7$

You can also draw the arrangement as crosses or dots on concentric circles (shells) around the nucleus, with up to 2 on the first circle and up to 8 on the next two.

Linking to the periodic table

For example, chlorine is $2,8,7$: it has $7$ outer electrons, so it is in Group 7, and $3$ occupied shells, so it is in Period 3. This connection is why elements in the same group react similarly: they all have the same number of outer electrons.

Why the outer shell matters most

It is the electrons in the outer shell that take part in chemical reactions, so the electronic structure does not just tell you where an element sits in the table, it predicts how reactive the element is and what ions it forms. An atom is most stable when its outer shell is full, so an element with one outer electron (Group 1) tends to lose it, an element with seven (Group 7) tends to gain one, and an element with a full outer shell (Group 0, the noble gases) does neither and is unreactive. Counting the outer electrons therefore links the electronic structure directly to the chemistry: the same single piece of information explains the group, the typical ion charge and the reactivity.

Working the other way

Examiners also ask you to work backwards: given a group and period, deduce the configuration. An element in Period 3, Group 2 must have 3 occupied shells with 2 electrons in the outer one, so it is $2,8,2$ (magnesium). Being able to move confidently in both directions, configuration to position and position to configuration, is the skill the higher-mark questions reward.

Try this

Q1. Write the electronic structure of magnesium (12 electrons). [1 mark]

• Cue. $2,8,2$.

Q2. An element has the electronic structure $2,8,6$. Give its group and period. [2 marks]

• Cue. Group 6, Period 3.

Q3. Write the electronic structure of argon (18 electrons) and explain why it is unreactive. [2 marks]

• Cue. $2,8,8$; its outer shell is full, so it does not need to gain, lose or share electrons.