How electrons fill shells, the electronic structures of the first twenty elements, and linking outer electrons to group and shells to period.

What this dot point is asking

WJEC Double Award Unit 2 wants you to describe how electrons fill shells, write the electronic structures of the first twenty elements, and link the structure to an element's group and period.

How electrons fill shells

For the first twenty elements, the rule is:

• the first shell holds up to 2 electrons,
• the second shell holds up to 8 electrons,
• the third shell holds up to 8 electrons (for the first twenty elements),
• then the fourth shell begins.

Writing electronic structures

To work one out, count the electrons (equal to the atomic number in a neutral atom) and fill the shells in order. Always check the numbers add up to the atomic number.

Linking to the Periodic Table

The electronic structure tells you where an element sits in the Periodic Table:

• The number of electrons in the outer shell equals the group number (for the main groups). Sodium has 1 outer electron, so it is in Group 1.
• The number of occupied shells equals the period. Sodium has 3 shells, so it is in Period 3.

Elements in the same group have the same number of outer electrons, which is why they have similar chemical properties: chemical reactions depend on the outer electrons.

Why full shells are stable

An important idea is that atoms are most stable when their outer shell is full. The noble gases (Group 0) already have full outer shells, which is why they are unreactive. Other atoms react to gain, lose or share electrons so that they end up with a full outer shell like the nearest noble gas. For example, sodium (2,8,1) loses its single outer electron to become 2,8, and chlorine (2,8,7) gains one electron to become 2,8,8. This drive to reach a full outer shell explains why elements form the ions and bonds they do.

Diagrams of electronic structure

Electronic structures are often drawn as shell diagrams: a central nucleus with circles (shells) around it, and crosses or dots for the electrons. The electrons are usually drawn in pairs around each shell, filling the inner shells first. A good diagram clearly shows the right number of electrons in each shell and matches the written structure (for example 2,8,1 for sodium has two electrons on the first circle, eight on the second and one on the third). Being able to move between the written form and the diagram is a common exam skill.

Try this

Q1. Write the electronic structure of oxygen (atomic number 8). [1 mark]

• Cue. 2,6.

Q2. An atom has the structure 2,8,1. Which group is it in? [1 mark]

• Cue. Group 1 (it has 1 outer electron).