9.04.4 Electronic Structure

Syllabus

The electrons in an atom occupy the lowest available energy levels (innermost available shells).
The electronic structure of an atom can be represented by numbers or by a diagram. For example, the electronic structure of Sodium is 2,8,1 or showing two electrons in the lowest energy level, eight in the second energy level and one in the third energy level. Students may answer questions in terms of either energy levels or shells.

Students should be able to represent the electronic structures of the first twenty elements of the Periodic Table in both numbers and as shell diagrams (eg to the right right).

What does this mean?

What is an electron configuration?

The electron configuration or electronic structure is the arrangement of electrons in an atom's shells (energy levels).

You can tell how many electrons an atom has by looking at its Atomic Number.

Not all shells can hold the same number of electrons.

The first holds 2

The next holds 8

The third holds 8

At GCSE we only study the first 20 elements.

So, by looking at the element opposite we can see that the electrons in a Potassium atom are arranged as (2,8,8,1) - this is its electron configuration (or electronic structure).

So, we should be able to give the electronic structure of any element given its Atomic Number (the number of electrons).

₁₇Cl - 2.8.7

20Ca - 2.8.8.2

₄Be - 2.2

Rules for filling shells

1 Shells are filled from the centre outwards.

2 The first shell must be completely filled before any electrons are placed in the second, the second must be filled before we start filling the third etc.

3 Electrons don't pair until there is no space for more single electrons.

4 Ignoring these rules won't make make you rebellious, cool or more attractive. It will make you wrong.