1.4 Electron Configuration

Page Contents

Quick Summary

Evidence for Electron Energy Levels

What are these Energy Levels?

The energy sub-levels

Filling the electron energy levels

The notation

Quick questions

Quick Summary…….

  • The atomic number of an element determines the number of electrons in its atom.

  • The number of electrons fixes the arrangement (or configuration) of these electrons.

  • This fixes the number of outermost electrons

  • These are responsible for the compound reaction in a specific manner.

So provided we know how many electrons are in an atom, it is possible to predict to some degree of accuracy the following:

  • The compounds they form

  • What formulae they will have

  • The properties of these compounds.

Evidence for electron energy levels

It is vital that you move away from the idea that energy (and light) is continuous (i.e. Analogue or continuously variable), but is made up of discrete packets of energy called quanta, rather like a Digital TV signal, a sequence of numbers 1, 2, 3 etc.

So even if the amount of energy available is 2.5, this idea of quantisation of energy only allows the quantum packet to have energy of 2.

Remember with the electromagnetic spectrum, high frequency or short wavelength = high energy (e.g. Gamma-rays and X-rays have higher energy than light or radio waves).

If the light from a hydrogen discharge tube is passed through a prism, the emission line spectrum of hydrogen is observed. This light may appear continuous to our eyes, but is in actual fact a collection of a large amount of packets of light with discrete energy levels.

When examined, this spectrum appears as a long thin black rectangle with discrete slits of light of fixed wavelengths on it.

So if some incident radiation falls onto a hydrogen atom, all this evidence is suggesting that there are only certain ways it can emit radiation as a result.


What are these energy levels??

At GCSE, electrons fill shells, in the order 2,8,8,2 as GCSE only takes us as far as calcium. The sequence for AS level is very similar, the main difference is – TERMINOLOGY!!

  • Main energy levels are numbered 1 (the lowest energy level), 2, 3 etc.
  • Coinciding with the periods of the table (e.g. H and He electron are in main level 1, the period beginning Li, Be have electrons in main level 2 etc.
  • These Main energy levels are split into sub-levels.
  • The number of sub-levels in each main level = the number of the main level. E.g. Main level 1 has only one sub-level, main level 2, 2 and so on. (This breaks down with later elements; so don’t take it as a rule).
  • The sub-levels are given really unhelpful letter, s, p, d and f, standing for sharp, principle, diffuse and fundamental.
  • Each sub-levels consists of one or more orbitals. There are always an odd number in each sub-level.
  • The s-sub-level has 1 orbital, the p- 3, the d- 5 and the f- 7.
  • Each orbital can only contain a maximum of 2 electrons.
  • If there are 2 electrons in an orbital they must have opposite spin. We don’t really understand spin of electrons, but the maths works!

Spinning Electrons

The energy sub-levels

1. Any electron can be described using the four quantum numbers, Main energy level, sub-level, orbital and spin. No 2 electrons have the same quantum number. This is called the Pauli Exclusion Principle.

2. It is possible to calculate how many electrons each energy level and sub-level can contain.


No. of orbitals

No. of electrons













So if we know how many electrons an atom has, we can now fill in the sub-levels and main levels – for the atom in is ground state (lowest energy state), and work out the electronic configuration. This is not as hard as it sounds!!!

NB Always fill electronic energy levels form the lowest first.

Filling the electron energy levels

Main Energy Number of electrons in

Level in each sub-level

This is a useful pattern of remembering how the levels are filled.

  • The parallel lines give the order of filling, beginning at the lowest energy level 1s.
  • Boron for example, atomic number 5, has 5 electrons.
  • Its electron arrangement can be written 2,3 showing 2 in the first shell and 3 in the second.
  • Following the arrows, this could be written 1s22s22p1.

How does this relate to the periodic table? Each different area or block corresponds to a different sub-shell.

The Notation.

Remember this notation


Some to try!!

Just remember the sequence, which goes……

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p exactly like you were counting up on the periodic table with a slight hiccup in that 4s comes before 3d!

Click on the radio buttons to try some!

1. Carbon (Click here for a Periodic Table)

1s2 2s2  


2. Fluorine (Click here for a Periodic Table)

1s2 2s2 2p5