1.4 Electron Configuration
So provided we know how many electrons are in an atom, it is possible to predict to some degree of accuracy the following:
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.
THIS IS BECAUSE EACH ATOM HAS DISCRETE POSITIONS FOR ELECTRONS, GIVING EVIDENCE FOR ENERGY LEVELS AND SPECIFIC ELECTRON CONFIGURATION
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!!
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.
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.
Main Energy Number of electrons in
Level in each sub-level
This is a useful pattern of remembering how the levels are filled.
How does this relate to the periodic table? Each different area or block corresponds to a different sub-shell.
Remember this notation
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)
2. Fluorine (Click here for a Periodic Table)