Chapter 1: Where are the electrons ?

Electron Configurations

The arrangement of electrons in the atomic orbitals of an atom is called the electron configuration. Electron configurations can be determined using a periodic table.

Terminology:

The ground state configuration is the lowest energy, most stable arrangement.	An excited state configuration is a higher energy arrangement (it requires energy input to create an excited state).

 

Valence electrons are the electrons utilised for bonding.  These are the 'outer' electrons (i.e. the electrons which travel furthest away from the nucleus) and the highest energy electrons.
Core electrons are electrons that under 'normal' reaction conditions are chemically inert and are in the completely filled inner energy levels.

You should be familiar with how to determine an electron configuration for an atom and identify the valence electrons.  You should be able to identify both ground and excited state electron configurations.

Example 1 : What is the ground state electron configuration of carbon ?

Answer : Carbon is the 6th element in the periodic table, hence the atomic number Z = 6, and therefore has 6 electrons.

simple	1s2 2s2 2p2
short form	[He] 2s2 2p2
orbital energy diagram

Example 2: What is the ground state electron configuration of aluminum, Al ?

Answer : Aluminum is the 13th element in the periodic table, atomic number Z = 13, and therefore has 13 electrons:

simple	1s22s22p63s23p1
short form	[Ne] 3s23p1
orbital energy diagram

You should now try some more questions.

© Dr. Ian Hunt, Department of Chemistry