Themes > Science > Chemistry > Inorganic Chemistry > More Information about Chemical Bonding > Chemical Bonding Index > Valence and core electrons


When we look at the electronic configuration of an atom we can divide up the electrons in the atom into two categories: valence and core electrons. The valence electrons are those that are used in chemical bonding: the core electrons are not involved in bonding.

A typical electron configuration looks something like this

1s22s22p2
This is the electron configuration of carbon. As a shorthand notation, we note that is2 is the electronic configuration of helium and thus write the electron configuration of carbon like
[He]2s22p2
In a similar vein, we can write the configuration of calcium like
[Ar]4s2
where the [Ar] stands for the configuration of argon 1s22s22p23s23p6.

Electronic configurations that are the same as noble gases are very stable. The electrons in the 1s orbital on carbon do not take part in bonding: they are the core electrons. The valence electrons are what is left: the 2s22p2 part are the valence electrons: these do bond.

In calcium, everything in the argon-like closed shell are the core electrons. The valence electrons are the 2 electrons in the 4s orbital.

Example: What are the core and valence electrons in cobalt?

Solution: Start by writing the electron configuration of cobalt, atom 27

1s22s22p23s23p64s23d7
However, argon has the electronic structure 1s22s22p23s23p6, so we can write the configuration of cobalt as
[Ar]4s23d7
The 2 electrons in the 4s orbital and the 7 electrons in the 3d are the valence electrons: all the others are core electrons.


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