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# 3 Electronic configurations revisited

Earlier you looked at the way electrons are arranged in the orbitals of atoms; there are two further features which will be introduced here to set you up for when you examine the way atoms link (bond) together to form molecules later in this module.

Consider for example sodium 11Na. What is the electronic configuration of this atom?

### Answer

1s2 2s2 2p6 3s1

In atoms it is the outer electrons, known as valence electrons which get involved in bonding. The outermost shell is often referred to as the valence shell.

In the case of sodium the 3s electron is the valence electron.

The remaining electrons (those which don’t get involved in bonding) are known as core electrons. In the case of sodium these are the 1s2 2s2 2p6 electrons.

What element has the 1s2 2s2 2p6 electronic configuration?

### Answer

Neon

So in fact for sodium, the core electrons correspond to the electronic configuration of the preceding noble gas neon. This may be represented as [Ne] and the electronic configuration of sodium may be simplified to [Ne] 3s1.

So now you have a sort of shorthand method of writing out electronic configurations.

Have a go at the following examples.

Using the shorthand notation described above, write the electronic configurations of the following elements, carbon (6C), aluminium (13Al) and potassium (19K).

### Answer

Carbon: [He] 2s2 2p2 , aluminium: [Ne] 3s2 3p1 , potassium: [Ar] 4s1

What is the similarity in terms of electronic structure, between sodium and potassium?

### Answer

They both have a single electron in the valence shell, in an s-subshell.

In fact the answer to the above question applies to all the elements in the Group 1- all the alkali metals have a single valence electron. However as you go down the group the principle quantum number (n) of the outer shell increases. So you can write the general valance electronic configuration as ns1.

Moving along to Group 2, the valence electrons are given by ns2. Starting from the top, beryllium will be 2s2, magnesium will be 3s2, calcium will be 4s2 and so on.

What is the valence electronic configuration of oxygen, phosphorus and bromine?

Oxygen: 2s2 2p4 phosphorus 3s2 3p3 bromine: 4s2 4p5

So to reiterate the key point; it is the valence electrons that are involved in bond formation during chemical reactions, and you will look at the nature of chemical bonding in electronic terms later in the course. But before this, in the next session you will be looking at the way atoms combine to form molecules and the huge richness and variety of chemical compounds.

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