Thread: What are valences?

I've read the term 'electron valences' in A Short History Of Nearly Everything, but it didn't actually go into what they are. I also had a friend mention 'valence electrons' to me in a conversation, but I didn't ask what they were (due to pride on my part I'm afraid. She goes to school in a different country, and I didn't want to admit her science lessons were better than mine ).

So basically, what are they?

The valency of an atom is the number of electrons found in its outermost energy level, or shell. The atoms in this shell are called the valence electrons. The valency of an atom affects how many bonds it forms, because an atom bonds to obtain a complete outer shell.

So for example, a calcium atom has a valency of two because it has two electrons in its outermost shell. This tells us that it will lose two electrons when it bonds. Or, if the number of electrons is closer to eight (the usually desired number of electrons in the valence shell) such as in oxygen which has a valency of six, then we know it will gain two electrons in bonding to complete its outer shell and gain a 'stable octet.' Make sense? I don't know how much chemistry you know already...

Valency can be thought of as the number of covalent bonds (shared pairs of electrons) a particular element is able to make, and is based on the electronic configuration of the element - in particular the number of electrons in the outer shell (called valence electrons).

For example carbon atoms are usually found to have 4 valence electrons, and therefore have a valency of +4 - it is said to be tetravalent (Hence the tetrahedral structures of allotropes like diamond) . This can however change depending on the particular compound an element is in.

More generally, valency gives a gauge of the reactivity of the element, including it's ability to form ionic bonds as well as covalent.

Right, thanks, that makes sense