Isn't It Ionic?

Ionic or Covalent? What is electronegativity?

This is a fairly confusing area of A level Chemistry (Alanis Morissette certainly found it hard) so I thought I would put together a blog post to explain it.

Think of it this way, we need to measure the ability of atoms to draw electrons towards them (i.e. electronegativity) the only way of doing this is on a comparative scale, that is, compared to other atoms. Then all somebody did (the man who did it was called Linus Pauling) was to give this scale values. The reactivity series from GCSE is the same principle except that nobody has ever bothered to give reactivities values so it is still just a list.

Anyhow, if you are going to compare atoms electronegativity (and therefore assign values) you need to get in to a situation where two atoms are both trying to pull electrons towards them and see which one has the bigger pull.

The only place this happens is in a covalent bond. In a covalent bond the electrons are shared between the two atoms but the electrons will spend more time with the atom that has the greater pull and therefore we say that atom is more electronegative (than the other one it is bonded to).

So, by comparing the covalent bonds between every possible pair of atoms you get a hierarchy of pulling ability (electronegativity) which Linus Pauling assigned numbers to. On Pauling's scale, Fluorine is the highest (best at pulling electrons) and is assigned a value of 4.0 and Francium is the lowest (poorest at pulling electrons) at 0.7. All of this measurement is done by comparing pulling ability in covalent bonds.

So the definition has to be something along the lines of...

"Electronegativity is a chemical property that describes the tendency of an atom to attract electrons towards itself in a covalent bond".

So, "How can compounds that form ionic bonds have electronegativities?" I hear you ask.

Electronegativity is a property of an element, if the value is high (e.g. Chlorine, 3.2) the element will form covalent bonds with other atoms of high electronegativity, because they are both pulling really hard on the electrons and so end up sharing them, but will form ionic bonds with atoms of low electronegativity (e.g. Na, 0.9) because the chlorine has so much of a bigger pull on the electrons compared to the sodium that it ends up pulling the electrons towards itself completely and forming a negative ion, whilst the sodium loses its electron and ends up being a positive ion.

So, don't think of electronegativity as happening "in bonds". It is a property that controls what sort of bonds an atom will form.

The only loose end to tie up is how do we know sodium's electronegativity if it never forms covalent bonds (pure sodium forms metallic bonds and sodium compounds have ionic bonds). The answer to this is that even though sodium doesn't naturally form covalent bonds, in a lab it can be made to share electrons (i.e. form a covalent bonds) with other atoms so that its electronegativity can be measured.

My analogy for all this is tug of war competitions, I won't expand on it now but it does work pretty well.

....and in future Alanis, just buy less spoons.