Bonding Types - What is that all about?

Answering bonding questions in CH2 is a really big problem. How do you know which bonding type a molecule has? How do you describe it? Here is a quick guide to enable you to at least start to answer these questions.

Lets think through first what type of bonds the molecule has.

If it has ONLY METAL atoms it has ONLY METALLIC bonds. Metallic is fairly straight forward higher charged ions = stronger attraction to electrons = higher BP/MP. Easy.

If it has METAL AND NON-METAL atoms it has ONLY IONIC bonds. Again straightforward you need to know your CsCl and NaCl stuff here and the forces of attraction and repulsion. Not too tricky.

If it has ONLY NON-METAL atoms it has COVALENT bonds IN the molecule and one/two or all three of the INTERMOLECULAR forces BETWEEN the molecules. Now this is the one that trips people up.

There are so many possible molecules here that you can't learn them but you can work out which sort of bonds they have.

Before I start there is a difference between what bonds a molecule has and which ones are important, for example water can do ID-ID but they are not important because the hydrogen bonds are so much stronger that only they matter. So here are the intermolecular bond types in order of importance from least to most.

1. Instantaneous Dipole-Induced Dipole (ID-ID)

If a molecule has...
...only one sort of atom (e.g. Cl2)
...all the same sort of atom on the outside (e.g. CH4)

...then the only sort of intermolecular force it can do is Instantaneous Dipole-Induced Dipole (ID-ID). This happens because all the electrons are swishing around and creating temporarily positive/negative ends of the molecule that then attract other molecules. As molecules/atoms get bigger there are more electrons and therefore stronger ID-ID, therefore, higher BP/MP. All molecules that have covalent in the molecule will have ID-ID between molecules but they are only important when the molecule can't do the other two types of force.

Don't forget then that all covalently bonded simple molecules can do this bond it is just that it only becomes important when it is the only bond that they can form (i.e. in non-polar molecules)


2. Dipole-Dipole (D-D)

If a molecule has a permanently positive and a permanently negative end then that molecule has a dipole, in other words it can attract molecules/atoms/ions of the opposite charge towards it...permanently. It is like the one above but it doesn't change. More electrons don't make a difference now because it is not down to swishing of gangs of electrons. This bond just gets stronger when dipoles get bigger because electronegativity differences become bigger.

On this one just be careful with shapes, e.g. NH3 might look non-polar because it has all the same type of atom on the outside but it is polar. When you look at the shape it is trigonal pyramidal with the N at the top point and all 3 Hs at the other three points.  So there is a negative end (the N) and 3  positive ends (the Hs) so it will do D-D (as it happens it will also hydrogen bond but that is a different story)

3. Hydrogen Bonding

This is the strongest of the three. This is a special case. IN THE MOLECULE, you need to have a very electronegative atom with an active lone pair directly attached to a hydrogen, i.e. in the molecule there must either be a H-F,H-O or H-N bond.

The bond will then be formed between the N, O or F of one molecule and the H of the other.

Your obvious examples of molecules that can Hydrogen bond are water, ammonia and HF but there are lots of other.

If you want a quick summary, here goes

Metal atoms only - Metallic

Metal and Non-Metal atoms - Ionic

Non-Metal atoms - Covalent IN the molecule and then BETWEEN the molecules...

Non-Polar molecules - only ID-ID
Polar molecules - D-D and ID-ID
A molecule that has N-H, O-H or F-H bonds - ID-ID, D-D and hydrogen bonds

(I have emboldened the one that matters in that last statement)

Simple? Not really that hard once you get your head around it.
Worth understanding and working on? Definitely as it is guaranteed to come up in CH2.