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.

Why are bonds different lengths?

(above) Not a bond

It must have crossed your minds at some point why are some bonds longer than others. Why is a hydrogen bond long, a covalent bond short and a double covalent bond shorter still. 

The answer is simple.

Bonds aren't physical things like pieces of string they are just attractions. The stronger the attraction the shorter the bond. So, simply covalent bonds are stronger than hydrogen bonds so they are shorter. 

Crystal Coordination Number

Background

Ionic compounds are made up of + and - ions (Na+ and Cl-) in a regular arrangement that is arranged so that + isn't next to a -. You would've thought that there is one or at best two or three ways of doing this, actually there are loads (visit this link to see 14 of them http://en.wikipedia.org/wiki/Crystal_structure#Lattice_systems of them) depending on the ratio of + to - ions and the relative sizes of + and - ions. Luckily for you, you only need to recognise and explain about two of them.

The two structures - NaCl and CsCl 

Think of a theoretical situation where all the ions are exactly the same size like in a box of bowling balls. When you have a box of bowling balls (or any identically sized spheres) the natural arrangement they will settle in will have each bowling ball being touched by 12 others (this is the maximum number of spheres that can touch a central sphere when they are all the same size, trust me its true!). 

Now to really exaggerate the situation imagine a similar scenario where you have a mixed bag of bowling balls and table tennis balls in a 1:1 ratio and for some weird reason when they arrange themselves naturally the bowling balls can only be touched by table tennis balls (and vice versa). Now there is a problem, because bowling balls are bigger than table tennis balls you can get maybe 50 around a bowling ball but in the reverse situation you can only get maybe 3 bowling balls to touch a table tennis. So in this theoretical box of bowling balls and table tennis balls there is a problem if you are to maintain this 1:1 ratio we can't have 50 of 1 and 3 of the other. So, the structure will have to take the smaller number of 3 as the number of each that touch each other. 

So now we have a pattern, when spheres are identically sized we can fit 12 around the opposite one, when they are different sizes this number decreases (down to possibly 3 for bowling balls and table tennis balls). This is called crystal coordination number. i.e. bowling balls have a crystal coordination number of 12. Mixed bowling balls and table tennis balls have a crystal coordination number of 3

Right to ions. 

Cs and Cl are v similar sizes (think bowling balls) so the crystal coordination number will be high (i.e. 8). Na and Cl are different sizes (think mixed bowling balls and table tennis balls) so the crystal coordination number must be lower (i.e. 6). 

This means that in the NaCl crystal there are 6 Nas next to every Cl and 6 Cls next to every Na.

The total crystal coordination number of the crystal is 6,6.  The crystal coordination number of sodium in the crystal is 6 and the crystal coordination of chloride in the crystal is 6.

What does NaCl and CsCl look like?

I am not going to try and draw this here so look at your notes or google them!

What are they going to ask you in the exam?

Firstly, there are the obvious questions like, what do they look like? what is the crystal coordination number? but what about the more descriptive questions that will involve a bit more thought.

1. Why do they have different crystal coordination numbers?

Cs is bigger than Na, so you can fit more Cl ions around a Cs than an Na. Hence CsCl is 8,8 and NaCl is 6,6

2. Why can't MgCl2 have a 6,6 coordination number?

In MgCl2 there are twice as many Cl ions as Mg ions and the crystal coordination number must reflect this, so the crystal coordination might be 4,8 or 6,12 or any structure that involves there being twice as many of one ion compared to another.