February 19, 2012 | 3
A while ago, I wrote a couple of posts describing some intra-molecular forces, forces that hold atoms and molecules together. I enjoyed writing them, and people come back to read them quite frequently, so I thought I’d continue and write about a couple more.
The previous posts covered van der Waals forces and hydrogen bonds (and dipoles!) These forces are both fun, and incredibly important in determining the properties of water (without which there would be no life) and petrol (without which there would be no cars) but they aren’t particularly strong forces. Both van der Waals and hydrogen bonds are relatively weak, and I think it’s about time I started covering the three Big Bads of molecular forces: ionic bonds, covalent bonds and metallic bonds.
Starting with ionic bonds. In order to properly cover these, I need just a little bit of background on what atoms are actually like. Now there are many different models for what atoms are ‘like’ but for the purposes of ionic bonds the easiest way to think of them is as a small nucleus with a positive charge surrounded by small whizzing negatively charged electrons. Electrons don’t just all fly around with no direction, they exist in orbitals, each a different distance from the nucleus. To find out how many electrons are in the outermost orbital, furthest away from the nucleus, you can look at the periodic table.
The outermost orbital is the most important because all the other electrons are a) in orbitals with the maximum number of electrons and b) unable to take much part in reactions due to the outermost electrons being in the way. Atoms in general are at their most stable when they have a full outer shell of electrons. As many atoms don’t have a full outer shell of electrons, they try to find ways of getting one.
Take sodium, for example, with the chemical symbol Na (blame the Greeks for that!) Now sodium is in the first column of the periodic table, so it has one electron in the outermost shell. All the other shells are full of electrons. So the quickest way to get a full outer shell is to loose that outermost electron, leaving you with a full shell underneath. Loosing one electron, however, requires energy, which means that unless this can be combined with a process which produces energy, it is not going to happen.
Loosing one electron also means that rather than having an equal number of positive nuclear charges and negative electron charges. the sodium now has one fewer negative charges. Instead of a neutral sodium atom, it is now a positively charged sodium ion.
So how does the sodium get the energy back? One way of producing a lot of energy in the world of molecular forces is to form a bond. Breaking bonds requires energy, but making them produces energy (ask a physicist about this if you’re unsure why, the explanation might not make you any more sure but it’s nice for physicists to have someone to talk to). Luckily, a positively charged ion is very good at forming bonds with a negatively charged ion.
To see how a negative ion forms, take a look at the other side of the periodic table at the element chlorine. Chlorine has seven electrons in its outer shell (a full shell is eight electrons for the smaller elements). In order to fill its outer shell it can take an electron gaining one extra negative charge and forming the negative chloride ion. It doesn’t just take that electron from anywhere, it will actually tear it away from the sodium atom. Leaving one negative and one positive ion; Na+ and Cl-
The bond formed between these ions is called an ionic bond.
The two ions don’t just form a single bond between them, ideally they want to make as many bonds as possible. Ionic bonds create giant ionic lattices of interchanging ions. These are incredibly strong, making ionic compounds very hard to break, melt, or crush.
Not only do ionic forces help to hold elements together, the formation of these bonds can completely change the properties of the elements they bind. Sodium is a soft grey metal (you can cut it with a knife!). Chlorine is a deadly green gas. The giant ionic lattice sodium chloride is salt, a white grainy edible food.
Credit for image 2