What is a Molecule?

When a group of atoms are tied together, they form molecules. You have no doubt heard the names of some types of molecule before, such as water (H2O) and carbon dioxide (CO2). Water molecules are made from three atoms: two hydrogen atoms (H) and one oxygen atom (O), while molecules of carbon dioxide are made from one carbon atom (C) and two oxygen atoms (O): you can see that the little "2" in H2O and CO2 goes with the atom written before it and not the one following - these molecules could also be written out as H2O1 and C1O2 but the "1" is almost always left out.
Hydrogen atoms are the lightest kind of atom (they have one proton, one electron and usually no neutron) and they can only make one bond using their single electron to tie themselves to other atoms. Oxygen atoms normally have eight electrons and eight neutrons to go with their eight protons, but they can only make two bonds with other atoms. Carbon atoms normally have six electrons and six neutrons to go with their six protons, but they can make four bonds with other atoms. I'll explain the strange numbers of bonds that different kinds of atoms can make with other atoms in a moment, but let's first look at a picture of some molecules which show just how these three kinds of atom link up with other atoms. You can see that each H has one bond, each O has two, and each C has 4 bonds:-

molecule diagrams

So what's going on? Well, it gets a little complicated, but the electrons in an atom are stored round the nucleus in a number of different layers called shells, with two electrons in the first shell, eight more electrons in the second shell, then it looks as if there might only be eight more electrons in the third shell too, but as soon as the fourth shell has a couple of electrons in it, the third shell is able to take in more electrons until it holds eighteen of them. You can watch these layers fill up with electrons for yourself if you move the curser from atom to atom in the periodic table of the elements click here. Start by going along the top row (which only has two atoms in it), then go along the second row, then go along the third, and so on: watch the electron numbers (shown in green) change as the shells fill up.

The final number in the electron arrangement box is the most useful one for working out how a particular atom is likely to behave, because if the outermost shell is 0, that atom won't be keen to form bonds with other atoms at all, whereas if it is 4, then the atom will be keen to make four bonds. If the number is 0, 1, 2, 3 or 4 then the atom will be keen to make 0, 1, 2, 3 or 4 bonds to other atoms, as you might expect: one for each of the electrons in its outermost shell. However, if the final number is 5, 6 or 7, then the number of bonds the atom will make is likely to be 3, 2 or 1 (in that order). If you put the curser over the "O" (oxygen) box, the final number in its electron arrangement is 6, so that converts to only 2 bonds to other atoms. It doesn't always work that way though: click on S (sulpher) and look at the structure of a sulphuric acid molecule, because the sulpher atom in it has six bonds, one for each of the six electrons in its outer shell. You'll find plenty of molecules that seem to break the rules, but don't worry: the rules are just a rough guide.
Let me give you an idea of why this happens. Atoms like to have a complete outer shell (with the next shell being completely empty), so if the final number is 0, that tells you that the atom is very stable and is not at all keen to bond to any other atom. If the final number is between 1 and 4, then there are 1 to 4 electrons sitting in the outer shell which the atom obviously wants to keep because they balance the number of protons in the atom, but at the same time the atom would like to offload them on another atom if possible to make its outer shell feel empty, though without actually letting go of those electrons completely. The result of this is that it likes to lend those electrons to other atoms while still holding on tightly, and this is how atoms tie themselves to each other to make molecules.
If the atom has 5, 6 or 7 electrons in its outer shell, it can make that shell feel full by borrowing 3, 2 or 1 electron(s) from another atom rather than by lending out any of its own electrons. This works very well with water molecules, because the two hydrogen atoms are happy to offload their electrons (one each) onto the oxygen atom which is equally keen to borrow two electrons: the result is a very stable molecule. Another stable molecule is carbon dioxide: the carbon atom is keen to offload the four electrons in its outer shell, and the two oxygen atoms are both keen to take in a couple of electrons each to complete their outer shells, so again it works out perfectly.
This is not the whole story, however, because the sharing out of electrons is not always so neat. It is possible for two oxygen atoms to bond together to make a molecule of oxygen gas (O2) - in this case, both try to borrow two electrons from each other to make their outer shell feel full, but it's not entirely successful because they have to lend two electrons to each other at the same time. The result of this is that oxygen is not a very stable molecule: indeed it is highly reactive and could even be described as highly explosive. In the same way, two hydrogen atoms combine to make a molecule of hydrogen gas (H2) - in this case one of them can lend its electron to the other, making its shell empty while the other atom's shell is made full, but they don't seem to be entirely happy with this arrangement because they would both like to lend their electron: the result is an explosive gas when it meets up with oxygen, most of this explosiveness probably being caused by the hydrogen.
When hydrogen gas and oxygen gas get together, their explosive natures can be revealed: 2 molecules of H2 and 1 molecule of O2 will burn, turning themselves into 2 molecules of H2O (water) in the process. The electrons in H2 and O2 molecules actually move with more energy than the ones in H2O, and it is this extra energy that is released as heat when these gases get together and explode, thereby turning into water. This also explains why you have to put a lot of energy into water to turn it back into oxygen and hydrogen gas: you have to make the electrons move a lot more energetically again before they can to hold those gas molecules together. Let's have another look at the diagram:-

molecule diagrams

You should now be able to identify four of these molecules for yourself: hydrogen gas, oxygen gas, water (a mix of oxygen and hydrogen) and carbon dioxide (a mix of carbon and oxygen), but there is a fifth molecule in the diagram, and that is methane (a mix of carbon and hydrogen) - if you use gas in your house for cooking, it will probably be methane. When methane is burned in air, one molecule of methane (CH4) and two oxygen molecules (2 O2) from the air are converted into one molecule of carbon dioxide (CO2) plus two molecules of water (2 H2O). It's funny to think that you can make water by burning gases, but it often happens: if you use a lot of gas for cooking and heating in winter, the water made when the gas burns can make your house damp.

In a molecule of methane, the carbon atom may be offloading the four electrons in its outer shell onto the four hydrogen atoms, thereby emptying its outer shell and filling the outer shells of the four hydrogen atoms, but it is more likely taht the four hydrogen atoms are offloading their electrons (one each) onto the carbon atom, thereby emptying their shells and enabling the carbon atom to fill its outer shell. Like hydrogen, methane is explosive when it meets up with oxygen, as are the heavier gases related to it (click on C in the periodic table to see them).

Don't try to memorise the details of the chemical reactions involved in burning hydrogen or methane. Just take a look at the above diagram every time you visit this page and try to learn to recognise the five molecules in it (water, oxygen, hydrogen, carbon dioxide and methane).

More             Home