 Here's a quick example of the importance of size. In most living organisms there's a family of proteins located in cell membranes. They're tube shaped and they're called pores, and their function is to allow ions in and out of the cell. They're vitally important, they help regulate cell processes, they drive chemical reactions, and in organisms with a nervous system they're the basis for nerve communications. To do their job these pores have to be able to separate different kinds of ions. Each one transports a very specific ion. For instance the KCSA pore in bacteria transports potassium ions but not sodium ions. The weird thing is potassium ions are bigger than sodium ions, so it's not just acting like a sieve. The mechanism has been the subject of a lot of research over the last 20 years, and it still isn't 100% certain, but it seems that the pore protein has evolved such that it in a tube precisely fits, that is it has stronger attractions with the larger potassium ions than it does with the smaller sodium ions. So potassium ions that drift in are separated from the surrounding water molecules, trapped and moved through to the other side, whereas sodium ions are more likely to stay surrounded by water molecules and drift straight back out again. So let's look at some sizes. Now the sodium and potassium in that example I just gave you were ions, and we're looking here at neutral atoms, but the same basic principles apply. The diagram here shows the periodic table with the radii of the elements in picometers. You can see that hydrogen is 53 picometers, and you may remember that a picometer is 10 to the minus 12 meters. So let's just start by observing what the trends are. If you look down each group on the periodic table you can see that the atoms get larger. If it seems to make sense, atoms with more stuff in them should surely be bigger. Now look across each period. You can see that the radii gets smaller. The atoms on the right of the table are smaller than those on the left. Does that seem a bit weird? Well let's think about the factors at play. There are three main factors to consider. The first is how many protons are in the nucleus of the atom, or in other words the atomic number. This determines how much positive charge the nucleus has, and because it is electrostatic attractions between the positive nucleus and the negative electrons that hold the atom together, the size of the nuclear charge, the charge on the nucleus, is important. The greater the charge the more tightly the electrons are pulled towards the nucleus. So the trend here is that the more protons there are in the nucleus the smaller the atom is. The second factor is the electron levels. Each electron shell in the atom is further from the nucleus than the last. So the more electrons an atom has, and the more electron shells are occupied, the larger the atom will be. The third factor also relates to the electron shells. Electrons in an outer shell don't feel the full force of attraction of the positive nucleus, because the electrons in lower levels form a sort of negatively charged shield. So in larger atoms electrons in the outer shells feel less attraction from the nucleus and so are not pulled as tightly towards it. This gives the same general trend as the second factor. So more electron shells, larger atom. So they kind of reinforce each other. So let's test our reasoning out. As you go down a group the number of protons in the atoms is increasing. So that would mean a smaller atom. But the number of electron shells is increasing because the number of electrons is increasing, which gives you the double whammy of bigger shells and more shielding. So atoms get bigger down a group. Going across a period the number of electron shells in the atoms stays the same, because they're all in the same period they all have the same valence shell. So no change there. But simultaneously the number of protons in the nucleus is increasing. So the electrons are pulled more tightly in and the atoms get smaller. Okay, so what you need to be able to do is justify differences in atom sizes and also be able to predict given two atoms which will be the larger. Test yourself, pick random elements from the periodic table, make a prediction, justify your choice to yourself and then check your answer on this table.