 Okay, let's have a look at the periodic table and what it can tell us about atomic structure. We already said that the elements are ordered by atomic number. You can see that at the top of each of the boxes, here hydrogen is number one, helium is number two and so on. That tells you how many protons are in the atom and it defines the identity of the atom. If somehow the number of protons in an atom changes and it takes a new clear reaction of some kind for that to happen, then the atom becomes a new element. But why not just have one long row of elements? Why make it into a table like this? And why are there only two elements in the first row, hydrogen and helium, and then eight in the second? Well, the rows of the periodic table, which are called periods, tell you which electron level is the outermost in that atom. Remember that in an ordinary neutral atom, the number of protons equals the number of electrons. So let's look at hydrogen. It has one proton in the nucleus. I'm not going to bother drawing it. But that means that has one electron in its shell. So we're going to put that in the first shell there. So that's hydrogen. Helium has two protons, so it must have two electrons. So we'll put them there. Now you can see that the outermost electron level that has any electrons in it for both of these atoms is the first electron level. And that's why they're in the first row or the first period. When you get to lithium, it has three protons, so therefore it must have three electrons. So we put in the first two electrons. But then the first electron level can hold only two electrons. So the third electron must go into the next electron level, the second one. So that's why lithium is the beginning of the second row or period. For all the elements lithium to neon, the second electron level is their outermost level that has electrons in it. So the horizontal period that an atom is in tells you the highest level that has electrons in it. For any atom, this highest filled level is called its valence shell. And the electrons that are in the valence shell are called valence electrons. Now let's have a quick look at sodium. It has 11 protons and therefore 11 electrons as well. Two go in the first level and eight in the next. And then we've still got one left over. But the second electron level can hold a maximum of eight electrons. So that one left over has to go into the third electron level. So sodium is in the third period. It's the beginning of the third row. And that means the third electron level is the highest one that has anything in it. And it has just one valence electron in that shell. Now let's compare lithium and sodium. They have different valence shell. For lithium the second electron level is its valence shell. And for sodium the third electron level is its valence shell. And that's reflected in the fact that they are in different periods of the table. But they both have one valence electron in that valence shell. And that's why they're both in the same vertical group on the table. The groups show you which elements have the same number of valence electrons in their valence shells. And if you think that's a bit arbitrary then collide that train of thought immediately with the immovable fact that this small act of classification is the basis of modern chemistry. So to summarize. The periods that's the horizontal rows tell you which electron level is the outermost for that atom. For instance for potassium the valence shell is the 1, 2, 3, 4th electron level because it's in the fourth period. And for lead over here it's the 1, 2, 3, 4, 5, 6th electron level is the valence shell. The vertical groups of the table tell you the number of electrons that are in the valence shell. Or in other words how many valence electrons the atom has. There are several group numbering systems. The newest one goes from 1 to 18 straight across. But there is an older system which I think is more useful at this stage which goes like this. It uses Roman numerals and it goes from 1 to 8. I'm going to ignore the transition metals here for now. We'll get into them in more detail later on. And we just number the other groups 1 to 8. Now this number then corresponds exactly to the number of valence electrons that the elements in each group have. So for instance boron, aluminium and gallium are all in group 3. They each have 3 electrons in their outermost shell. While for instance fluorine, chlorine and bromine they're in group 7. They all have 7 valence electrons.