Atomic Structure and Periodic Properties

I'll show you some of the details of atomic structure using an interactive periodic table applet, which, I hope, should now have opened in another window.

 After Mendeleev's time, scientists discovered what you already know: an atom consists of a positively charged nucleus, made of neutrons and protons, and some negatively charged electrons swarming around it.

But what exactly is the configuration of those electrons? That's the key to understanding why each element behaves the way it does.

"Configuration"? I'm not sure I understand what that means. Does it have something to do with that chart in the applet, the one that says "s p d" at the top?

Yes; that chart shows how the electrons are arranged in the selected element. I'd be happy to explain in detail how the electrons organize themselves; if you'd prefer, I can also give you a short crash course in interpreting the chart.

Now that we've talked about the structure of atoms, can you answer my question about their sizes?

There are two patterns to be explained: atoms get bigger as you go down a group, and smaller as you go to the right across a period. The reason for the first one shouldn't be so hard to see now; look again down the column of alkali metals in the applet.

Each time you move down, you add another primary level--lithium's highest electron is in a 2s state, for sodium it's 3s, and so on.

Exactly. And the higher an electron's energy, the farther from the nucleus it is.

So the atoms get bigger as you add electrons to higher energy levels--that makes sense. But why do they get smaller as you move to the right?

Well, you'll notice that within a period, the outermost electrons are all in the same primary level--that is, at (roughly) the same distance from the nucleus. But as you move to the right, the elements increase in atomic number; each element has one more proton than its left-hand neighbor. The more protons in the nucleus, the more strongly the valence electrons are pulled in...

...and so the atoms shrink! Also, I can see from the chart that the ionization energies get larger as you go to the right; that must be for the same reason.

Very good! Similarly, the ionization energies decrease as you move down a group.