Simple explanation of atomic radii and ionization energies?

There are exceptions, but the general trend is this:

As you scan to the left in any row on the periodic table, the atomic radii increase.

As you scan down in any column on the periodic table, the atomic radii increase.

The largest atoms will be in the lower left corner.

Ionization energies will be just the opposite. They increase as you scan to the right and up.

The highest IE will be in the upper right corner.

If you are looking instead for why this trend exists, add a comment and I'll help you out.

OK. Think of Na and Mg. Mg is to the right of Na. Mg has 1 more electron and 1 more proton than Na. The extra electron tends to repel the other electrons and expand the Mg atom slightly compared to the Na. But the extra proton tends to do the opposite, to pull the electrons a bit closer to the nucleus, making the atom smaller than Mg. You might think that the two effects would cancel each other out, but the extra proton actually has more effect than the extra electron because on average, the proton is closer to the 12 electrons than the extra electron is. So Mg is a bit smaller than Na. This same argument can be used for any two elements in any row.

Now think of Na and K. K is below Na, and its outer electron shell is the 4s level compared to Na's outer shell being 3s. 4s is significantly bigger than 3s, which explains why K is larger than Na. This same argument can be used for any two elements in any column.

The reason that IE trends are the opposite of atomic radius trends, is pretty simple. As atoms get smaller, their electrons get closer to the nucleus and are held more tightly. It then takes more energy to remove the electrons.

Moving from left to right on the periodic table results in decreasing atomic radii.

Moving from bottom left to top right on the periodic table results in increasing ionization energy.

On a separate note, electronegativity works the same way as ionization energy; increasing electronegativity going from bottom left to top right.

Here is a link to my website where I explain the notion behind ionization energy, why ionization energy is a periodic property and how it relates to the shape of the periodic table.

http://c7chemistry.wikispaces.com/Electrons+and+bo...