Re: Why do diagonal relationships in the Periodic Table occur?

Diagonal relationships occur because of the directions in the trends of various properties as you move across or down the periodic table.  Many of the chemical properties of an element are related to the size of the atom.    You may have learned in your chemistry class that the trends in size in the periodic table are as follows:

Size decreases moving left to right across a row and increases moving right to left
Size decreases moving up a period and increases moving down

Comparing Li to Mg, as in your example, as you go down the column the size increases, then as you go across the row the size decreases.  Thus it might not be surprising that the size of Mg atoms is about the same as that of Li atoms.

Why does this matter?   It matters because a great number of important chemical properties depend on the size of an atom.  Most important, perhaps, is that the size of an atom influences how strongly the outer electrons are held.   A great deal of chemistry depends on this, because chemical reactions depend on the transfer or sharing of electrons.

If you look at the sizes of the atoms arranged in the periodic table, you will see that each atom is about the same size as the element one row down and one column to the right.  A terrific source for this kind of information - and lots of information about the properties of all the elements  - can be found at a site called WebElements at www.webelements.com.  Here is a table from WebElements showing the relative sizes of the atoms:

I said I would talk about the reasons for the size trends.   It makes sense that atoms get larger as you go down a column, because you are adding not only more protons to the nucleus but also more electron shells at farther distances from the nucleus.  It seems a little odd, though, that atoms should get smaller as you go across a row.  After all, you are adding more protons and electrons.  But the key is that as you go across a row you add more protons, but the added electrons are placed in the same shell.  The added protons in the nucleus cause the electron shell to be smaller with each successive atom across a row.

I hope this is helpful.  Please feel free to email me.

David Reibstein, Princeton Materials Institute, Princeton University