Periodic trends play a huge role in chemistry. Regular changes in atomic size and other variables across allow us to make systematic predictions about the behavior of similar .

Covalent radius is a convenient measure of atomic size. The figure below shows the covalent radii of metals in groups 4-10. Note that the row numbers refer to the transition series only, not to the Periodic Table as a whole. What jumps out at us from this graph?

1. As we move down a group (from row 1 to 2), covalent radius increases. The trend makes sense, for the simple reason that the principal quantum number increases and orbitals get larger as we move down a Group.

2. As we move across the periodic table from left to right, the covalent radius decreases. This trend also makes sense. As we move to the right across the periodic table, protons are added to the nucleus but, because of shielding, the added electrons don’t exactly balance the proton’s charge. The net result is that effective nuclear charge increases as we move left-to-right across the periodic table. But there is still something amiss. The atoms in row 3 are almost the same size as their counterparts in row 2! Normally, we expect atoms to get bigger row by row, as additional layers of electrons are filled in. Not so for the third row of transition metals. To see the probable reason for that, we have to look at the whole Periodic Table and remember that the lanthanides and actinides — the two orphaned rows at the bottom — actually fit in the middle of the periodic table. Let’s look at a complete Periodic Table. The lanthanides, in particular lanthanum to ytterbium, go in between lutetium and hafnium. Now you can see why we put the lanthanides and actinides at the bottom of the Periodic Table. Including them gives a very long table.

As a result, the third row of transition metals contains many more protons in their nuclei, compared to the second row transition metals of the same column. Silver has ten more protons in its nucleus than rubidium, the first atom in the same row as silver, but gold has twenty four more than cesium. The third row “contracts” because of these additional protons. This effect is called the “lanthanide contraction”.