Atomic Radii Trends in the Periodic Table

In order to talk about the radius of an atom, we have to make an arbitrary decision about where the edge of the atom is. It is arbitrary because the electron orbitals do not end sharply.

Nevertheless, we can do like we did with the 3D contour plots of the orbitals and just arbitarily choose the radius that the electron spends 90% of its time inside. The electrons spend 90% of the time inside the black line.

Using this definition consistently, we can look at the trends of the atomic radii as a function of position in the periodic table.

That trend is... In general the size of the atom depends on how far the outermost valence electron is from the nucleus. With this in mind we understand two general trends...

Size increases down a group:
The increasing principle quantum number of the valence orbitals means larger orbitals and an increase in atomic size.
Size generally decreases across a period from left to right:
To understand this trend it is first important to realize that the more strongly attracted the outermost valence electron is to the nucleus then the smaller the atom will be. While the number of positively charged protons in the nucleus increases as we move from left to right the number of negatively charged electrons between the nucleus and the outer most electron also increases by the same amount. Thus you might expect there to be no change in the radius of the outermost electron orbital since the increasing charge of the nucleus would be canceled by the electrons between the nucleus and the outermost electron. This, however, is not the case. The ability of an particular inner electron to cancel the charge of the nucleus for the outermost electron depends on the orbital of that inner electron. Remember that electrons in the s-orbital have a greater probability of being near the nucleus than a p-orbital, so the s-orbital does a better job of canceling the nuclear charge for the outermost electron than an electron in a p-orbital. Likewise, an electron in a p-orbital is does a better job than a d-orbital. Thus, as we move across a given period the ability of the inner electrons to cancel the increasing charge of the nucleus diminishes and the outermost electron is more strongly attracted to the nucleus. Hence the radius decreases from left to right.