The trends in atomic radii of the main group elements are very regular.   Atomic radii decrease across a period and increase down a group.  We can explain these trends by considering the two major factors that affect the size:  the effective nuclear charge and the principal quantum number of the orbitals holding the outermost electrons.

The decrease in size across a row in the d block is not as pronounced as with the main group elements.  An increase in radius occurs between the fourth- and fifth-period transition metals, as expected from the increase in principal quantum number of the outermost electrons.  In contrast, there is an unexpected similarity between the radii of the fifth- and sixth-period transition elements.

The location of the lanthanide series of elements (from cerium, atomic number 58, to lutetium, atomic number 71), between the elements lanthanum and hafnium in the sixth period, explains this phenomenon.  The electrons in the completely filled 4f subshell in hafnium and the other elements of the sixth-row transition metals do not completely shield the valence electrons from the increase in the nuclear charge, thus causing a larger effective nuclear charge for the outermost electrons.  A result of this increase in effective nuclear charge is a reduction in the atomic radii, called the lanthanide contraction.  This contraction nearly cancels the expected increase in size between the fifth- and sixth-period transition-metal elements.

An important consequence of the lanthanide contraction is that many of the fifth- and sixth-period transition elements show remarkable similarities in their physical and chemical properties.  For example, hafnium is so similar to zirconium in atomic radius and chemical behavior that it took more that 100 years after the discovery of zirconium for chemists to realize that hafnium was present as an impurity in every sample.    Until 1923, when hafnium was finally identified, every published atomic mass for zirconium was wrong.  All the physical constants that were published for zirconium actually applied to a naturally occurring mixture of zirconium and hafnium.   Even with today's superior techniques, the two elements are difficult to separate from each other.

The lanthanide contraction has other consequences.  One physical property that it influences directly is the density of the sixth-period elements.  These elements have unusually high densities because their metallic radii are virtually the same as those of the fifth-period elements in the same group, while their atomic masses are almost twice as large.  Osmium and its neighbor iridium have the highest densities of any naturally occurring elements.  The lanthanide contraction also influences the chemical reactivity of the sixth-period elements.  Because of the high effective nuclear charge experienced by their valence electrons, sixth-period elements such as platinum, gold, and mercury are relatively inert.  As a result of this chemical inactivity, platinum and gold are among the few metallic elements that occur in nature in the uncombined state.

Reger/Goode/Mercer:  Chemistry Principles and Practice,  2/e,  pp. 304–305