Final answer:
The atomic radius of gallium is less than that of aluminium due to increased effective nuclear charge (Zeff) and the stable pseudo noble gas electron configuration of its Ga³+ ion. The filled 3d±° subshell in gallium provides additional electron screening, resulting in a smaller atomic and ionic radius as compared to aluminium.
Step-by-step explanation:
Why is the Atomic Radius of Gallium Less Than Aluminium?
The atomic radius of gallium is less than that of aluminium due to a phenomenon known as the effective nuclear charge (Zeff). Gallium (Ga), which lies just after the first row of transition metals in the periodic table, has an electron configuration of [Ar]4s²3d±4p². When gallium forms a Ga³+ ion by losing electrons, the resulting electron configuration is [Ar]3d±°, which is particularly stable. This pseudo noble gas configuration leads to a lower energy state for the Ga³+ ion compared to the Al³+ ion. Moreover, the filled 3d±° subshell of gallium provides an increased screening effect, which enables the 4p electron to be removed more easily than might be expected. As a result, gallium has a smaller atomic radius than aluminum, aligning with the general trend for cations having a smaller radius than their neutral atoms due to lost electrons, as seen in Figure 4.4.3. Additionally, the effective nuclear charge felt by the remaining electrons in gallium is higher, pulling them closer to the nucleus and reducing the atomic radius further.
The comparison of ionic radii shows that a cation has fewer electrons than its parent atom and is hence smaller. For instance, the ionic radius of Al³+ is much less than that of a neutral aluminum atom because the removal of valence electrons allows the core electrons to be more strongly attracted to the nucleus due to an increased Zeff. This concept also explains why the radius of gallium is smaller than that of aluminum, conforming to the variation in ionic radii.