Final answer:
The slight increase in atomic radius of d block elements at the end of a period is due to the increase in electron-electron repulsion, which slightly pushes out the outer electron shell despite the increase in nuclear charge, leading to a small increase in size.
Step-by-step explanation:
The observation that there is a slight increase in the atomic radius of d block elements at the end of each period can be explained by periodic trends. Generally, the atomic radius decreases across a period due to the increase in the number of protons in the nucleus (atomic number Z), which enhances the nuclear charge and attracts the electrons more strongly. However, at the transition from d block to p block elements, subtle changes in electron-electron repulsion and differences in electron shielding slightly counteract this attraction.
Within the d block, electrons are added to the inner d subshell, which provides an increase in electron-electron repulsion more than it increases the shielding effect. This repulsion slightly pushes out the outer electron shell, causing a small increase in the atomic radius. This trend is overall consistent with the general observation that atomic radii decrease from left to right across a period in the periodic table.