why Fe , Co, and Ni have comperable size ? expain brifely.
Atomic radius is inversely proportional to the effective nuclear charge. As we move from left to right in a period the effective nuclear charge increases. This will decrease the radius of an atom. At the same time, in transition elements the number of electrons in the 3d sub-shell will increase. This will repel the already present 4s electrons. This repulsion will increase the atomic radius. In Fe, Co, Ni & Cu both the effects nearly balance each other keeping the atomic radius same.