Why does atomic size of manganese, iron, cobalt and nickel remains constant whereas of scanidum, titanium, vanadium and chromium decreases?

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, due to more attraction from nucleus. 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 Mn, Fe, Co, Ni & Cu both the effects nearly balance each other keeping the atomic radius same.

In scandium titanium vanadium, the rise in effective nuclear charge is predominant and hence,  and hence their size decreases​

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