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From left to right across a period, effective nuclear charge increases in the transition series, just like in the p block, and so atomic size decreases. Here the electrons are being added to the d orbital, where shielding is relatively poor compared to the p orbital. It should follow that the increase in the effective nuclear charge is more significant for the transition elements than the p block in a given period, leading to greater size variation among successive members - but the size variation is smaller in the transition elements instead. In fact atomic size increases from Ni to Zn. Is there any logical explanation for the fact?

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From Ni to Zn there is increase in size because of the completely filled 3d and 4s orbitals. Here the nuclear charge is increased with atomic number but due to more electron -electron repulsion in 3d and 4s orbitals as compared to force of attraction between nucleus and outer orbital electrons there is little expansion in size as compared to other elements in 3d series.

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From Ni to Zn, Z$_{eff}$ decreases and the shielding effect increases . Z$_{eff}$ is inversely proportional to the size, so if Z$_{eff}$ decreases, size will increase. That's why there is some uncertainty in the order of size for 3d series.

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In case of d block elements as we move from left to right across the period, atomic number increases. The nuclear charge increases. But the electrons are added to penultimate i.e. (n-1) shell, hence the electron cloud density of inner shells increases which increases the screening effect. Thus nuclear charge increases and screening effect increases. Hence there is decreases in the atomic radius but the extent of variation is very small compared to s block and p block elements. The extent of variation is so small that all of them can be considered to have almost equal atomic radii.

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