In which of the following pairs is the differ | vedclass

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(A) The covalent radii decrease as we move from left to right across a period due to an increase in effective nuclear charge. For the pair $K$ $(19)$

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$K, Ca$

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(A) The covalent radii decrease as we move from left to right across a period due to an increase in effective nuclear charge. For the pair $K$ $(19)$ and $Ca$ $(20)$,both are $s$-block elements where the shielding effect is less effective,leading to a significant decrease in atomic size. In contrast,for transition elements like $Mn, Fe, Co, Ni,$ and $Cr$,the electrons are added to the penultimate $(n-1)d$ subshell. This $(n-1)d$ subshell provides a screening effect that offsets the increase in nuclear charge,resulting in a very small change in atomic radii across the transition series. Therefore,the difference in covalent radii is maximum for the pair $K$ and $Ca$.

In which of the following pairs is the difference between the covalent radii of the two metals maximum?

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