Assertion :Across the period effective nuclear charge decreases. Reason: The third period contains only 8 elements and not 18 like 4th period.

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

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Assertion is incorrect but Reason is correct

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Both Assertion and Reason are incorrect

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Solution

The correct option is C Assertion is incorrect but Reason is correct
Across the period effective nuclear charge increases.
The third period contains only 8 elements and not 18 like 4th period.
The energy of 3d orbital is higher than the energy of 4s orbital and hence, 4s orbital must be filled completely before the filling of 3d starts.
Hence, after filling of 3p orbital, the next electron goes into 4s orbital and the new (fourth) shell stats.
Thus, the third period has only 4 orbitals (one s and three p) and thus 8 elements although the third shell can accommodate 18 electrons in 9 shells.
Hence assertion is incorrect but reason is correct.

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