Question

$K_2Cr_2O_7 + KI \xrightarrow{H^+} I_2 + Cr^{3+}$
Statement-I : Size of $O^{2-}$ is smaller than $F^-$.
Statement-II: Second ionization energy of $\text{Na}$ is greater than second ionization energy of $\text{Mg}$.

• A. Both statements are correct.
• B. Both statements are incorrect.
• C. Statement I is correct while Statement II is incorrect.
• D. Statement I is incorrect while Statement II is correct.

Hint:

Always analyze isoelectronic species based on nuclear charge ($Z$): higher $Z$ means stronger pull on electrons, resulting in smaller size. Ionization energy jumps drastically when an electron is removed from a completely filled inner shell (noble gas configuration).

Answer 1

The Correct Option is D

Statement I:
Step 1: $O^{2-}$ and $F^-$ are isoelectronic species (10 electrons each).
Step 2: In an isoelectronic series, size decreases with increase in nuclear charge.
Step 3: Nuclear charge: \[ Z(O)=8,\quad Z(F)=9 \] Step 4: Hence $O^{2-}$ is larger than $F^-$.
So, Statement I is incorrect.
Statement II:
Step 5: Second ionization energy means removal of electron from the cation.
Step 6: $Na^+$ has noble gas configuration, so removal of electron is very difficult.
Step 7: $Mg^+$ still has one electron in 3s orbital.
Thus, \[ IE_2(Na)>IE_2(Mg) \] Statement II is correct.
Hence, correct option is (4).