Q.29

The incorrect statement is option (C), as the first ionization energy order K < Mg < Ca does not align with periodic trends.
Mg has a higher first ionization energy than Ca due to stable electron configuration and smaller size effects within group 2.

Option Analysis

A: MgO and Na₂O basic, SiO₂ acidic
Na₂O (group 1) and MgO (group 2) form basic oxides by reacting with acids to produce salts and water, as metals on the left of the periodic table yield electron-donating oxides. SiO₂ (group 14) acts as an acidic oxide, reacting with bases like NaOH to form silicates, following the trend where oxide acidity increases across a period.

B: Atomic radius decreases left to right
Across a period, effective nuclear charge rises due to added protons with minimal shielding, pulling electrons closer and reducing atomic radius. This holds for all periods, like from Li to Ne.

C: Ionization energies K < Mg < Ca (incorrect)
First ionization energies (kJ/mol): K ≈ 419 (low due to large size, group 1 ns¹ electron), Mg ≈ 738 (high, group 2 ns² stability), Ca ≈ 590 (lower than Mg due to larger size down group 2). Actual order: K < Ca < Mg.

D: Bond energies C−C < Si−O < N≡N
Typical values (kJ/mol): C−C ≈ 348 (single bond), Si−O ≈ 452 (strong due to size mismatch), N≡N ≈ 941 (triple bond strength). The increasing order matches periodic bond strength trends.

Periodic trends MCQ questions test core concepts like atomic radius decrease across periods, ionization energy variations, oxide acidity, and bond energies—key for CSIR NET Life Sciences and chemistry exams. This periodic trends MCQ analysis breaks down “which of the following is NOT correct” options on MgO Na₂O basic SiO₂ acidic nature, atomic radius trends, K < Mg < Ca ionization order, and C−C < Si−O < N≡N bond energies.

Why Periodic Trends Matter for Exams

Periodic trends govern element properties: atomic radius shrinks left-to-right due to nuclear charge; ionization energy drops down groups but spikes at stable configurations; left-side oxides (MgO, Na₂O) stay basic, right-side (SiO₂) acidic. Bond energies rise with multiplicity or polarity like Si-O strength. Master these for competitive exams.

Detailed MCQ Solution

• Correct: Basic/acidic oxides trend—MgO, Na₂O react with acids; SiO₂ with bases.

• Correct: Atomic radius periodic decrease—Electrons pull tighter across periods.

• Incorrect: K < Mg < Ca ionization—K (419 kJ/mol) < Ca (590) < Mg (738); Mg’s full 3s² resists loss more than Ca’s larger 4s².

• Correct: Bond energy order—C-C (348) < Si-O (452) < N≡N (941 kJ/mol).

Practice similar periodic trends MCQ to ace CSIR NET—focus on exceptions like group 2 ionization dips.