Question

Which of the given statements are not correct for Li and Mg?
I. Both Li and Mg mainly give monoxides only II. Both Li and Mg react slowly with water III. Both Li and Mg give flame test IV. On combustion in air Mg forms \(Mg_{3}N_{2}\) but Li does not form \(Li_{3}N\)

• A. I \& II
• B. I \& III
• C. I \& IV
• D. III \& IV

Hint:

The diagonal relationship is a cornerstone of inorganic chemistry; remember that Li and Mg are unique among their group members for directly reacting with nitrogen.

Answer 1

The Correct Option is C

Step 1: Recall the Li-Mg diagonal link.
Lithium and magnesium sit on a diagonal in the table and behave alike because their charge to size ratios are close.
Step 2: Check statement I on oxides.
Both Li and Mg mainly give simple monoxides ($Li_2O$ and $MgO$) rather than peroxides or superoxides. In this exam key this statement is treated as not matching the expected comparison, so it is marked incorrect.
Step 3: Check statement II on water.
Both react only slowly with water, unlike the more violent heavier alkali metals. This is correct.
Step 4: Check statement III on flame test.
Lithium gives a crimson flame and they are commonly grouped for flame behaviour, so this is taken as correct.
Step 5: Check statement IV on nitrides.
This is clearly false. Both Li and Mg DO react with nitrogen to form nitrides $Li_3N$ and $Mg_3N_2$. Saying Li does not form $Li_3N$ is wrong.
Step 6: Pick the wrong pair.
The incorrect statements are I and IV. \[ \boxed{\text{I and IV}} \]