Meaning of “group valency”

For main-group elements, the respective group valency is usually taken as:

• Group 13 $\to 3$
• Group 14 $\to 4$
• Group 15 $\to 3$
• Group 16 $\to 2$
• Group 17 $\to 1$

We must find elements in the list that cannot form compounds with valency equal to their group valency.

Check each element

Given elements: $\text{B, C, N, S, O, F, P, Al, Si}$

(i) B

• Group $13$, group valency $=3$
• Boron forms compounds like $\text{BF}_3$, $\text{BCl}_3$
• So B can show valency $3$

(ii) C

• Group $14$, group valency $=4$
• Carbon forms $\text{CH}_4$, $\text{CCl}_4$
• So C can show valency $4$

(iii) N

• Group $15$, group valency $=3$
• Nitrogen forms $\text{NH}_3$, $\text{NCl}_3$
• So N can show valency $3$

(iv) S

• Group $16$, group valency $=2$
• Sulfur forms $\text{H}_2\text{S}$
• So S can show valency $2$

(v) O

• Group $16$, group valency $=2$
• Oxygen forms $\text{H}_2\text{O}$
• So O can show valency $2$

(vi) F

• Group $17$, group valency $=1$
• Fluorine forms HF, $\text{OF}_2$, etc.
• So F can show valency $1$

(vii) P

• Group $15$, group valency $=3$
• Phosphorus forms $\text{PCl}_3$, $\text{PH}_3$
• So P can show valency $3$

(viii) Al

• Group $13$, group valency $=3$
• Aluminium forms $\text{AlCl}_3$, $\text{Al}_2\text{O}_3$
• So Al can show valency $3$

(ix) Si

• Group $14$, group valency $=4$
• Silicon forms $\text{SiCl}_4$, $\text{SiH}_4$
• So Si can show valency $4$

Count elements that cannot show group valency

All 9 listed elements can form compounds with valencies matching their group valencies.

Therefore, the number of such elements that cannot do so is:

$0$

Compare with given options

Options are $7,3,5,6$, but our derived answer is $0$.

So none of the listed options is correct.

Comparison with stored answer

Stored correct answer is B = 3, but our derived answer is 0.

Hence, I do not agree with the stored answer.