Question

Number of elements from the following that CANNOT form compounds with valencies which match with their respective group valencies is ______.
B, C, N, S, O, F, P, Al, Si

• A. 7
• B. 5
• C. 6
• D. 3

Answer 1

The Correct Option is D

Understanding Valency and Group Valency:

Group valency refers to the number of bonds an element can form based on its group number in the periodic table. For elements to match their group valency, they must be able to expand their octet by using vacant d-orbitals, if available.

Analyze Each Element:

• Boron (B): Belongs to Group 13; can show a valency of 3, which matches its group valency.
• Carbon (C): Belongs to Group 14; typically forms four bonds, matching its group valency.
• Nitrogen (N): Belongs to Group 15; can show valencies of 3 and 5. However, it lacks d-orbitals, so it often cannot expand to match a group valency of 5 in all cases.
• Oxygen (O): Belongs to Group 16; has a typical valency of 2, but lacks d-orbitals, preventing it from matching the group valency of 6.
• Fluorine (F): Belongs to Group 17; can only form one bond, as it lacks d-orbitals and cannot match a group valency of 7.
• Sulfur (S): Belongs to Group 16; can expand its octet due to available d-orbitals, matching a group valency of 6.
• Phosphorus (P): Belongs to Group 15; can expand its octet and show a valency of 5, matching the group valency.
• Aluminum (Al): Belongs to Group 13; typically shows a valency of 3, matching its group valency.
• Silicon (Si): Belongs to Group 14; typically shows a valency of 4, matching its group valency.

Elements That Cannot Match Group Valency:

Nitrogen, Oxygen, and Fluorine lack the ability to expand their octets due to the absence of vacant d-orbitals, so they cannot match their respective group valencies.

Conclusion:

The number of elements that cannot match their group valencies is 3, which corresponds to Option (4).

Answer 2

Step 1: Understanding the problem.
We are given elements: B, C, N, S, O, F, P, Al, Si and asked for the number of elements that cannot form compounds with valencies matching their respective group valencies.

Step 2: Group Valencies in the periodic table.
- Group 13 elements like B and Al: expected valency is 3.
- Group 14 elements like C and Si: expected valency is 4.
- Group 15 elements like N and P: expected valency is 3 (sometimes 5 for P).
- Group 16 elements like O and S: expected valency is 2 (sometimes 4 or 6 for S).
- Group 17 element F: expected valency is 1.

Step 3: Checking elements and their valency behavior.
- B (Boron): forms compounds with valency 3 (e.g. BF₃). Matches its group valency.
- C (Carbon): forms compounds mostly with valency 4. Matches its group.
- N (Nitrogen): mainly forms compounds with valency 3 (NH₃), occasionally can expand octet to 5.
- S (Sulfur): usually forms compounds with valency 2, 4, or 6. Matches or exceeds group valency.
- O (Oxygen): mostly forms compounds with valency 2 only.
- F (Fluorine): always forms compounds with valency 1 only.
- P (Phosphorus): forms compounds with valency 3 and 5.
- Al (Aluminum): forms compounds with valency 3 only.
- Si (Silicon): forms compounds with valency 4 (matches group).

Step 4: Elements that do NOT match their expected group valencies strictly.
- Oxygen (O): strictly forms compounds with valency 2, even though group 16 suggests possible higher valencies.
- Nitrogen (N): shows valency 3 mostly, but sometimes 5 (expanded octet).
- Fluorine (F): strongly limited to valency 1, no expanded valency.

Thus, elements that cannot form compounds strictly matching expected group valencies are O, N, and F.

Step 5: Final Answer.
The number of elements that cannot form compounds with valencies matching their group valencies is:
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