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

Valency and Group Valency Explained:

Group valency is determined by an element's group number in the periodic table, indicating the number of bonds it can form. To achieve their group valency, elements may need to expand their octet using available vacant d-orbitals.

Element Analysis:

• Boron (B): In Group 13, typically exhibits a valency of 3, aligning with its group valency.
• Carbon (C): In Group 14, commonly forms four bonds, matching its group valency.
• Nitrogen (N): In Group 15, can display valencies of 3 and 5. However, the absence of d-orbitals limits its ability to always achieve a group valency of 5.
• Oxygen (O): In Group 16, usually has a valency of 2. Lacking d-orbitals prevents it from matching a group valency of 6.
• Fluorine (F): In Group 17, forms only one bond due to its lack of d-orbitals, thus cannot match a group valency of 7.
• Sulfur (S): In Group 16, can expand its octet using available d-orbitals, achieving a group valency of 6.
• Phosphorus (P): In Group 15, can expand its octet to show a valency of 5, aligning with its group valency.
• Aluminum (Al): In Group 13, typically shows a valency of 3, matching its group valency.
• Silicon (Si): In Group 14, typically shows a valency of 4, matching its group valency.

Elements Unable to Match Group Valency:

Nitrogen, Oxygen, and Fluorine cannot expand their octets because they lack vacant d-orbitals, thus they cannot achieve their respective group valencies.

Conclusion:

There are 3 elements that cannot match their group valencies, corresponding to Option (4).