Question

The correct statement is

• A. Aluminium has five valence orbitals.
• B. Boron has a maximum covalency of four.
• C. Beryllium has three valence orbitals.
• D. Magnesium has a maximum covalency of four.

Hint:

Important facts: \[ BF_3 + F^- \rightarrow BF_4^- \] Boron can expand its coordination number to 4 by accepting a lone pair, giving a maximum covalency of 4.

Answer 1

The Correct Option is B

Concept: Maximum covalency refers to the maximum number of covalent bonds that an atom can form. Elements of the second period cannot expand their octet because vacant d-orbitals are absent. However, boron is capable of forming four covalent bonds in species such as tetrafluoroborate ion \((BF_4^-)\).

Step 1: Check statement (1). Aluminium electronic configuration: \[ 1s^2\,2s^2\,2p^6\,3s^2\,3p^1 \] Valence shell contains: \[ 3s,\,3p_x,\,3p_y,\,3p_z \] Total valence orbitals = 4 Not 5. Hence statement (1) is incorrect.

Step 2: Check statement (2). Boron generally forms three covalent bonds. Example: \[ BF_3 \] However, boron can accept one lone pair and form: \[ BF_4^- \] where boron becomes tetracoordinate. Thus maximum covalency of boron is 4. Hence statement (2) is correct.

Step 3: Check statement (3). Beryllium valence shell: \[ 2s,\,2p_x,\,2p_y,\,2p_z \] Total valence orbitals = 4. Therefore statement (3) is incorrect.

Step 4: Check statement (4). Magnesium belongs to Group 2. Its usual covalency is 2. The statement that magnesium has maximum covalency 4 is not correct in this context. Hence statement (4) is incorrect. Therefore, only statement (2) is correct. \[ \boxed{(2)\ \text{Boron has a maximum covalency of four}} \]