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

Step 1: Define maximum covalency.
Maximum covalency is the largest number of covalent bonds an atom can form, which depends on the orbitals available in its valence shell.
Step 2: Test statement (1) on aluminium.
Al is \([Ne]3s^2 3p^1\); its valence shell offers \(3s, 3p_x, 3p_y, 3p_z\), that is 4 orbitals, not 5. Statement (1) is wrong.
Step 3: Test statement (2) on boron.
Boron normally makes 3 bonds as in \(BF_3\), but by accepting a lone pair it forms \(BF_4^-\), where it is bonded to four groups. So boron's maximum covalency is 4. Statement (2) is correct.
Step 4: Test statement (3) on beryllium.
Be has the valence orbitals \(2s, 2p_x, 2p_y, 2p_z\), which is 4 orbitals, not 3. Statement (3) is wrong.
Step 5: Test statement (4) on magnesium.
Mg is a Group 2 metal with a usual covalency of 2; claiming a maximum covalency of 4 here is not correct. Statement (4) is wrong.
Step 6: Conclude.
Only statement (2) survives.
\[ \boxed{\text{Boron has a maximum covalency of four.}} \]