What is the overall formal charge of BH4? (It abides by the octet rule)
The correct answer and explanation is:
The correct answer is: –1
Explanation:
To determine the overall formal charge of the tetrahydroborate ion (BH₄⁻), it is essential to apply the concept of formal charge, which is a bookkeeping method used in chemistry to determine the charge distribution in a molecule.
Step 1: Determine the valence electrons
- Boron (B) has 3 valence electrons (Group 13).
- Each hydrogen (H) has 1 valence electron.
- Since there are 4 hydrogen atoms: 4 × 1 = 4 electrons from hydrogen.
- The molecule has a negative charge (–1), which means there is 1 extra electron.
Total valence electrons:
= 3 (from B) + 4 (from H) + 1 (extra for negative charge)
= 8 valence electrons
Step 2: Assign electrons in the structure
BH₄⁻ has a central boron atom with 4 single bonds connecting to 4 hydrogen atoms. Each bond consists of 2 shared electrons, so there are 4 bonds × 2 electrons = 8 electrons used in bonding. This satisfies the octet rule for boron (though boron often is an exception, in this ion it obeys the octet).
Step 3: Determine formal charge for each atom
Formal charge formula:
Formal Charge = Valence Electrons – (Nonbonding Electrons + ½ Bonding Electrons)
For Boron:
= 3 – (0 nonbonding + ½ × 8 bonding)
= 3 – 4 = –1
For each Hydrogen:
= 1 – (0 nonbonding + ½ × 2 bonding)
= 1 – 1 = 0
Step 4: Calculate overall formal charge
- Boron: –1
- 4 Hydrogens: 0 × 4 = 0
- Total: –1
Conclusion:
The overall formal charge of BH₄⁻ is –1. This result aligns with the fact that the ion is negatively charged and obeys the octet rule.