Show that [BrF4] – is a non-polar molecule.
The correct answer and explanation is:
Answer:
The BrF4−\text{BrF}_4^- ion is a non-polar molecule.
Explanation:
To understand why BrF4−\text{BrF}_4^- is non-polar, we need to analyze its molecular geometry, the distribution of electron density, and the symmetry of the molecule.
1. Lewis Structure and Electron Count:
- Bromine (Br) is the central atom.
- Fluorine (F) atoms: 4 atoms bonded to Br.
- Charge: −1-1 (extra electron).
Bromine has 7 valence electrons, each fluorine has 7, and there’s one extra electron due to the negative charge.
Total valence electrons = 7+(4×7)+1=7+28+1=367 + (4 \times 7) + 1 = 7 + 28 + 1 = 36 electrons.
2. Electron Pair Arrangement:
- Br forms 4 single bonds with F atoms.
- Remaining electrons form lone pairs on Br.
- Bromine uses 8 electrons for bonding (4 bonds × 2 electrons).
- Leftover electrons: 36 total – 8 bonding = 28 electrons.
- These 28 electrons are placed as lone pairs: 3 lone pairs on Br (6 electrons) and the rest on fluorines (which complete their octet).
So, Br has 4 bonding pairs and 2 lone pairs.
3. Molecular Geometry:
With 4 bonded atoms and 2 lone pairs, the electron geometry is octahedral (6 electron pairs around Br), but the molecular shape (considering only atoms) is square planar because the two lone pairs occupy positions opposite each other to minimize repulsion.
4. Symmetry and Polarity:
- The square planar shape is highly symmetrical.
- The four fluorine atoms are arranged evenly around bromine in a plane.
- The lone pairs are above and below the plane, cancelling any dipole moment they might create.
- The dipole moments of the Br–F bonds cancel out due to the symmetry.
Hence, the net dipole moment is zero.
5. Conclusion:
Since the BrF4−\text{BrF}_4^- ion has a symmetric square planar shape with opposing lone pairs cancelling dipoles, it is non-polar.
Summary:
- BrF4−\text{BrF}_4^- has 4 bonding pairs and 2 lone pairs on Br.
- Electron geometry is octahedral; molecular shape is square planar.
- The symmetry causes the bond dipoles to cancel out.
- Therefore, BrF4−\text{BrF}_4^- is a non-polar molecule despite having polar Br–F bonds.