1. Count valence electrons:
- Phosphorus (P) has 5 valence electrons.
- Bromine (Br) has 7 valence electrons.
- Total valence electrons: 5 + (7 x 5) = 40
2. Central atom: Phosphorus (P) is the least electronegative atom, making it the central atom.
3. Single bonds: Connect each bromine atom to the phosphorus atom with a single bond. This uses up 10 electrons (5 bonds x 2 electrons per bond).
4. Remaining electrons: You have 30 electrons remaining. Distribute these as lone pairs around each bromine atom. Each bromine atom will have 3 lone pairs (6 electrons).
5. Formal charges: Calculate the formal charge on each atom to ensure the structure is the most stable. In this case, all atoms will have a formal charge of 0.
Lewis Structure of PBr5:
```
Br
/ \
Br Br
/ \ /
P Br
\ / \
Br Br
```
Key points:
* The phosphorus atom is surrounded by 5 bonding pairs, giving it a steric number of 5.
* The trigonal bipyramidal geometry is due to the five electron domains around the central atom.
* The axial bonds (those pointing directly at the phosphorus atom) are slightly longer than the equatorial bonds.
This structure is consistent with the experimentally observed geometry of phosphorus pentabromide.
