Draw a valid Lewis electron dot structure for phosphorus trichloride (PCl3) accounting for all valence electrons. State the shape of PCl3 molecules. How many ‘lone pairs’ does this molecule have
The Correct Answer and Explanation is:
Lewis Electron Dot Structure for Phosphorus Trichloride (PCl₃):
To draw the Lewis structure for PCl₃:
- Count total valence electrons:
- Phosphorus (P) has 5 valence electrons.
- Each Chlorine (Cl) atom has 7 valence electrons.
- Total = 5 (from P) + 3 × 7 (from 3 Cl) = 26 valence electrons
- Choose a central atom:
- Phosphorus is less electronegative than chlorine, so it becomes the central atom.
- Create single bonds:
- Connect each Cl atom to the P atom using single bonds.
- Each bond uses 2 electrons, so 3 bonds = 6 electrons used.
- Remaining electrons = 26 − 6 = 20 electrons
- Distribute remaining electrons:
- Place 6 electrons (3 lone pairs) around each chlorine to complete their octets: 3 × 6 = 18 electrons.
- Remaining electrons = 20 − 18 = 2 electrons
- Place leftover electrons on phosphorus:
- Place the final 2 electrons (1 lone pair) on the phosphorus atom.
The Lewis structure looks like this:
lessCopyEdit Cl
|
Cl - P - Cl
..
Here, the dots represent a lone pair on phosphorus.
Molecular Shape of PCl₃:
The shape of the PCl₃ molecule is trigonal pyramidal. This shape arises because phosphorus has three bonding pairs and one lone pair of electrons. According to VSEPR theory (Valence Shell Electron Pair Repulsion), the electron pairs around the central atom repel each other and adopt a geometry that minimizes this repulsion. In this case, the presence of one lone pair pushes the bonding pairs down, forming a three-sided pyramid.
Number of Lone Pairs:
- Phosphorus: 1 lone pair
- Each Chlorine: 3 lone pairs × 3 = 9 lone pairs
Total lone pairs in the molecule: 1 + 9 = 10 lone pairs
However, the number of lone pairs on the molecule itself (that is, on central atom P) is 1.
