How many lone pairs (the pair number, not total number of electrons) are on P in PCl3?

To determine the number of lone pairs on P in PCl3, we need to follow these steps:

1. Identify the number of valence electrons for Phosphorus (P). Phosphorus is in group 15 of the periodic table, so it has 5 valence electrons.

2. Identify the number of atoms bonded to Phosphorus. In PCl3, there are 3 Chlorine atoms bonded to the Phosphorus atom.

3. Each bond to the Phosphorus atom represents a pair of electrons. So, the 3 bonds to the Chlorine atoms account for 6 of the valence electrons on Phosphorus.

4. Subtract the number of electrons involved in bonds from the total number of valence electrons. This gives 5 - 6 = -1. However, we cannot have a negative number of electrons. This indicates that we need to add 3 more electrons to the Phosphorus atom to make it have 8 electrons, which is the full number for the outer shell.

5. These additional electrons form a lone pair. So, there is 1 lone pair on the Phosphorus atom in PCl3.