Draw a Lewis Structure of a PCl3 Molecule

To draw the Lewis structure of phosphorus trichloride (PCl₃), we will follow these steps:

1. Count the total number of valence electrons: Phosphorus (P) is in group 15 and has 5 valence electrons. Chlorine (Cl), which is in group 17, has 7 valence electrons. Since there are three chlorine atoms in PCl₃, we calculate the total valence electrons as follows:
   • 1 × 5 (for P) + 3 × 7 (for Cl) = 5 + 21 = 26 valence electrons.
2. Determine the central atom: Phosphorus is less electronegative than chlorine, making it the central atom of the molecule.
3. Draw single bonds: Connect the phosphorus atom to each of the three chlorine atoms with single bonds. Each single bond accounts for 2 electrons.
   • 3 bonds × 2 electrons/bond = 6 electrons used.
4. Distribute remaining electrons: After using 6 electrons for the bonds, we have:
   • 26 total electrons – 6 used = 20 electrons remaining.

   Now we distribute these 20 electrons to the chlorine atoms. Each chlorine atom needs 8 electrons to complete its octet:

   • 3 Cl atoms × 6 electrons (3 lone pairs on each) = 18 electrons placed.
5. Check the electron count: Phosphorus has 6 electrons around it (3 bonds), while each chlorine atom now has 8 electrons total (2 from the bond and 6 as lone pairs). PCl₃ satisfies the octet rule for chlorine and expands the octet for phosphorus, which can hold more than 8 electrons.

In summary, the final Lewis structure for PCl₃ shows phosphorus at the center with three single bonds to each chlorine atom, and each chlorine has three lone pairs of electrons:

       ..   ..   ..
      :Cl:  :Cl:  :Cl:
       ..   ..   ..
         \   |   /
          P

This representation helps us understand how atoms bond in phosphorus trichloride and the distribution of electrons in the molecule.