Question 8.6: Draw the Lewis structure for phosphorus trichloride, PCl3.
Draw the Lewis structure for phosphorus trichloride, PCl_{3}.
Analyze and Plan We are asked to draw a Lewis structure from a molecular formula. Our plan is to follow the five-step procedure just described.
Solve First, we sum the valence electrons. Phosphorus (Group 15) has five valence electrons, and each chlorine (Group 17) has seven. The total number of valence electrons is therefore
5 + (3 × 7) = 26
Second, we arrange the atoms to show which atom is connected to which, and we draw a single bond between them. There are various ways the atoms might be arranged. It helps to know, though, that in binary compounds the first element in the chemical formula is generally surrounded by the remaining atoms. So we proceed to draw a skeleton structure in which a single bond connects the P atom to each Cl atom:
Cl-\underset{\underset{Cl}{|} }{P} -Cl(It is not crucial that the Cl atoms be to the left of, right of, and below the P atom—any structure that shows each of the three Cl atoms bonded to P will work.)
Third, we add Lewis electron dots to complete the octets on the atoms bonded to the central atom. Completing the octets around each Cl atom accounts for 24 electrons (remember, each line in our structure represents two electrons):
\overset{\cdot \cdot }{\underset{\cdot \cdot }{:Cl}} -\underset{\underset{\overset{ }{\underset{\cdot \cdot }{:Cl:}}}{|} }{P} -\overset{\cdot \cdot }{\underset{\cdot \cdot }{Cl:}}Fourth, recalling that our total number of electrons is 26, we place the remaining two electrons on the central atom, P, which completes its octet:
\overset{\cdot \cdot }{\underset{\cdot \cdot }{:Cl}} -\underset{\underset{\overset{ }{\underset{\cdot \cdot }{:Cl:}}}{|} }{\overset{\cdot \cdot }{P} } -\overset{\cdot \cdot }{\underset{\cdot \cdot }{Cl:}}This structure gives each atom an octet, so we stop at this point. (In checking for octets, remember to count a single bond as two electrons.)