Question 6.12: Draw the Lewis structures for PCl3, phosphorus trichloride, ...
Draw the Lewis structures for PCl_{3}, phosphorus trichloride, used commercially to prepare insecticides and flame retardPCl_{3}ants.
| ANALYZE THE PROBLEM | Given | Need | Connect |
| PCl_{3} | Lewis structure | total valence electrons |
STEP 1 Determine the arrangement of atoms. In PCl_{3}, the central atom is P because there is only one P atom.
Cl \underset{\overset{\quad}{Cl}}{P} ClSTEP 2 Determine the total number of valence electrons. We use the group number to determine the number of valence electrons for each of the atoms in the molecule.
| Element | Group | Atoms | Valence Electrons | Total |
| P | 5A(15) | 1 P \times | 5 e^{-} = | 5 e^{-} |
| Cl | 7A(17) | 3 Cl \times | 7 e^{-} = | 21 e^{-} |
| Total valence electrons for PCl_{3} = | 26 e^{-} | |||
STEP 3 Attach each bonded atom to the central atom with a pair of electrons. Each bonding pair can also be represented by a bond line.
\begin{array}{r c}\begin{matrix} Cl_{\bullet }^{\bullet }\underset{\overset{\bullet \bullet }{Cl} }{P_{\bullet }^{\bullet }} Cl \text{ or } \end{matrix} & Cl \overline{\quad}\underset{\underset{Cl}{|} }{P} \overline{\quad}Cl \end{array}Six electrons (3 \times 2 e^{-}) are used to bond the central P atom to three Cl atoms. Twenty valence electrons are left.
26 valence e^{-} – 6 bonding e^{-} = 20 e^{-} remaining
STEP 4 Use the remaining electrons to complete octets. We use the remaining 20 electrons as lone pairs, which are placed around the outer Cl atoms and on the P atom, such that all the atoms have octets.
\begin{array}{r c}\begin{matrix} {}_{\bullet }^{\bullet }\overset{\bullet \bullet }{\underset{\bullet \bullet }{Cl_{\bullet }^{\bullet } }} \underset{{}_{\bullet }^{\bullet }\underset{\bullet \bullet }{Cl_{\bullet }^{\bullet }} }{\overset{\bullet \bullet }{\underset{\bullet \bullet }{P_{\bullet }^{\bullet } }}} \overset{\bullet \bullet }{\underset{\bullet \bullet }{Cl_{\bullet }^{\bullet } }} \text{ or } \end{matrix} & {}_{\bullet }^{\bullet }\overset{\bullet \bullet }{\underset{\bullet \bullet }{Cl }}\overline{\quad} \underset{\overset{|}{ {}_{\bullet }^{\bullet }\underset{\bullet \bullet }{Cl_{\bullet }^{\bullet }} } } {\overset{\bullet \bullet }{P }}\overline{\quad} \overset{\bullet \bullet }{\underset{\bullet \bullet }{Cl_{\bullet }^{\bullet } }} \end{array}