Many elements in the third row and beyond in the periodic table may form more than four bonds and thus appear to have "expanded octets." phosphorus and sulfur, for example, may form five and six covalent bonds. count up the total number of valence electrons in pcl5 and draw its lewis structure. how many valence electrons are "counted" toward the central p atom?

See the sketch attached for a Lewis cross-and-dot structure of PCl₅.

The central P atom ends up with ten valence electrons.

Unlike P, Cl does not form expanded octets. Each Cl atom in PCl₅ will share one electron with a P atom. Each will form one P-Cl covalent bond and end up with a typical octet of eight electrons. Six of the eight electrons come from three non-bonding pairs (a.k.a. "lone pairs").

P is found in IUPAC group 15 of a modern periodic table. A neutral P atom has five valence electrons. Each of them goes to a P-Cl bond in PCl₅. The central P atom will end up forming five P-Cl bonds. That will gives the atom a total of ten valence electrons. That is two electrons more than a typical octet- hence the name "expanded octet".