The chlorine atoms obey the octet rule, but the phosphorus atom does not. In a molecule of phosphorus pentachloride, #PCl_5#, each chlorine atom ends up with an octet of electrons (four pairs), but the phosphorus atom ends up with ten electrons (five pairs).
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To understand this, we need to look at the electron configuration of a phosphorus atom, which is #[Ne]3s^"2"3p^"3"#, in which each of the three p electrons are unpaired and available for bonding by sharing three electrons.
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However, in #PCl_5#, the phosphorus atom shares five electrons, one each with the five chlorine atoms. The phosphorus atom can do this because it can promote one of the #3s# electrons into a #3d# orbital, which now makes five unpaired electrons available for bonding.
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Experiments have shown that all five of the bonds with chlorine are identical. This indicates that the orbitals have hybridized, forming five #sp^3d# hybrid orbitals.
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