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The most common exception to the octet rule is a molecule or ion with with at least one atom that possesses more than an octet of electrons. These compounds are found for elements of period 3 and beyond. For example, sulfur can accommodate more than eight valence electrons by using one or more d orbitals. Sulfur and oxygen are the most common elements seen to exceed the octet rule and have an expanded valence shell.
The octet (8 e-) comes from the full s- and p- subshells; s2p6 totals 8 electrons. Atoms in period 3 or higher have valence shell d-orbitals, so they can accommodate more electrons than the standard 8 for atoms with lower atomic numbers
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