For PH3, describe the central atom and number of unshared pairs of electrons in the molecule, if any.

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Phosphine (PH3) has a central phosphorus atom combined with 3 hydrogen atoms. It is a trigonal pyramidal shaped molecule.

Phosphorus has an electron configuration of [Ne]3s2 3p3, the outer most shell has 5 electrons. To gain the electron configuration of a noble gas, 3 electrons are shared with electrons of other atoms.  Hydrogen has an electron configuration of 1s1. Each atom of hydrogen can share 1 electron with an electron of phosphorus. When phosphine is formed there are three electrons shared between the phosphorus and hydrogen atoms. This results in phosphorus achieving a stable number of electrons in its outermost shell of 8 and each of the hydrogen atoms achieving a stable electron configuration of 1s2.

There is one pair of electrons in the phosphorus atom that is not shared.