The symbol for the ion which has 15 protons, 17 neutrons, and 18 electrons
is P³⁻.
Phosphorus has a valence electron of 5 in which it can gain 3 electrons to
attain the octet configuration. It can also lose the 5 valence electrons in order to become stable.
This gives rise to the electrons becoming 18 instead of 15 and thereby
becomes P³⁻
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