Answer
$^{31}_{15}$P$^{3-}$
Work Step by Step
We know that the mass number is the sum of the number of protons and neutrons in an atom. If we are given that the element we are working with has a mass number of 31 and that the number of neutrons in the atom's nucleus is 16, then we can subtract the number of neutrons from the mass number to get the number of protons, which, in this case, is 15.
The number of protons in an atom is also its atomic number, which identifies that atom as a member of a certain element. In this case, the element with atomic number 15 is phosphorus (P). In a neutral atom, the number of protons and the number of electrons equal one another, but since this atom contains three more electrons than a neutral atom, this isotope is an anion with a charge of 3-.
We write the mass number as a superscript to the left of the chemical symbol and the atomic number as a subscript to the left of the chemical symbol. The charge is a superscript to the right of the chemical symbol. Therefore, the complete chemical symbol for this atom of phosphorus is $^{31}_{15}$P$^{3-}$.
