When writing the symbol of an element, the correct way is:
ᵇXₐ
Where b is the mass number of the element, which is equivalent to the sum of the number of protons and neutrons, while a is the atomic number, equal to the number of protons. X is the symbol of the element.
In this case, the atomic number 15 belongs to phosphorus, with symbol P. This becomes:
³¹P₁₅
