answer is the only element that has 15 - i.e. Phosphorous - as its
ATOMIC NUMBER (which means the number of protons it has). The number of
protons + number of neutrons equals atomic mass. The typical P has a
mass of 31 which means it has 16 neutrons. An ISOTOPE of P, i.e. P-30,
would be the right element to get 15 neutrons.
Usually, P has
the same number of electrons as protons, i.e. 15. In this case, 3 extra
electrons are needed to get 18. Therefore, the answer Phosphorous 30 P
Basically, once you know the proton number, all you have
to do is look at the periodic table to find the element with that number
on the top of the symbol.