Write the symbol for the atom or ion that has 15 protons, 15 neutrons and 18 electrons. Enter your answer with the mass number followed by the symbol of the element followed by any charge on the ion.

The
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
3-

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.