One electron(s) are added to an phosphorus atom. Enter the chemical symbol of the element.

S (sulphur)

Explanation:

In a neutral atom, the number of electrons and the number of protons in an element are equal. These numbers are called the ATOMIC NUMBER. That is, in a neutral atom;

proton no. = electron no. = Atomic no.

Hence, in this case where phosphorus atom has an electron number of 15. It means that there are 15 protons and 15 electrons in a phosphorus atom. If one electron is added to these electrons i.e. 15 +1, there will be 16 electrons in the atom, hence, this atom becomes SULPHUR element, which has 16 electrons in its atom, with chemical symbol S.

the electron configuration that you gave me has 16 electrons ergo it has 16 protons ergo it is the element sulfur.