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.

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