When a radioactive phosphorus atom decays by emitting a positron, one of its protons in the nucleus is converted into a neutron, and a positron and a neutrino are emitted. As a result, the atomic number of the nucleus decreases by one, while the mass number remains unchanged. Therefore, the resulting nucleus is an isotope of the element with atomic number one less than phosphorus, which is silicon (Si).

Here's the nuclear reaction that takes place:

Radioactive Phosphorus (30P) -> Silicon (30Si) + Positron (e+) + Neutrino (v)

The atomic number of phosphorus (P) is 15, while that of silicon (Si) is 14. The mass number of the original phosphorus atom (30P) is the same as the mass number of the resulting silicon isotope (30Si).