Final answer:
A phosphorus atom in the ground state has five electrons in its outermost shell (third shell): two in the 3s subshell and three in the 3p subshell.
Step-by-step explanation:
The element phosphorus (P) with an atomic number of 15 has a total number of electrons equal to its atomic number. Thus, it contains 15 electrons. According to the Aufbau principle which dictates the order of filling of the energy levels, the ground-state electron configuration for phosphorus is 1s²2s²2p⁶3s²3p³. This configuration reveals that in the outermost shell, which is the third shell for phosphorus, there are a total of five electrons: two in the 3s subshell and three in the 3p subshell.