The atomic radius refers to the distance from the nucleus of an atom to a. it's proton cloud b. it's electron cloud c. it's neutron cluster d. the nucleus of another atom

B. Its electron cloud (the boundary of it). This is the outside of the atom. Just as we measure a circle's radius from the centre to the circumference, we measure an atom's radius from its nucleus to its cloud of electrons.

B? i feel like this is a trick  

The ground state electron configuration of a phosphorous atom has  three unpaired electrons. they're in the 3p sublevel. the electron configuration is: , for a total of  15 electrons.