phosphorus has atomic number #15#, and is in group #5# and period #3# of the Periodic Table. it is in period #3#, because it has orbits at #3# energy levels. there are #2# electrons at the first energy level, and #8# at the second energy level. there are #15# electrons in total, there must be #5# electrons at the third energy level. read more
According to the periodic table above, phosphorus belongs to Group 5A. Therefore, Its valence electrons should be 5. The outermost orbitals, 3s2 3p3 , contains 5 electrons. read more