Which of the following is not a possible electronic configuration of the P atom? (A) 1s2 2s2 2p6 3s2 3p3 3d0, (B) 1s2 2s2 2p6 3s1 3p4 3d0, (C) 1s2 2s2 2p6 3s2 3p2 3d2, (D) 1s2 2s2 2p6 3s1 3p3 3d1, or (E) 1s2 2s2 2p6 3s0 3p4 3d1.
Before we go any further, let’s just reassure ourselves of the name for this type of atom. It’s a phosphorus atom. As with all elements, we can find phosphorus on the periodic table. Phosphorus could be found in group 15, sometimes called group five, and in the third row, otherwise called period three. And phosphorus is just below nitrogen. The question is about what’s a feasible electronic configuration of the phosphorus atom. Here, we expect four of the candidates to be possible and one of the candidates to be impossible.
There is a possibility of being tripped up on this question because some of these electronic configurations are highly, highly unlikely. But we’re looking for the one configuration that is fundamentally definitionally impossible. From the period number, we get the valence shell number — in this case, three. In this case, that isn’t particularly helpful. None of the options can be eliminated on this basis because all the electron configurations here have their valence electrons in the third shell. The group number of phosphorus tells us the number of valence electrons we can expect. In this case, it’s five.
All five of our possible answers have 1s2 at the beginning, indicating that their inner shell is completely full. All of the electron configurations also have 2s2 2p6. That’s eight electrons in total in the second shell. So that’s full as well. So the only way we can tell these configurations apart is by looking at the valence shell. The first thing we can do is check that the valence shell for each one has the right number of electrons. For electron configuration (A), there are five electrons in total in the valence shell — two in the 3s subshell, three in the 3p subshell, and none in the 3d subshell.
Electron configuration (B) also has five electrons in the valence shell. But this time, there’s only one in the 3s subshell and four in the 3p subshell. But alarm bells should be ringing for statement (C) because there are six electrons in the valence shell — two in the 3s subshell, two in the 3p subshell, and two in the 3d subshell. Atoms are neutral, which means they shouldn’t have any extra electrons. Atoms are neutral, which means they shouldn’t have extra electrons. Therefore, this particular electron configuration is not possible for a phosphorus atom.
We can see that option (D) is feasible because it has five electrons in the valence shell. And it’s the same case for option (E). Options (A), (B), (D), and (E) all have five electrons in their valence shell. And, therefore, they’re possible but not necessarily likely electron configurations of the phosphorus atom. Option (C), however, is not possible because there are more electrons than a phosphorus atom has. So our final answer is 1s2 2s2 2p6 3s2 3p2 and 3d2.