Which of the following best describe the ground-state electron configuration of a phosphorus atom? (I) It has half-filled 3p orbitals. (II) It has five electrons in the valence shell. (III) It can form up to five covalent bonds. (A) I only, (B) II only, (C) I and II, (D) II and III, (E) I, II, and III.
To answer this question, we need to check statements (I), (II), and (III) to see if they do indeed apply to the ground-state electron configuration of a phosphorus atom. Phosphorus is a nonmetallic element. It’s located in group 15 or five of the periodic table. And it has an atomic number of 15. This piece of information tells us straight away that phosphorus contains 15 protons in its nucleus. It also therefore contains 15 electrons found in the shells outside the nucleus. So we need to establish the ground-state electron configuration of these 15 electrons.
Ground state simply means that all the electrons are in the lowest possible energy levels. Electrons will occupy the lowest energy levels first. The lowest energy level is energy level one. And this is closest to the nucleus. This energy level contains an s orbital. An s orbital is a spherical region of space that can contain up to two electrons. Energy level two is the next energy level. And it can contain an s orbital and some p orbitals. There are three p orbitals. They are aligned along the 𝑥-, 𝑦-, and 𝑧-axes in three-dimensional space. Each p orbital can be filled with two electrons, giving a total of six electrons, if these p orbitals are completely filled.
Energy level three is the next highest energy level. It can contain s, p, and d orbitals. There are five separate d orbitals. Each d orbital can contain two electrons. This gives a total of 10 electrons when filled completely. Since phosphorus is found in period three of the periodic table, the orbitals it contains are found in energy level three. So we can stop here. We don’t need to consider any higher energy level orbitals to find the ground-state electron configuration of phosphorus.
To get the ground-state electron configuration for phosphorus, we simply follow the arrows in the diagram from top to bottom. Which gives away the order in which these orbitals fill from lowest energy to highest energy. So we find that the ground-state electron configuration for a phosphorus atom is 1s2 2s2 2p6 3s2 3p3. This accounts for the 15 electrons total in the phosphorus atom. And notice that the 3p orbital is not completely filled.
Notice that, in phosphorus, the 3p orbital is half filled. It has three unpaired electrons in the 3p orbital. Statement (I) is therefore true. The next statement suggests that phosphorus contains five electrons in its valence shell. Valence shell here simply means the outer shell. The valence shell or outer shell for phosphorus contains electrons in energy level three. Energy level three does contain five electrons. So the second statement is also correct.
Five electrons in the valence shell for phosphorus also correlates with the fact that it’s in group 15, also known as group five. The third statement suggests that phosphorus can form up to five covalent bonds. A covalent bond is a shared pair of electrons. Since phosphorus is a nonmetal, it can react with other nonmetals to form covalently bonded simple molecules. Since phosphorus has five electrons in its outer shell, it would theoretically be possible for phosphorus to share these five electrons and make five covalent bonds with other atoms.
PCl5 is an example of a compound where phosphorus has indeed formed five covalent bonds. PCl5 is a stable compound. Notice in this compound that phosphorus has more than eight electrons in its outer shell. This type of compound is described as hypervalent or hypercoordinated. This compound exceeds the octet rule by having more than four pairs of electrons around the central atom. Phosphorus achieves this by using some d orbitals that are available to elements in period three to form the extra bonds. So our third statement here is also correct. Phosphorus can form up to five covalent bonds.
Statements (I), (II), and (III) are therefore all correct.