\(\mathrm{P}\) is immediately under \(\mathrm{N}\) in the periodic table. Both react with three atoms of chlorine to form \(\mathrm{NCl}_{3}\) and \(\mathrm{PCl}_{3} .\) However, phosphorus also forms \(\mathrm{PCl}_{5}\), whereas nitrogen does not. Draw dot diagrams for all three molecules. In doing so, you will have to violate the octet rule for \(\mathrm{PCl}_{5}\). That is OK. Do it. Then explain why phosphorus can do this but nitrogen can't. (Hint: Nitrogen has only \(2 \mathrm{~s}\) and \(2 p\) orbitals in its valence shell, which together can hold a maximum of eight electrons. Read section \(5.8\) about expanded octet atoms.) Then postulate what kind of atoms might be expected to exceed the octet rule.

To draw the dot diagrams (Lewis structures) for these molecules, we will first find the valence electrons for nitrogen, phosphorus, and chlorine atoms, and then connect them to form the respective compounds. - Nitrogen (N) belongs to Group 15 and has 5 valence electrons - Phosphorus (P) belongs to Group 15 too, but is in period 3, has 5 valence electrons - Chlorine (Cl) belongs to Group 17 and has 7 valence electrons NCl3: 1. Create a central nitrogen (N) atom with 5 valence electrons. 2. Surround the N atom with 3 chlorine (Cl) atoms, each having 7 valence electrons. 3. Place a pair of electrons between N and each Cl atom to form single bonds. PCl3: 1. Create a central phosphorus (P) atom with 5 valence electrons. 2. Surround the P atom with 3 chlorine (Cl) atoms, each having 7 valence electrons. 3. Place a pair of electrons between P and each Cl atom to form single bonds. PCl5 (violating the octet rule for P): 1. Create a central phosphorus (P) atom with 5 valence electrons. 2. Surround the P atom with 5 chlorine (Cl) atoms, each having 7 valence electrons. 3. Place a pair of electrons between P and each Cl atom to form single bonds.

Phosphorus (P) can form PCl5, in which it has 10 electrons in its valence shell, whereas nitrogen (N) cannot. The reason for this difference lies in the fact that nitrogen belongs to period 2 and has only 2s and 2p orbitals in its valence shell. These orbitals can hold a maximum of 8 electrons. As a result, nitrogen cannot accommodate more than 3 chlorine atoms to form NCl3, obeying the octet rule. Phosphorus belongs to period 3 and has 3s, 3p, and 3d orbitals available. The 3d orbitals have the capability to accommodate additional electrons beyond the octet rule, allowing for the expansion of valence shells. This is why phosphorus can form PCl5, in which it has 10 electrons in its valence shell, violating the octet rule.

Atoms that could exceed the octet rule typically belong to period 3 and beyond in the periodic table. This is because they have access to vacant d (and f) orbitals, which can accommodate additional electrons, allowing them to expand beyond the 8 electrons in their valence shell. Examples of such elements include phosphorus (P), sulfur (S), and chlorine (Cl), which can form expanded octets, such as PCl5, SF6, and ClF5, respectively.