Final answer:
Elements starting from the third period of the periodic table tend to have expanded valences due to the availability of d orbitals. These larger atoms like sulfur, phosphorus, and others can form hypervalent molecules by sharing more than four pairs of electrons with other atoms.
Step-by-step explanation:
The group of elements that would tend to have expanded valences are mainly found starting from the third period of the periodic table and onward. These elements can exceed the 'octet rule', which typically limits elements to eight electrons in their valence shell. The reason for this capability lies in the availability of d orbitals in these larger atoms, such as sulfur (S), phosphorus (P), selenium (Se), and tellurium (Te), among others.
For example, in the case of phosphorous pentachloride (PCl5), phosphorus shares five pairs of electrons, thus having a total of ten electrons in its valence shell. This is possible because phosphorus, being in the third period, has empty d orbitals available in the same shell, allowing it to have an 'expanded octet' or hypervalent molecule configuration.
Expanded Valence in Nonmetals and Transition Metals
Main-group nonmetals from groups 15 (Group 5A), 16 (Group 6A), and 17 (Group 7A) of the periodic table may also handle more electrons than the typical octet. For instance, halogens like chlorine (Cl) can form hypervalent compounds by accepting additional electrons into their valence shell. Transition metals, found in the d-block of the periodic table, have a different method of filling their electron orbitals that can lead to a variety of valence states, but they do not usually form hypervalent compounds in the same manner as nonmetals do.