Which of these molecules has an expanded octet

Question

Accepted Answer

The correct option is (4):

An expanded octet occurs when a central atom in a molecule can accommodate more than eight electrons in its valence shell. This is typically observed in elements from Period 3 and beyond in the periodic table, as they have accessible d-orbitals that can participate in bonding. Let's analyze $PF_5$. Phosphorus (P) is the central atom and is in Period 3. It has 5 valence electrons. In $PF_5$, phosphorus forms five single bonds with five fluorine atoms. Each single bond involves two electrons, so the phosphorus atom is surrounded by $5 	imes 2 = 10$ electrons. Since 10 electrons are more than 8, phosphorus in $PF_5$ exhibits an expanded octet, utilizing its empty d-orbitals for bonding.

The incorrect option is (1):
In $CCl\_4$, carbon (C) is the central atom. Carbon is in Period 2 and can only accommodate a maximum of 8 electrons in its valence shell. It forms four single bonds with chlorine atoms, resulting in 8 electrons around the carbon atom, which obeys the octet rule.

The incorrect option is (2):
In $CO\_2$, carbon (C) is the central atom, which is in Period 2. Carbon forms two double bonds with two oxygen atoms. This results in 8 electrons around the carbon atom, satisfying the octet rule.

The incorrect option is (3):
In $CH\_4$, carbon (C) is the central atom. As discussed, carbon is a Period 2 element and adheres to the octet rule. It forms four single bonds with hydrogen atoms, with 8 electrons around the carbon, thus not having an expanded octet.