Question

Which one of the following molecules contains an incomplete octet of the central atom?

• A. SF₆
• B. AICI₃
• C. CH₄
• D. PF₅
• E. H₂O

Answer 1

The Correct Option is B

The octet rule states that atoms in molecules tend to have eight electrons in their valence shell, like the noble gases. However, some molecules do not follow this rule.

SF\(_6\): Sulfur has more than eight electrons in its valence shell, as it is able to accommodate more than eight electrons due to the availability of d-orbitals.

AlCl\(_3\): Aluminum in AlCl\(_3\) only has six electrons in its valence shell, which is an example of an incomplete octet, meaning it does not satisfy the octet rule.

CH\(_4\): Carbon in CH\(_4\) follows the octet rule as it shares four electrons with four hydrogen atoms, completing its octet.

PF\(_5\): Phosphorus in PF\(_5\) has ten electrons in its valence shell, which is allowed as phosphorus can expand its octet.

H\(_2\)O: Oxygen in H\(_2\)O has eight electrons in its valence shell, fulfilling the octet rule.

Thus, the molecule AlCl\(_3\) contains an incomplete octet of the central atom.

The correct option is (B) : \(AlCI_3\)

Answer 2

To identify the molecule with an incomplete octet of the central atom, we must check if the central atom has fewer than 8 electrons in its valence shell. 

• SF₆: Sulfur has 6 valence electrons and forms 6 bonds → 12 electrons → expanded octet ✅
• AlCl₃: Aluminium has 3 valence electrons and forms 3 bonds → 6 electrons → incomplete octet ❌
• CH₄: Carbon has 4 bonds → 8 electrons → complete octet ✅
• PF₅: Phosphorus forms 5 bonds → 10 electrons → expanded octet ✅
• H₂O: Oxygen has 2 bonds + 2 lone pairs → 8 electrons → complete octet ✅

Final Answer: AlCl₃ (central atom has incomplete octet)