Question

In which one of the following compounds, there is complete octet of central atom

• A. BF\(_3\)
• B. BeH\(_2\)
• C. SCl\(_2\)
• D. AlCl\(_3\)
• E. LiCl

Hint:

Elements in period 3 and below (like S, P, Cl) can expand their octet, but they can also have a complete octet.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The octet rule states that atoms tend to bond in such a way that they each have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. We need to examine the Lewis structure of the central atom in each compound to see if it is surrounded by eight valence electrons.
Step 2: Detailed Explanation:
Let's analyze each option:

(A) BF\(_3\): Boron (B) is the central atom. It is in Group 13 and has 3 valence electrons. It forms three single covalent bonds with three fluorine atoms. The total number of electrons around Boron is \(3 \times 2 = 6\). This is an "electron-deficient" molecule with an incomplete octet.

(B) BeH\(_2\): Beryllium (Be) is the central atom. It is in Group 2 and has 2 valence electrons. It forms two single covalent bonds with two hydrogen atoms. The total number of electrons around Beryllium is \(2 \times 2 = 4\). This is another example of an incomplete octet.

(C) SCl\(_2\): Sulfur (S) is the central atom. It is in Group 16 and has 6 valence electrons. It forms two single covalent bonds with two chlorine atoms, using 2 of its valence electrons. The remaining 4 valence electrons exist as two lone pairs on the sulfur atom. The total number of electrons around Sulfur is (2 bonding pairs \(\times\) 2 electrons/pair) + (2 lone pairs \(\times\) 2 electrons/pair) = 4 + 4 = 8. Sulfur has a complete octet.

(D) AlCl\(_3\): Aluminum (Al) is the central atom. It is in Group 13 and has 3 valence electrons. It forms three single covalent bonds with three chlorine atoms. The total number of electrons around Aluminum is \(3 \times 2 = 6\). It has an incomplete octet.

(E) LiCl: Lithium chloride is an ionic compound. Li loses an electron to form Li\(^+\) (duet configuration) and Cl gains an electron to form Cl\(^-\) (octet configuration). The concept of a central atom with a shared octet is not applicable here in the same way as for covalent molecules.

Step 3: Final Answer:
Among the given options, only in SCl\(_2\) does the central atom (Sulfur) have a complete octet of eight electrons. This corresponds to option (C).