Question

Identify the electron rich hydrides from the following

• A. B₂H₆, AlH₃
• B. NaH, MgH₂
• C. HCl, H₂S
• D. CH₄, SiH₄

Hint:

To quickly classify covalent hydrides, look at the group of the central atom. Group 13 \(\rightarrow\) deficient. Group 14 \(\rightarrow\) precise. Group 15, 16, 17 \(\rightarrow\) rich (due to lone pairs).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Hydrides can be broadly classified into ionic, covalent, and metallic hydrides. Covalent hydrides are further classified based on the number of electrons around the central atom relative to a complete octet.
- Electron-deficient hydrides: The central atom has fewer than 8 valence electrons. Typically formed by Group 13 elements.
- Electron-precise hydrides: The central atom has exactly 8 valence electrons (a complete octet). Typically formed by Group 14 elements.
- Electron-rich hydrides: The central atom has a complete octet and one or more lone pairs of electrons. Typically formed by Group 15, 16, and 17 elements.
Step 2: Analyzing the Options:
- (A) B₂H₆, AlH₃: Boron and Aluminum are in Group 13. Their hydrides, diborane and alane, are electron-deficient. B in B₂H₆ does not have a complete octet.
- (B) NaH, MgH₂: Sodium and Magnesium are Group 1 and Group 2 metals. They form ionic or saline hydrides, which consist of metal cations (Na⁺, Mg²⁺) and hydride anions (H⁻). They are not classified under the electron-deficient/precise/rich scheme for covalent hydrides.
- (C) HCl, H₂S: Chlorine (Group 17) and Sulfur (Group 16) are p-block non-metals. In HCl, the Cl atom has 3 lone pairs of electrons in addition to the bonding pair. In H₂S, the S atom has 2 lone pairs in addition to the two bonding pairs. Since the central atoms have excess electrons in the form of lone pairs, these are electron-rich hydrides.
- (D) CH₄, SiH₄: Carbon and Silicon are in Group 14. In methane (CH₄) and silane (SiH₄), the central atom forms four single bonds and has a complete octet with no lone pairs. These are electron-precise hydrides.
Step 3: Final Answer:
HCl and H₂S are the hydrides where the central atom has lone pairs, making them electron-rich. Therefore, option (C) is correct.