The number of molecules from the following which contain only two lone pair of electrons is _______
H$$_2$$O, N$$_2$$, CO, XeF$$_4$$, NH$$_3$$, NO, CO$$_2$$, F$$_2$$

To determine the number of molecules containing only two lone pairs of electrons, we analyze the total lone pairs (on both central and terminal atoms) for each given molecule:

Molecule-by-Molecule Analysis:

• H2O: Oxygen has 6 valence electrons. It forms 2 single bonds with Hydrogen atoms, leaving 4 non-bonding electrons.
  👉 Total Lone Pairs = 2 (on Oxygen)
• N2: Each Nitrogen atom shares 3 electrons to form a triple bond (N≡N), leaving 1 lone pair on each atom.
  👉 Total Lone Pairs = 2 (1 on each N)
• CO: Carbon and Oxygen share a triple bond (:C≡O:).
  👉 Total Lone Pairs = 2 (1 on C, 1 on O)
• NO: Nitric oxide is an odd-electron molecule (11 valence electrons total). It contains a double bond, 2 lone pairs (1 on N, 1 on O), and 1 single unpaired electron.
  👉 Total Lone Pairs = 2 (Note: Often excluded in standard textbook counting due to its radical/unpaired electron nature).
• NH3: Nitrogen forms 3 single bonds with Hydrogen, leaving 1 lone pair on the Nitrogen atom.
  👉 Total Lone Pairs = 1
• CO2: Carbon forms double bonds with both Oxygen atoms ($$O=C=O$$). Each Oxygen retains 2 lone pairs.
  👉 Total Lone Pairs = 4
• F2: Two Fluorine atoms share a single bond. Each Fluorine atom has 3 lone pairs.
  👉 Total Lone Pairs = 6
• XeF4: Xenon has 2 lone pairs, and each of the 4 Fluorine atoms has 3 lone pairs ($$2 + 4 \times 3$$).
  👉 Total Lone Pairs = 14

Summary Table:

Conclusion:

Excluding the odd-electron radical ($$\text{NO}$$), the molecules containing exactly two lone pairs of electrons are H2O, N2, and CO.

Correct Answer: 3