Which of the following molecules has only one lone (non-bonding) pair in its Lewis dot structure?

• A. NH3
• B. CO2
• C. H2O
• D. CH4

Answer: (A)

Verified step by step guidance

1. Recall that a lone pair (non-bonding pair) of electrons is a pair of valence electrons that are not shared between atoms in a bond but belong exclusively to one atom.
2. Draw or visualize the Lewis dot structure for each molecule to count the number of lone pairs on the central atom or relevant atoms:
3. For NH3 (ammonia), the nitrogen atom has 5 valence electrons. It forms 3 single bonds with hydrogen atoms, using 3 electrons, leaving 2 electrons (1 lone pair) on nitrogen.
4. For CO2 (carbon dioxide), the carbon atom forms two double bonds with oxygen atoms and has no lone pairs on carbon; oxygen atoms have lone pairs but the question focuses on the molecule as a whole and the central atom.
5. For H2O (water), the oxygen atom has 6 valence electrons, forms 2 single bonds with hydrogen atoms, and has 2 lone pairs (4 electrons) remaining on oxygen.