Which molecule has a central atom with two single bonds and two lone pairs of electrons, representing an exception to the octet rule in its Lewis dot structure?

• A. H2O (water)
• B. BF3 (boron trifluoride)
• C. CO2 (carbon dioxide)
• D. NH3 (ammonia)

Answer: (A)

Verified step by step guidance

1. Identify the central atom in each molecule and determine its valence electrons based on its group in the periodic table.
2. Draw the Lewis dot structure for each molecule, showing all bonding pairs (single, double bonds) and lone pairs of electrons around the central atom.
3. Count the total number of electrons around the central atom in each structure to check if it follows the octet rule (8 electrons) or is an exception.
4. For the molecule with two single bonds and two lone pairs on the central atom, verify that the central atom has exactly 8 electrons (4 pairs) and note if this arrangement is common or an exception to the octet rule.
5. Compare all molecules: H2O has a central oxygen atom with two single bonds and two lone pairs, satisfying the octet rule but with a bent shape; BF3 has an incomplete octet; CO2 has double bonds; NH3 has three single bonds and one lone pair.