Questions: Lewis Structures

Question 1

A student drawing the Lewis structure for water (H₂O) places hydrogen as the central atom with the two oxygen atoms bonded to it. What is the error?

• A. No error — hydrogen can be the central atom when there are only two bonds
• B. Hydrogen must always be a terminal atom; the central atom should be the least electronegative non-hydrogen atom (oxygen in this case)
• C. The error is that oxygen and hydrogen cannot form polar covalent bonds
• D. The error is that oxygen should have no lone pairs in the correct structure

Answer: (B)

Hydrogen has only one valence electron and can form only one bond, making it incapable of serving as a central atom — it can never satisfy an octet or connect to more than one other atom. The central atom rule is: use the least electronegative atom that isn't hydrogen. In H₂O, oxygen is the only candidate. This is a common error because students sometimes confuse 'least electronegative central atom' with 'most electronegative central atom' — but the central atom forms the most bonds, which requires being willing to share electrons generously (lower electronegativity).

Question 2

After drawing single bonds from the central atom to all outer atoms and distributing remaining electrons as lone pairs, the central atom in your Lewis structure has only 6 electrons instead of 8. What is the correct next step?

• A. Add 2 more electrons to the structure to complete the central atom's octet
• B. Convert one lone pair from an adjacent outer atom into a bonding pair, creating a double bond to the central atom
• C. Accept the structure as complete — not all central atoms need a full octet
• D. Move the central atom to a terminal position and start over with a different central atom

Answer: (B)

When the central atom is short of an octet after initial electron distribution, the fix is to convert lone pairs from adjacent atoms into bonding pairs (forming double or triple bonds) — not to add electrons that weren't in your original count. Adding electrons would violate the total valence electron count. Converting a lone pair to a bond moves electrons already in the structure to a position where they now count toward both atoms. This is exactly what happens in CO₂: each oxygen donates a lone pair to form a double bond, giving carbon its octet.

Question 3

When drawing the Lewis structure for the sulfate ion (SO₄²⁻), you must add 2 electrons to the total valence electron count to account for the 2− charge.

• T. True
• F. False

Answer: True

Ion charges directly modify the total valence electron count. Each unit of negative charge represents one extra electron that has been gained; each unit of positive charge represents one electron that has been lost. For SO₄²⁻: sulfur contributes 6, each oxygen contributes 6 (×4 = 24), plus 2 for the 2− charge, giving 6 + 24 + 2 = 32 total valence electrons. Forgetting to adjust for charge is one of the most common Lewis structure errors, especially for polyatomic ions.

Question 4

The central atom in a Lewis structure is generally the most electronegative atom, because electronegative atoms attract more electrons and therefore form the most bonds.

• T. True
• F. False

Answer: False

This is backwards. The central atom is typically the LEAST electronegative atom (excluding hydrogen, which is always terminal). Highly electronegative atoms prefer to hold their electrons as lone pairs rather than share them broadly — they are 'greedy' with electrons. The central atom must form multiple bonds to different surrounding atoms, which requires willingness to share electrons with many partners. Carbon, nitrogen, and sulfur serve as central atoms more readily than oxygen or fluorine in most molecules.

Question 5

Explain why, after the initial lone-pair distribution step in drawing a Lewis structure, you might need to convert lone pairs into bonding pairs, and what chemical problem this step solves.

Model answer: After distributing lone pairs to satisfy outer atoms' octets, the central atom may be left with fewer than 8 electrons. Since you cannot add electrons beyond the original valence electron total, the only way to give the central atom more electrons is to move existing lone pairs from adjacent atoms into the bond between them and the central atom. This creates a double (or triple) bond, which counts 4 electrons toward the central atom instead of 2, without increasing the total electron count. It solves the octet deficiency while respecting conservation of electrons.

This step reflects real chemistry: multiple bonds form precisely because they allow both bonding atoms to achieve stable electron configurations simultaneously. In CO₂, converting to double bonds gives carbon 8 electrons and keeps each oxygen at 8 — neither the single-bond nor the double-bond structure is forced arbitrarily. The procedure encodes the principle that electron distribution optimizes the stability of all atoms in the molecule.