CHEM1011 · Fundamentals Of Chemistry 1a
Lewis Structures and VSEPR
This chapter is the bonding-and-shape core of CHEM1011: you draw a valid Lewis structure for a small molecule or ion, then read its 3-D shape, bond angles and polarity straight off it using VSEPR. It carries real exam weight because the short-answer paper asks you to draw structures with all lone pairs, assign formal charges, and state geometry — and the periodic table is provided, so the marks reward method, not memorised facts.
What this chapter covers
- 011. The octet rule and the boron sub-octet exception (H needs 2)
- 022. Counting valence electrons from the group number (adjust for ionic charge)
- 033. The 5-step Lewis build: central atom, count, single bonds, lone pairs, multiple bonds
- 044. Bond order vs bond length vs bond strength (single < double < triple)
- 055. Resonance and the resonance hybrid (fractional bond orders)
- 066. Formal charge as the tie-breaker: FC = valence e⁻ − lone-pair e⁻ − ½ bonding e⁻
- 077. VSEPR geometries: linear 180°, trigonal planar 120°, tetrahedral 109.5°
- 088. Lone-pair distortion (NH₃ ~107°, H₂O ~104.5°) and molecular polarity
Build the Lewis structure and predict the shape of the ammonium ion NH₄⁺
- +1Choose the central atom: N has lower electronegativity and higher valence than H, and H can never be central (it forms one bond only). So N sits in the centre with four H atoms around it.
- +1Count valence electrons, adjusting for charge: N contributes 5 and each of the four H contributes 1, giving 9; the overall +1 charge removes one electron, so total valence e⁻ = 5 + 4 − 1 = 8 (four pairs).
- +1Place single bonds and check the octet: draw four N–H single bonds; these four bonding pairs use all 8 electrons, so there is no lone pair left and N is surrounded by 4 × 2 = 8 electrons — a full octet. Enclose the whole ion in brackets with the +1 outside.
- +1Apply VSEPR: the central N has 4 bonding domains and 0 lone pairs, so the four N–H bonds spread out as far apart as possible — a tetrahedral arrangement at the ideal angle (no lone pair to squeeze it).
Key terms
- Octet rule
- Atoms gain, lose or share electrons to reach a full valence shell of 8 electrons (the noble-gas configuration). H is the exception that needs only 2; the rule always holds for C, N, O and F, while B is electron-deficient and is stable with only 6 (e.g. in BF₃).
- Valence electrons
- The electrons in an atom's outer shell that take part in bonding. For a main-group atom the count equals the old group number: H = 1, B = 3, C = 4, N = 5, O = 6, F/Cl = 7, read straight off the provided periodic table.
- Bond order
- The number of shared electron pairs between two specific atoms: 1 for a single bond, 2 for a double, 3 for a triple. As bond order rises the bond becomes shorter and stronger (e.g. C–C 150 pm < C=C 133 pm < C≡C 120 pm).
- Resonance
- When a species cannot be drawn with one valid Lewis structure (e.g. NO₃⁻), it is represented as several contributing structures linked by a ↔ arrow. The real molecule is the resonance hybrid — a single weighted average with equal, intermediate (often fractional) bond orders, not a structure that flips between forms.
- Formal charge (FC)
- A bookkeeping charge on an atom in a Lewis structure: FC = (valence e⁻) − (lone-pair e⁻) − ½(bonding e⁻). The preferred structure has formal charges closest to zero, with any negative FC on the most electronegative atom; FCs sum to 0 for a molecule and to the charge for an ion.
- VSEPR
- Valence-Shell Electron-Pair Repulsion: electron domains around a central atom arrange to minimise repulsion. Each bond (single, double or triple) is ONE domain, plus each lone pair. 2 domains → linear (180°), 3 → trigonal planar (120°), 4 → tetrahedral (109.5°); lone pairs repel harder and shrink the angle below ideal.
Lewis Structures and VSEPR FAQ
What's the 5-step method for drawing any Lewis structure?
(1) Pick the central atom — the lowest-electronegativity / highest-valence atom (never H or F). (2) Count total valence electrons by summing group numbers, then subtract for a positive charge or add for a negative one. (3) Draw a single bond from the centre to each outer atom. (4) Add lone pairs to the outer atoms first, then any leftover to the centre. (5) If the central atom is short of an octet, convert adjacent lone pairs into double or triple bonds until it reaches 8 (stop at 6 if the centre is boron). Do the same five steps every time — the marks are in the order.
Why is CO₂ non-polar but H₂O polar when both have polar bonds?
A molecule is polar only if it has polar bonds AND a geometry that does not cancel them. CO₂ is linear, so its two equal C=O bond dipoles point in opposite directions and sum to zero — non-polar overall. H₂O is bent (two lone pairs on O), so its two O–H bond dipoles add rather than cancel, giving a net dipole — polar. Always check both bond polarity and shape.
How do lone pairs change the bond angle in VSEPR?
A lone pair repels more strongly than a bonding pair, so it pushes the bonding pairs closer together and the bond angle drops below the ideal. Starting from the tetrahedral 109.5°: CH₄ has 0 lone pairs and keeps 109.5°; NH₃ has 1 lone pair and is pyramidal at ~107°; H₂O has 2 lone pairs and is bent at ~104.5°.
Does a double or triple bond count as more than one VSEPR domain?
No. For shape, a double or triple bond counts as a SINGLE electron domain — it's the number of atoms/lone pairs bunched around the central atom that matters, not the number of shared pairs. That's why CO₂, with two C=O double bonds, has only 2 domains on carbon and is linear rather than bent.
What does this topic look like on the CHEM1011 exam?
Short-answer items ask you to draw a valid Lewis structure for a small molecule or polyatomic ion, show all lone pairs, assign formal charges, and state whether an atom obeys or breaks the octet, then often give the VSEPR shape, angle and polarity. Multiple-choice questions test counting valence electrons and ranking bond order against bond length or strength. The exam is closed-book but the periodic table is provided, so memorise the method and the octet target rather than specific numbers.
Exam move
Treat this chapter as a single chain: Lewis structure → VSEPR shape → polarity, and drill it in that order because the marks track each link. The load-bearing step is the electron count — if you get total valence electrons (and the charge sign) right, the correct number of lone pairs, the shape and the polarity all fall out of it. Practice by running the same fixed 5-step build on a rotation of small species (H₂O, CO₂, NH₃, NH₄⁺, BF₃, CH₄, NO₃⁻) until it is automatic, then for each one immediately count domains for the angle and check both bond polarity and symmetry for polarity. Memorise the method, the octet target, the boron sub-octet exception and the three core geometries (180° / 120° / 109.5°, dropping below for each lone pair) — the periodic table is provided, so never waste effort memorising group numbers or datasheet bond lengths.
