Difficulty: Intermediate | Prerequisites: Electron configuration, valence electrons, electronegativity basics
Lewis structures are the starting point for predicting molecular shape, polarity, and reactivity. Once you draw the structure, VSEPR theory tells you the geometry, and geometry tells you the bond angles. This sequence (Lewis structure → electron geometry → molecular geometry → bond angle) appears on virtually every AP Chemistry exam. The S₂Cl₂ molecule in this problem is a good test case because it has lone pairs that influence geometry without being obvious at first glance.
Draw the Lewis structure by distributing valence electrons as bonds and lone pairs. Count electron groups around each atom to determine geometry using VSEPR. For S₂Cl₂, each sulphur atom has a bent/tetrahedral electron arrangement, giving Cl-S-S bond angles of approximately 109° (the answer key accepts 107° as well).
Lewis electron-dot structure
A diagram showing all valence electrons in a molecule as dots (lone pairs) or lines (bonding pairs). Think of it as a map of where the electrons live.
Valence electrons
The electrons in the outermost shell of an atom, which participate in bonding. Sulphur has 6, chlorine has 7, and you add them all up to get the total for the molecule.
Lone pair (non-bonding pair)
A pair of valence electrons not involved in bonding, sitting on a single atom. In simple terms, electrons that belong to one atom and are not shared.
VSEPR theory (Valence Shell Electron Pair Repulsion)
The model that predicts molecular geometry by assuming electron groups around a central atom repel each other and arrange themselves to be as far apart as possible. Think of it as balloons tied together: they push away from each other into a predictable shape.
Tetrahedral electron geometry
The arrangement when four electron groups surround a central atom, producing bond angles of approximately 109.5°. If some of those groups are lone pairs rather than bonds, the molecular shape is different (bent, trigonal pyramidal), but the underlying electron geometry is still tetrahedral.
Bond angle
The angle formed between two bonds that share a common atom. In simple terms, how wide the "V" is between two neighbouring atoms.
Total valence electrons: 2 sulphur atoms × 6 = 12, plus 2 chlorine atoms × 7 = 14. Total = 26 electrons.
The skeleton is Cl-S-S-Cl (a chain, not a ring). Each bond uses 2 electrons, so 4 bonds = 8 electrons used.
Remaining electrons: 26 − 8 = 18 electrons = 9 lone pairs.
Distribute lone pairs to satisfy the octet rule:
Each Cl gets 3 lone pairs (6 electrons each, completing its octet).
Each S gets 2 lone pairs (4 electrons each, completing its octet with the 2 bonds it already has).
Check: 4 bonds (8 e⁻) + 3 lone pairs on each Cl (12 e⁻) + 2 lone pairs on each S (8 e⁻) = 8 + 12 + 8 - wait, let me recount.
3 lone pairs × 2 Cl atoms = 6 lone pairs = 12 electrons
2 lone pairs × 2 S atoms = 4 lone pairs = 8 electrons
4 bonding pairs = 8 electrons
Total: 12 + 8 + 8 = 28. That is too many.
Correct count: each Cl has 3 lone pairs (6 e⁻), each S has 2 lone pairs (4 e⁻).
Bonds: 3 (Cl-S, S-S, S-Cl) = 6 electrons
Cl lone pairs: 3 × 2 = 6 lone pairs = 12 electrons
S lone pairs: 2 × 2 = 4 lone pairs = 8 electrons
Total: 6 + 12 + 8 = 26 ✓
The structure looks like: :Cl-S-S-Cl: with lone pairs on every atom.
Focus on one sulphur atom: it has 4 electron groups (1 bond to Cl, 1 bond to the other S, and 2 lone pairs).
4 electron groups = tetrahedral electron geometry.
The molecular geometry around each S is bent (2 bonds + 2 lone pairs, like water).
Ideal tetrahedral angle is 109.5°. Lone pairs compress the bond angle slightly.
The Cl-S-S bond angle is approximately 109° (the AP answer key gives 107°; both are accepted).
Lone pairs occupy more space than bonding pairs because they are held closer to the nucleus and spread out more.
This extra repulsion pushes bonding pairs closer together, reducing the bond angle below the ideal 109.5°.
For S₂Cl₂, the compression is modest, bringing the angle to roughly 107-109°.
Total valence electrons = sum of valence electrons for all atoms (adjust for charge if an ion)
Electron groups around an atom = number of bonds + number of lone pairs on that atom
For S₂Cl₂:
:Cl - S - S - Cl:
Each atom carries lone pairs (3 on each Cl, 2 on each S), giving 26 total electrons.
Lewis structures and VSEPR predictions are how chemists figure out whether a molecule is polar (which affects solubility, boiling point, and biological activity) without needing to run an experiment. Pharmaceutical chemists use molecular geometry to predict how a drug molecule will fit into a protein's active site.
Students often draw S₂Cl₂ with a double bond between the sulphur atoms. There is no need for a double bond here; single bonds satisfy all octets with the available electrons.
A frequent mistake is forgetting to count lone pairs as electron groups when determining geometry. Lone pairs are invisible in the molecular shape but absolutely count for VSEPR.
Some students confuse electron geometry (tetrahedral) with molecular geometry (bent). The exam may ask for either, so read the question carefully.
Answering "109.5°" is usually accepted, but writing "120°" (trigonal planar) is a common error that results from miscounting electron groups.
⚠️ The AP exam often asks you to draw a complete Lewis structure with all lone pairs shown. Missing lone pairs lose marks.
⚠️ Bond angle questions require you to identify the number of electron groups first. State the electron geometry, then give the angle.
⚠️ If the question says "include both angles if they are not equal," it is hinting that symmetry matters. For S₂Cl₂, both Cl-S-S angles are equivalent by symmetry.
Fill in the blank: Sulphur has ________ valence electrons.
True or false: Lone pairs do not count as electron groups in VSEPR theory.
Fill in the blank: Four electron groups around a central atom give a ________ electron geometry.
True or false: Lone pairs compress bond angles below the ideal value.
Fill in the blank: The molecular shape around each S in S₂Cl₂ is ________.
Answers: 1. Six. 2. False (they absolutely count). 3. Tetrahedral. 4. True. 5. Bent.
Q: How many total valence electrons are in S₂Cl₂?
A: 2(6) + 2(7) = 26 valence electrons.
Q: What is the electron geometry around each sulphur atom in S₂Cl₂?
A: Tetrahedral (4 electron groups: 2 bonds and 2 lone pairs).
Q: Why is the Cl-S-S bond angle slightly less than 109.5°?
A: Lone pairs on the sulphur atom repel the bonding pairs more strongly than bonding pairs repel each other, compressing the bond angle to approximately 107-109°.
Q: Draw the Lewis structure of water (H₂O) and predict its bond angle. How does this compare to S₂Cl₂?
A: Water has 2 bonding pairs and 2 lone pairs around oxygen, giving a bent shape with a bond angle of about 104.5°. S₂Cl₂ has the same electron geometry (tetrahedral) around each S, but the bond angle is slightly larger (~107-109°) because the atoms bonded to S are larger.
Lewis structures feed directly into discussions of formal charge (used to evaluate which resonance structure is most stable), hybridisation (sp³ for tetrahedral electron geometry), and polarity. The geometry you determine here also connects to intermolecular forces: a molecule's shape determines whether it has a net dipole moment, which in turn affects its boiling point and solubility.
Lewis structure, electron-dot diagram, VSEPR, valence shell electron pair repulsion, tetrahedral, bent geometry, bond angle, lone pair, bonding pair, S₂Cl₂, molecular geometry, electron geometry, octet rule, AP Chemistry free response, molecular shape prediction