Unit 2 · Topic 2.7 Beta

VSEPR and Hybridization

VSEPR predicts shape from the Lewis diagram: the electron domains on the central atom (each bond or lone pair counts once) spread as far apart as possible, giving linear, trigonal planar, tetrahedral, trigonal bipyramidal or octahedral arrangements.

Practice 1: Models and RepresentationsPractice 4: Model AnalysisPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Carbon dioxide and water both have a central atom between two others, yet one is a straight line and the other is bent like a boomerang. That single difference is why one of them can be a polar molecule. The cause is invisible in the formula and plain in the Lewis diagram: lone pairs.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. How many lone pairs does the O atom in water have?

  1. 2
  2. 1
  3. 0
  4. 3
Show the answer

H₂O has 8 valence electrons: two O–H bonds use 4 and the other 4 are two lone pairs on O.

  • Correct: 2:
  • 1:
  • 0:
  • 3:

2. In a polar bond, the dipole arrow points

  1. toward the more electronegative atom
  2. toward the less electronegative atom
  3. toward the larger atom
  4. in no particular direction
Show the answer

The shared electrons spend more time near the more electronegative atom, which becomes δ−.

  • Correct: toward the more electronegative atom:
  • toward the less electronegative atom:
  • toward the larger atom:
  • in no particular direction:

3. Which atoms can have more than eight electrons around them in a Lewis diagram?

  1. Atoms in period 3 and below, such as S and Xe
  2. Any atom bonded to fluorine
  3. Period 2 atoms such as C and N
  4. Only metal atoms
Show the answer

Expanded octets occur from period 3 on; period 2 atoms hold at most eight.

  • Correct: Atoms in period 3 and below, such as S and Xe:
  • Any atom bonded to fluorine:
  • Period 2 atoms such as C and N:
  • Only metal atoms:

Part 4 · See it

See it first

Six ball-and-stick shapes around a central atom A, with lone pairs drawn as lobes. Linear, 180 degrees. Trigonal planar, 120 degrees. Bent with three domains, slightly less than 120 degrees. Tetrahedral, 109.5 degrees. Trigonal pyramidal, about 107 degrees as in NH3. Bent with four domains, about 104.5 degrees as in H2O. A table below matches 2 domains to sp, 3 to sp2 and 4 to sp3.
Two, three or four electron domains around a central atom (A) spread as far apart as possible. Lone pairs (lobes) are part of the arrangement but not of the shape's name, and they squeeze the bond angles. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Bonding pairs and lone pairs on the central atom are regions of negative chargethey repel and spread as far apart as possible (VSEPR)
  2. Each bond (single, double or triple) and each lone pair is one electron domainthe number of domains sets the arrangement: 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral
  3. Lone pairs occupy positions but are not atomsthe molecular geometry differs from the arrangement (NH₃ pyramidal, H₂O bent)
  4. A lone pair is held by one nucleus and spreads widerit pushes bonding pairs together, so bond angles shrink (109.5° to 107° to 104.5°)
  5. Bond dipoles add like arrows in the 3-D shapesymmetrical shapes cancel them (nonpolar); lopsided shapes leave a net dipole (polar)

Part 6 · Key ideas

Key ideas

  • VSEPR: electron domains (each bond or lone pair counts once) spread as far apart as possible.
  • Domains set the arrangement; molecular geometry names the atom positions only.
  • Lone pairs repel more than bonding pairs and shrink bond angles: CH₄ 109.5°, NH₃ about 107°, H₂O about 104.5°.
  • Molecular polarity: polar bonds plus an unsymmetrical shape give a net dipole.
  • Hybridization matches domains: 2 sp, 3 sp², 4 sp³. Single bond 1 σ; double 1 σ + 1 π; triple 1 σ + 2 π.

Part 7 · Misconception

A common mistake

The wrong idea: A molecule with polar bonds is always a polar molecule.

What actually happens: Polarity needs polar bonds and a shape in which their dipoles do not cancel. CO₂ and BF₃ have polar bonds, but their symmetrical shapes make the bond dipoles cancel, so the molecules are nonpolar.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Model

Lewis diagrams of four fluorides

Lewis diagrams of four molecules, each with a central atom bonded to fluorine atoms.

BFFFMolecule 1BF3NFFFMolecule 2NF3ClFFFMolecule 3ClF3XeFFMolecule 4XeF2

1. What is the molecular geometry of Molecule 2, NF₃?

  1. Trigonal planar
  2. Trigonal pyramidal
  3. Trigonal bipyramidal
  4. Tetrahedral
Show the answer

N has four electron domains: three bonds and one lone pair. They point to the corners of a tetrahedron; with one corner taken by the lone pair, the atoms form a pyramid with N at the top.

  • Trigonal planar: Trigonal planar needs three domains and no lone pair, like BF₃. NF₃'s lone pair is a fourth domain.
  • Correct: Trigonal pyramidal: Right: four domains, one of them a lone pair.
  • Trigonal bipyramidal: Trigonal bipyramidal needs five domains with no lone pairs, as in PCl₅; N has only four domains.
  • Tetrahedral: Tetrahedral is the electron-domain arrangement; the molecular geometry names only the atoms' positions.

2. BF₃ and NF₃ each have three F atoms bonded to the central atom, but different shapes. Which explanation is best?

  1. The N–F bonds are double bonds, while the B–F bonds are single bonds
  2. N is more electronegative than B, so it pulls the F atoms closer and bends the molecule
  3. B is smaller than N, so the F atoms around B have more room and spread into a plane
  4. N's lone pair is a fourth domain, which bends the bonds into a pyramid
Show the answer

Shape comes from the number of electron domains. B has three (trigonal planar, 120°). N has four, one a lone pair, so the domains are arranged tetrahedrally and the atoms form a trigonal pyramid.

  • The N–F bonds are double bonds, while the B–F bonds are single bonds: All the bonds in both diagrams are single bonds.
  • N is more electronegative than B, so it pulls the F atoms closer and bends the molecule: Electronegativity affects polarity, not the number of domains that sets the shape.
  • B is smaller than N, so the F atoms around B have more room and spread into a plane: B is in fact larger than N; size is not what decides the shape here.
  • Correct: N's lone pair is a fourth domain, which bends the bonds into a pyramid: Right: the lone pair is the difference.

3. What is the molecular geometry of Molecule 3, ClF₃?

  1. T-shaped
  2. Trigonal planar
  3. Trigonal pyramidal
  4. Seesaw
Show the answer

Cl has five domains (three bonds, two lone pairs) in a trigonal bipyramidal arrangement. The two lone pairs take equatorial positions, where they have more room, leaving the three F atoms in a T shape.

  • Correct: T-shaped: Right: five domains, two lone pairs.
  • Trigonal planar: Trigonal planar has three domains and no lone pairs; Cl has five domains.
  • Trigonal pyramidal: Trigonal pyramidal has four domains with one lone pair; Cl has five domains.
  • Seesaw: A seesaw has five domains with only one lone pair, as in SF₄.

4. Which of the four molecules are polar? Select all that apply.

  1. Molecule 2, NF₃
  2. Molecule 3, ClF₃
  3. Molecule 1, BF₃
  4. Molecule 4, XeF₂
Show the answer

Every bond is polar (F is more electronegative than each central atom). In NF₃ (pyramidal) and ClF₃ (T-shaped) the bond dipoles do not cancel. BF₃ (trigonal planar) and XeF₂ (linear, lone pairs around the middle) are symmetrical, so their bond dipoles cancel.

  • Correct: Molecule 2, NF₃: Right: the pyramid's dipoles add up to a net dipole.
  • Correct: Molecule 3, ClF₃: Right: the T shape is not symmetrical, so the dipoles do not cancel.
  • Molecule 1, BF₃: The three B–F dipoles point 120° apart in a plane and cancel.
  • Molecule 4, XeF₂: The two Xe–F dipoles point in opposite directions and cancel; the three lone pairs sit evenly around Xe.

5. Carbon dioxide has two polar C=O bonds, yet it is a nonpolar molecule. Which explanation is best?

  1. The lone pairs on C point away from the O atoms and cancel the bond dipoles
  2. C and O have the same electronegativity, so the bonds are not actually polar
  3. Double bonds are nonpolar, since the four shared electrons are spread evenly
  4. It is linear, so the two equal bond dipoles point opposite ways and cancel
Show the answer

Polarity depends on bond polarity and shape. Each C=O bond is polar (O is more electronegative), but in a linear molecule the two dipoles point in opposite directions with equal size, so the net dipole is zero.

  • The lone pairs on C point away from the O atoms and cancel the bond dipoles: C in CO₂ has no lone pairs: its four bonding pairs are in the two double bonds.
  • C and O have the same electronegativity, so the bonds are not actually polar: O (3.4) is more electronegative than C (2.6), so each bond is polar.
  • Double bonds are nonpolar, since the four shared electrons are spread evenly: A double bond between different atoms is polar; the electrons are pulled toward O.
  • Correct: It is linear, so the two equal bond dipoles point opposite ways and cancel: Right: the shape makes the dipoles cancel.

6. Which molecule has a bent molecular geometry?

  1. SO₂
  2. CO₂
  3. BeCl₂
  4. HCN
Show the answer

SO₂ has 18 valence electrons; S has two bonding domains and one lone pair, three domains in all. With one domain a lone pair, the O–S–O shape is bent (about 119°).

  • Correct: SO₂: Right: three domains, one a lone pair.
  • CO₂: CO₂'s C has two domains (two double bonds) and no lone pairs: linear.
  • BeCl₂: Be in BeCl₂ has two bonds and no lone pairs: linear.
  • HCN: C in HCN has two domains, a single and a triple bond: linear.

7. What does VSEPR theory say determines the shape of a molecule?

  1. Bonds point toward the most electronegative neighboring atoms
  2. Atoms arrange themselves so that the heaviest atoms are farthest apart
  3. Electron domains on the central atom repel and spread as far apart as they can
  4. Each bond angle is 90°, because orbitals are at right angles
Show the answer

Valence shell electron pair repulsion: bonding pairs and lone pairs are regions of negative charge that repel, so they take positions as far apart as possible.

  • Bonds point toward the most electronegative neighboring atoms: Electronegativity affects bond polarity, not the direction the bonds point.
  • Atoms arrange themselves so that the heaviest atoms are farthest apart: Mass plays no part in VSEPR; the repulsion of electron domains does.
  • Correct: Electron domains on the central atom repel and spread as far apart as they can: Right: domains spread apart to reduce repulsion.
  • Each bond angle is 90°, because orbitals are at right angles: Real angles are 180°, 120°, 109.5°, 90° and in-between values, depending on the number of domains.

Part 9 · Summary

Summary

VSEPR predicts shape from the Lewis diagram: the electron domains on the central atom (each bond or lone pair counts once) spread as far apart as possible, giving linear, trigonal planar, tetrahedral, trigonal bipyramidal or octahedral arrangements. The molecular geometry names only the atoms, so lone pairs change it, and because lone pairs repel more strongly they shrink bond angles. A molecule is polar when its bond dipoles do not cancel in its shape. Two, three and four domains correspond to sp, sp² and sp³ hybridization; a single bond is one σ bond, and each extra bond in a double or triple bond is a π bond.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections