General Chemistry · Bonding and structure
Molecular shape, polarity and intermolecular forces
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What you'll learn
- Predict electron-domain geometry, molecular shape and approximate bond angles from a Lewis structure using VSEPR.
- Assign sp, sp² or sp³ hybridization to atoms and count σ and π bonds.
- Decide whether a molecule has a net dipole moment by combining bond dipoles with its shape.
- Identify the intermolecular forces in a substance and use them to rank boiling points.
Free lesson
The text of the worked lesson. In Arrow Lab each step comes with its structures and arrows drawn out.
Read the free lesson (6 parts)
Count electron domains
VSEPR starts from the Lewis structure. Count the electron domains around the central atom: each bonded atom is one domain whatever the bond order, and each lone pair is one domain. Two domains point to a line (180°), three to a triangle (120°), four to a tetrahedron (109.5°), five to a trigonal bipyramid (90° and 120°) and six to an octahedron (90°). Ammonia has three bonds and one lone pair, so its four domains are tetrahedral, but the shape named from the atoms alone is trigonal pyramidal.
Lone pairs change shape and squeeze angles
Lone pairs repel more strongly than bonding pairs, so they compress the angles between bonds: CH₄ 109.5°, NH₃ about 107°, H₂O about 104.5°. With five domains, lone pairs sit in the roomier equatorial positions: one lone pair gives a seesaw (SF₄), two give a T shape (ClF₃) and three give a linear molecule (XeF₂). With six domains, two lone pairs sit opposite each other and leave a square planar molecule (XeF₄).
Hybridization follows the domain count
Two domains need two hybrid orbitals (sp, 50% s character), three need sp² (33% s) and four need sp³ (25% s). Every single bond is a σ bond; a double bond is one σ plus one π bond and a triple bond is one σ plus two π bonds, made from unhybridized p orbitals. In acrylonitrile the CH₂ and CH carbons are sp², the nitrile carbon is sp, and there are six σ and three π bonds. An atom whose lone pair is delocalized, such as an amide nitrogen, is sp² and planar. Gaining a bond changes the picture: planar sp² BF₃ accepts a fluoride lone pair into its empty p orbital and becomes the tetrahedral sp³ ion BF₄⁻, and NH₃ turns its lone pair into a fourth N–H bond to give tetrahedral NH₄⁺.
Bond dipoles add as vectors
A bond is polar when its atoms differ in electronegativity; the dipole points toward the more electronegative atom. The molecular dipole is the vector sum of the bond dipoles and lone pairs. In linear CO₂ the two C=O dipoles cancel, as do the bond dipoles of symmetric BF₃, CCl₄, SF₆ and XeF₄, so these molecules are nonpolar. Bent H₂O, pyramidal NH₃, seesaw SF₄ and CH₂Cl₂ do not cancel and are polar.
Intermolecular forces set boiling points
Every molecule feels London dispersion forces, which grow with the number of electrons and the contact surface: F₂ and Cl₂ are gases, Br₂ a liquid and I₂ a solid, and unbranched pentane boils higher than its branched isomers. Polar molecules add dipole–dipole attraction. Hydrogen bonding needs an H on N, O or F and a lone pair on N, O or F of a neighbour; it is why ethanol (78 °C) boils far above dimethyl ether (−24 °C), why H₂O and HF boil far above the heavier hydrides of their groups, and why NH₃ (−33 °C) boils far above PH₃ (−88 °C). The number of hydrogen bonds per molecule matters as well: water, with two H atoms and two lone pairs, boils above HF, which has only one H. The same forces govern solubility: a compound mixes with water when it can hydrogen-bond with water and its hydrocarbon part is small, so ethanol is miscible with water but octan-1-ol is not.
What to carry forward
Draw the Lewis structure, count domains to get the geometry and hybridization, name the shape from the atoms, add the bond dipoles as vectors, then compare dispersion, dipole–dipole and hydrogen bonding to explain physical properties.
Practice problems (40)
Free to practice with an account.
Draw the mechanism 2
VSEPR shapes and angles
- Ammonia takes a proton from hydrogen chlorideDraw the proton transfer from hydrogen chloride to ammonia and relate it to the change from a trigonal pyramidal molecule to a tetrahedral ion.
Hybridization and bonding
- Boron trifluoride and fluoride form tetrafluoroborateDraw fluoride adding to boron trifluoride and relate it to boron changing from trigonal planar sp² to tetrahedral sp³.
Predict the product 5
Hybridization and bonding
- Geometry of allenePredict the relative orientation of the terminal CH₂ groups in a cumulated diene from the orbitals of the central carbon.
Polarity and dipole moments
- Polarity of expanded-octet moleculesPredict which expanded-octet molecule is polar from its VSEPR shape.
VSEPR shapes and angles
- Shape of sulfur tetrafluoridePredict the shape of an AX₄E molecule with an expanded octet.
- Shape and angle of waterPredict the shape and approximate bond angle of water from its domain count.
- Shape of xenon difluoridePredict the shape of an AX₂E₃ molecule.
Classify 8
VSEPR shapes and angles
- Shape of ammoniaName the molecular shape of a molecule with three bonding domains and one lone pair.
Hybridization and bonding
- Find the sp carbonIdentify which of several structures contains an sp-hybridized carbon.
- Hybridization of a nitrile carbonAssign the hybridization of a carbon bearing a triple bond from its domain count.
- Where is pyridine's lone pair?Assign the orbital holding the lone pair of an aromatic ring nitrogen that already contributes one π electron.
Intermolecular forces
- Forces between methane moleculesClassify the intermolecular forces present in a nonpolar hydrocarbon.
- Which pure liquid is hydrogen-bonded?Identify which compound can both donate and accept hydrogen bonds in its pure liquid.
Polarity and dipole moments
- Polar bonds, no dipoleRecognize a molecule whose identical bond dipoles cancel because of a linear shape.
- Pick the polar moleculeIdentify the polar molecule among symmetric molecules whose bond dipoles cancel.
Compare 7
Intermolecular forces
- Rank alkane boiling pointsRank alkane boiling points using chain length and branching as measures of dispersion forces.
- Rank group 16 hydride boiling pointsRank the boiling points of a hydride series where hydrogen bonding overrides the dispersion trend.
- Rank hydrogen halide boiling pointsRank the hydrogen halides by boiling point, weighing dispersion against hydrogen bonding.
- Highest-boiling C₃H₈O isomerCompare boiling points of isomers using hydrogen bonding and molecular shape.
VSEPR shapes and angles
- Rank bond anglesRank the bond angles of five small molecules using domain count and lone-pair repulsion.
Polarity and dipole moments
- Which dichloroethene is polar?Compare alkene stereoisomers and decide which has a nonzero dipole moment.
- Most polar H–X bondCompare bond polarity from electronegativity differences.
Calculate 6
Hybridization and bonding
- Count π bonds in acrylonitrileCount the π bonds in a molecule containing both a double and a triple bond.
- Count σ bonds in acrylonitrileCount σ bonds, including C–H bonds, knowing every multiple bond contains exactly one σ bond.
- s character of an sp² orbitalCalculate the percent s character of a hybrid orbital from the orbitals that are mixed.
Polarity and dipole moments
- Add two bond dipolesCalculate a molecular dipole as the vector sum of two equal bond dipoles at a known angle.
VSEPR shapes and angles
- Electron domains in triiodideCount the electron domains on the central atom of a polyatomic anion, including its charge.
- Lone pairs on xenon in XeF₄Count the lone pairs on the central atom of a noble-gas fluoride from its total valence electrons.
Find the error 1
- Find the error about ammoniaDiagnose the error in equating the electron-domain angle with the bond angle when a lone pair is present.
Interpret the evidence 2
Polarity and dipole moments
- Read hydrogen halide dipolesInterpret a dipole-moment series and draw only the conclusion the data support.
Intermolecular forces
- Boiling points at nearly equal massInterpret boiling-point data for compounds of similar molar mass to rank the strength of intermolecular forces.
Work backward 1
- Which molecule is square planar?Work back from an observed square planar shape to a formula with six domains and two lone pairs.
Plan a synthesis 1
- Choose a water-miscible solventChoose a solvent that mixes with water by matching hydrogen bonding and hydrocarbon size.
Explain why 7
Intermolecular forces
- Why acetic acid boils higher than propanolExplain the higher boiling point of a carboxylic acid than an alcohol of the same molar mass.
- Why iodine is a solidExplain the change from gas to solid down the halogen group using dispersion forces.
- Water versus hydrogen fluorideExplain why water boils higher than HF although fluorine is more electronegative than oxygen.
Hybridization and bonding
- Hybridization of an amide nitrogenExplain why a nitrogen with three bonds and a lone pair is sp² when the lone pair is delocalized.
VSEPR shapes and angles
- Why ClF₃ is T-shapedExplain the T shape of ClF₃ from the placement of lone pairs in a trigonal bipyramid.
Polarity and dipole moments
- Why NF₃ is barely polarExplain how bond dipoles and the lone-pair contribution combine differently in two molecules of the same shape.
- Is ozone polar?Explain how a molecule made of a single element can have a dipole moment.
Stuck on molecular shape, polarity and intermolecular forces? Scan the problem.
Snap a photo of the reaction from your homework or textbook and draw its mechanism with every arrow checked. 1 free scan now, no sign-up; 1 more with a free account.
Then Scan Standard, /month