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5B: Nature of Molecules and Intermolecular Interactions

Lewis structures, VSEPR shapes, hybrid orbitals, the whole of stereochemistry, and the four forces that decide how molecules stick together.

22 topics · 89 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

5B. Lewis Electron Dot Structures

  1. How it worksWalk me through drawing the Lewis structure of SO2 starting from the valence electron count.
  2. Worked exampleDraw the Lewis structure of the nitrate ion NO3- and account for all 24 valence electrons.
  3. Common mistakeWhy doesn't every atom in a finished Lewis structure end up with exactly eight electrons around it?
  4. Going furtherWhat changes about the drawing rules for an element like phosphorus that can expand its octet?
Your turneasy1 of 3
A polyatomic anion is built from 2 carbon atoms and 3 oxygen atoms and carries a charge of 2-. How many valence electrons must its Lewis structure account for?

5B. Formal Charge

  1. How it worksHow does formal charge bookkeeping split every shared pair evenly between the two bonded atoms?
  2. Worked exampleFind the formal charge on every atom in the two competing Lewis structures of the cyanate ion OCN-.
  3. Worked exampleFind which structure of CO2 wins on formal charge, the one with two double bonds or the triple-bonded one.
  4. Common mistakeWhy isn't formal charge the same as oxidation state, when both hang a number on an atom?
Your turneasy1 of 3
An atom with 6 valence electrons appears in a Lewis structure making 3 bonds, with the rest of its octet filled by lone pairs. What is its formal charge?

5B. Resonance and Delocalized Electrons

  1. How it worksHow do curved arrows shift a π bond from one position to another to make a resonance structure?
  2. Common mistakeWhy isn't a resonance hybrid a molecule flickering back and forth between two real structures?
  3. Worked exampleDraw every resonance structure of the carbonate ion and rank them by formal charge.
  4. Going furtherWhat happens to a molecule's stability as the number of reasonable resonance structures climbs?
  5. Ties togetherHow does delocalization in a carboxylate ion explain why carboxylic acids beat alcohols on acidity?
Your turneasy1 of 3
A central atom is bonded to 3 equivalent oxygen atoms. Each resonance structure shows one of those bonds as a double bond and the rest as single bonds. What is the average bond order?

5B. Electronegativity and Partial Ionic Character

  1. How it worksHow does the electronegativity gap across a bond decide where the shared electrons actually sit?
  2. Worked exampleCompare the bonds in HF, HCl and Cl2 and rank them by how much ionic character each one carries.
  3. Common mistakeWhy isn't there a clean line where covalent bonding stops and ionic bonding takes over?
Your turnhard1 of 3
A diatomic molecule has a bond length of 173 pm and a measured dipole moment of 1.41 D. A full unit of charge separated by 100 pm would give 4.803 D. What is the bond's percent ionic character?

5B. Dipole Moment and Molecular Polarity

  1. How it worksHow do individual bond dipoles add together like vectors into one net molecular dipole?
  2. Worked exampleFind the net dipole of NH3 and of BF3 and show how the geometry alone flips the answer.
  3. Going furtherWhat changes about the net dipole when one hydrogen on methane is swapped for a chlorine?
  4. Common mistakeWhy doesn't a molecule packed with polar bonds have to be a polar molecule?
Your turnmedium1 of 3
A molecule is shaped so that two bond dipoles, one of 2 D and one of 1.15 D, meet at right angles. What is the magnitude of the net dipole moment?

5B. Sigma and π Bonds

  1. How it worksHow does head-on orbital overlap build a sigma bond while side-by-side overlap builds a π bond?
  2. Worked exampleFind the number of sigma bonds and π bonds in acetonitrile, CH3CN.
  3. Common mistakeWhy isn't a double bond simply twice as strong as a single bond between the same two atoms?
  4. Going furtherWhat happens to the π system when two double bonds sit right next to each other in butadiene?
Your turnmedium1 of 3
An acyclic molecule contains 10 atoms in total, and 2 of its bonds are double bonds while the rest are single. How many sigma bonds does it contain?

5B. Hybrid Orbitals and Their Geometries

  1. How it worksHow do one s orbital and three p orbitals blend into four identical sp3 orbitals?
  2. How it worksHow does leaving one p orbital unmixed give sp2 carbon its flat trigonal shape and its π bond?
  3. Worked exampleFind the hybridization at every carbon, nitrogen and oxygen in the amino acid glycine.
  4. Common mistakeWhy doesn't hybridization mean an atom physically promoted an electron before it bonded?
Your turneasy1 of 3
An atom is surrounded by 3 electron domains. Its hybrid orbitals are built from one s orbital and how many p orbitals?

5B. VSEPR and the Prediction of Molecular Shape

  1. How it worksHow does counting electron domains and then subtracting lone pairs hand you the molecular shape?
  2. How it worksHow does a lone pair squeeze bond angles below the ideal value the geometry would predict?
  3. Worked exampleFind the electron geometry and the molecular shape of SF4 and show where the seesaw comes from.
  4. Worked exampleFind the shape and bond angle of CH4, NH3 and H2O and show why all three start from one count.
  5. Common mistakeWhy isn't the molecular shape the same as the electron domain geometry once lone pairs appear?
Your turneasy1 of 3
A central atom is bonded to 3 other atoms and carries 1 lone pairs of its own. How many electron domains decide its electron-domain geometry?

5B. Bond Order, Bond Length and Bond Dissociation Energy

  1. How it worksHow does bond order fix bond length and bond strength at once, and in opposite directions?
  2. Worked exampleCompare the carbon-oxygen bonds in CO, CO2 and the carbonate ion and rank them by length.
  3. Going furtherWhat happens to bond order when resonance smears one double bond over three equivalent positions?
  4. Common mistakeWhy isn't the strongest bond in a molecule always the last one to break during a reaction?
Your turnmedium1 of 3
In the resonance hybrid of an oxoanion, 6 bonding pairs are shared equally over 3 equivalent central-atom-to-oxygen positions. What is the bond order of each of those bonds?

5B. Multiple Bonding and Molecular Rigidity

  1. How it worksHow does breaking the π bond become the price of rotating around a carbon-carbon double bond?
  2. Worked exampleFind the energy cost of rotating about the double bond in 2-butene and compare it to a single bond.
  3. Ties togetherHow does the rigidity of the peptide bond arise from exactly the same π overlap that stiffens an alkene?
Your turneasy1 of 3
Rotation about a C-C single bond has a barrier of about 15 kJ/mol, while rotation about the C=C bond of 2-butene costs about 263 kJ/mol. How many times larger is the alkene barrier?

5B. Structural Isomers

  1. How it worksHow do you generate every structural isomer of a formula without accidentally drawing one twice?
  2. Worked exampleDraw all five structural isomers of C6H14 and give each one its systematic name.
  3. Common mistakeWhy isn't a molecule drawn backwards on the page a different isomer from the same one drawn forwards?
  4. Going furtherWhat changes about the isomer count for C4H8 once you allow rings and not only open chains?
Your turneasy1 of 3
A straight-chain alkane with 7 carbon atoms and every branched isomer of it share one molecular formula. How many hydrogen atoms does that formula contain?

5B. Chirality and Stereocenters

  1. How it worksHow does a carbon carrying four different groups create a mirror image that cannot be superimposed?
  2. Worked exampleFind every stereocenter in glucose and count how many stereoisomers that number allows.
  3. Common mistakeWhy isn't a molecule with a stereocenter in it automatically chiral?
  4. Going furtherWhat happens to chirality in a molecule that has no stereocenter but still has no internal mirror plane?
Your turneasy1 of 3
A molecule has 5 stereocentres and no internal symmetry. What is the maximum number of stereoisomers it can have?

5B. R and S Configuration

  1. How it worksHow does ranking four groups by atomic number and sighting down the lowest one produce R or S?
  2. Worked exampleFind whether the alpha carbon of L-alanine is R or S by assigning all four priorities.
  3. Worked exampleFind the configuration at both stereocenters of 2-bromo-3-chlorobutane.
  4. Common mistakeWhy isn't the D and L labelling used for sugars the same system as R and S?
Your turneasy1 of 3
A stereocentre carries -H, -CH3, -OH and a fourth group whose attached atom has atomic number 16. What is the priority order, and how is the centre then read?

5B. Optical Activity and Specific Rotation

  1. How it worksHow does a chiral molecule in solution rotate the plane of polarized light passing through it?
  2. Common mistakeWhy doesn't an R configuration tell you which direction the molecule will rotate light?
  3. Worked exampleFind the enantiomeric excess of a sample reading +6.6 degrees when the pure enantiomer reads +13.2.
  4. Going furtherWhat happens to the observed rotation when two enantiomers are mixed in exactly equal amounts?
Your turneasy1 of 3
A sample rotates plane-polarised light by 9 degrees under conditions where the pure enantiomer reads 22 degrees. What is the enantiomeric excess, in percent?

5B. Diastereomers and Meso Compounds

  1. How it worksHow does flipping only one of two stereocenters turn an enantiomer into a diastereomer?
  2. Common mistakeWhy isn't a molecule carrying two stereocenters always chiral?
  3. Worked exampleDraw the four stereoisomers of tartaric acid and show which one is meso.
  4. Going furtherWhat happens to the physical properties of two diastereomers that lets you separate them by distillation?
  5. Ties togetherHow does the internal mirror plane of a meso compound explain its zero optical rotation?
Your turnmedium1 of 3
A molecule has 4 stereocentres and no internal symmetry. Taking one particular stereoisomer, how many of the others are diastereomers of it?

5B. Cis-Trans and E-Z Isomerism

  1. How it worksHow does restricted rotation around a double bond lock substituents onto one side or the other?
  2. Worked exampleFind whether 2-bromo-2-butene is E or Z by applying priority rules at each end of the double bond.
  3. Common mistakeWhy isn't cis always another word for Z?
  4. Going furtherWhat changes about cis-trans naming when the two groups sit on a ring instead of a double bond?
Your turneasy1 of 3
A straight-chain polyene has 4 carbon-carbon double bonds, and each one can be either cis or trans. How many geometric isomers does the chain allow?

5B. Newman Projections and Conformational Isomers

  1. How it worksHow does a Newman projection let you sight straight down a bond and watch the back carbon turn?
  2. How it worksHow does butane's energy rise and fall through anti, gauche, eclipsed and fully eclipsed positions?
  3. Worked exampleFind the highest and lowest energy conformations of butane as its central bond turns a full 360 degrees.
  4. Common mistakeWhy aren't two conformations of butane different compounds you could bottle separately?
Your turneasy1 of 3
Each eclipsed hydrogen-hydrogen interaction in ethane costs about 4.2 kJ/mol. What is the total torsional barrier between the staggered and eclipsed conformations?

5B. Chair Conformations of Cyclohexane

  1. How it worksHow does a chair flip swap every axial position into an equatorial one and back again?
  2. Worked exampleFind which chair of methylcyclohexane is favored and estimate how lopsided that equilibrium gets.
  3. Worked exampleDraw the more stable chair of cis-1,4-dimethylcyclohexane and explain the axial penalty.
  4. Going furtherWhat happens to the ring when a bulky tert-butyl group pins one chair permanently in place?
Your turnmedium1 of 3
An axial methyl group on a cyclohexane ring suffers two gauche-butane interactions with the ring, each costing about 4 kJ/mol. What is the energy penalty for the axial chair relative to the equatorial one?

5B. Hydrogen Bonding

  1. How it worksHow does a hydrogen bolted to N, O or F get bare enough to grab a neighbor's lone pair?
  2. Worked exampleCompare the boiling points of ethanol, dimethyl ether and propane, which weigh almost the same.
  3. Common mistakeWhy isn't a hydrogen bond just an unusually strong dipole-dipole attraction given a fancy name?
  4. Going furtherWhat changes about hydrogen bonding when the donor hydrogen hangs off carbon instead of oxygen?
Your turneasy1 of 3
Three compounds of nearly equal molar mass boil at -42, -22 and 80 degrees C. They are propane, dimethyl ether and ethanol. Which one boils at 80 degrees C, and why?

5B. Dipole-Dipole and Ion-Dipole Interactions

  1. How it worksHow does a dissolved ion pull the matching end of every nearby water molecule into a shell?
  2. Worked exampleCompare the energy of an ion-dipole interaction, a dipole-dipole one and a real covalent bond.
  3. Going furtherWhat changes about an ion-dipole interaction when the solvent is hexane instead of water?
Your turneasy1 of 3
An ion-dipole interaction in water is worth about 68 kJ/mol and a dipole-dipole interaction between two small polar molecules about 5 kJ/mol. How many times stronger is the ion-dipole interaction?

5B. London Dispersion Forces

  1. How it worksHow does a momentary flicker of electron density on one atom induce a matching dipole next door?
  2. Worked exampleCompare the boiling points of pentane and neopentane, two molecules with identical formulas.
  3. Common mistakeWhy isn't dispersion always the weakest force, when iodine is a solid and water is only a liquid?
  4. Going furtherWhat happens to dispersion strength as a molecule grows longer, flatter and more polarizable?
Your turneasy1 of 3
Pentane boils at 37 degrees C and neopentane, its branched isomer, boils at 11 degrees C. Why the gap?

5B. How Intermolecular Forces Set Boiling Point, Solubility and Viscosity

  1. How it worksHow does the strength of intermolecular attraction convert directly into a boiling temperature?
  2. Worked exampleCompare butane, acetone, 1-propanol and propanoic acid by boiling point and justify every step.
  3. Worked exampleFind why glycerol pours like syrup while ethanol pours like water, using their hydrogen bond counts.
  4. Common mistakeWhy isn't like-dissolves-like a rule about molecular weight rather than about the forces at work?
  5. Ties togetherHow do the forces that set a boiling point also decide how far a spot climbs on a chromatography plate?
Your turnmedium1 of 3
Butane, acetone, 1-propanol and propanoic acid boil at 1, 57, 99 and 140 degrees C in some order. Which compound boils at 140 degrees C, and why?

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