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Química Geral · Exam review

Revisão de ligação e estrutura

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O que você vai aprender

  • Draw Lewis structures of molecules and ions, assign formal charges and choose the best structure or skeleton.
  • Use resonance to find average bond orders and count resonance contributors correctly.
  • Predict electron-domain geometry, molecular shape, bond angles, hybridization and polarity with VSEPR.
  • Fill valence molecular orbital diagrams of diatomic species to find bond order, bond strength trends and magnetism.
  • Rank lattice energies from ion charge and size, and calculate lattice energies with a Born–Haber cycle.

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Lewis structures and formal charge

Count valence electrons from group numbers, add one per negative charge and subtract one per positive charge. Put the least electronegative atom in the centre (never H or F), join atoms with single bonds, complete the outer octets, then put leftover electrons on the central atom and turn lone pairs into multiple bonds until second-row atoms have octets. Formal charge = valence electrons − nonbonding electrons − number of bonds. Choose the structure with the fewest and smallest formal charges, with any negative charge on the most electronegative atom and no like charges on neighbouring atoms. Worked example: N₂O has 2 × 5 + 6 = 16 electrons on the skeleton N–N–O (drawn). N≡N⁺–O⁻ and ⁻N=N⁺=O each carry only +1 and −1 and are the two important contributors; ²⁻N–N⁺≡O⁺ carries three charges, one of them +1 on oxygen, and is negligible. Third-row and heavier atoms (P, S, Cl, Xe, I) may hold more than eight electrons; second-row atoms (C, N, O, F) never do.

Resonance and bond order

Resonance contributors keep every atom in place and differ only in where π electrons and lone pairs sit. The real species is one structure, the hybrid; it does not flip between contributors. Bond order of a link = bonding pairs in that link averaged over equivalent contributors. Carbonate (drawn) has three equivalent contributors, each with one C=O and two C–O⁻, so every C–O bond has order 4/3 ≈ 1.33. A C–OH bond joins in only through a minor, charge-separated contributor, so it stays essentially single. Count contributors carefully: separate resonance units multiply, so a species with two independent carboxylate groups has 2 × 2 = 4 contributors. It is not 2 (the count for one group alone), and not 6 (every way of placing two negative charges on four oxygens), because putting both charges on one carbon's oxygens leaves that carbon with three bonds and the other with five. Higher bond order means a shorter, stronger bond.

VSEPR and hybridization

Count electron domains on the central atom: each lone pair and each bond counts once, whether single, double or triple. Domains set the electron-domain geometry and the hybridization: 2 linear (sp), 3 trigonal planar (sp²), 4 tetrahedral (sp³), 5 trigonal bipyramidal (sp³d), 6 octahedral (sp³d²). The molecular shape names only the atoms. Three domains with one lone pair give a bent shape (below 120°); four domains give trigonal pyramidal with one lone pair and bent with two (near or below 109.5°). As a VSEPR rule of thumb, a bent molecule with four domains has a smaller angle than a bent one with three (H₂O 104.5° vs SO₂ 119°). With five domains the lone pairs go equatorial: one lone pair gives a seesaw (SF₄, drawn), two a T shape, three a linear molecule. With six domains, one lone pair gives a square pyramid and two give a square plane. Lone pairs compress the remaining angles. Hybridization follows the domain count, not the shape. A π bond forms from side-on overlap of unhybridized p orbitals, which must stay parallel, so the atoms around a C=C bond lie in one plane.

Polarity

A bond is polar when its atoms differ in electronegativity; the dipole points toward the more electronegative atom. A molecule is polar when its bond dipoles do not cancel. They cancel when identical outer atoms sit symmetrically: linear AX₂ (also AX₂E₃), trigonal planar AX₃, tetrahedral AX₄, square planar AX₄E₂, trigonal bipyramidal AX₅ and octahedral AX₆. A lone pair that breaks the symmetry (NH₃, SO₂, SF₄) or different outer atoms (dichloromethane, drawn; OCS) give a net dipole. Polar bonds do not guarantee a polar molecule, and a molecule with only weakly polar bonds can still be slightly polar if its shape is unsymmetrical.

Molecular orbitals, bond order and magnetism

Two atomic orbitals combine into one bonding and one antibonding MO. For the period 2 diatomics up to N₂, the valence order is σ2s < σ*2s < π2p < σ2p < π*2p < σ*2p; for O₂, F₂ and Ne₂, σ2p lies below π2p. Fill electrons by Aufbau, Pauli and Hund. Bond order = (bonding electrons − antibonding electrons)/2; zero means no stable molecule. Any unpaired electron makes the species paramagnetic (attracted into a magnetic field); all paired means diamagnetic. Worked examples: O₂ has 12 valence electrons, σ2s² σ*2s² σ2p² π2p⁴ π*2p², so bond order (8 − 4)/2 = 2 with two unpaired electrons in the two π*2p orbitals, which is why liquid oxygen sticks to a magnet even though the Lewis structure O=O shows every electron paired. F₂ has 14, bond order (8 − 6)/2 = 1, diamagnetic. Removing an electron from a bonding MO weakens a bond; removing one from an antibonding MO strengthens it. For species with 10 or more valence electrons the σ2p/π2p order does not change bond order or magnetism, because both orbitals are full. For a heteronuclear diatomic such as CO or NO, count all valence electrons and fill the same diagram.

Lattice energy

Here lattice energy is the energy needed to separate one mole of an ionic solid into gaseous ions, so it is positive. By Coulomb's law it scales as Q₁Q₂/d, where d = r₊ + r₋. Charge dominates: MgO (2+/2−) has about four times the lattice energy of NaF (1+/1−). Among ions of the same charge, smaller ions give a larger lattice energy: LiF > NaF > KF and NaF > NaCl > NaBr > NaI. A Born–Haber cycle gets it from Hess's law: lattice energy = −ΔH°f + ΔH(sublimation of the metal) + ionization energies + energy to make the gaseous nonmetal atoms + electron affinity (negative when energy is released). Worked example for NaCl (drawn): ΔH°f = −411, sublimation of Na +107, IE₁ of Na +496, ½ Cl–Cl bond energy ½ × 243 = +122, electron affinity of Cl −349 kJ/mol, so lattice energy = 411 + 107 + 496 + 122 − 349 = 787 kJ/mol. A liquid nonmetal such as Br₂ must be vaporized before its bond is broken.

What to carry forward

Count electrons, draw octets and let formal charges pick the best structure; average equivalent resonance contributors for bond order. Count domains for geometry and hybridization, then let lone pairs and outer atoms decide shape and polarity. In MO theory, fill the diagram and use bond order = (bonding − antibonding)/2, with unpaired electrons meaning paramagnetism. For ionic solids, charge first, then size: lattice energy scales as Q₁Q₂/(r₊ + r₋).

Problemas de prática (40)

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Molecular orbitals and magnetism

  • Which bond gets stronger when an electron is added?Predict how adding an electron changes bond order by locating the lowest molecular orbital with room for it in each diatomic.

Shapes, hybridization and polarity

  • Describe iodine pentafluoride completelyCombine electron counting, VSEPR, hybridization and polarity into a full structural description of an interhalogen.
  • Bond angles in phosphorus pentachloridePredict the full set of Cl–P–Cl angles in a trigonal bipyramidal molecule.

Classificar 10

Shapes, hybridization and polarity

  • Which molecule is completely flat?Use hybridization and the geometry of π bonds to decide whether every atom of a molecule, hydrogens included, lies in one plane.
  • Which chlorinated benzene is nonpolar?Use the vector sum of C–Cl bond dipoles on a planar ring to identify a nonpolar substituted benzene.
  • When does the hybridization change?Track electron-domain counts through Lewis acid–base reactions to decide whether the central atom's hybridization changes.
  • Find the sp³d central atomAssign hybridization from the total electron-domain count rather than from the molecular shape.

Lewis structures and resonance

  • Which species has equivalent resonance contributors?Recognize which one-carbon oxygen species is described by two equivalent resonance contributors.
  • The octet structure of sulfur dioxideIdentify the Lewis structure of sulfur dioxide that gives every atom an octet and check its formal charges.

Lattice energy and ionic bonding

  • Ionic and covalent bonding in one compoundRecognize a salt of a polyatomic ion as containing both ionic and covalent bonds.
  • Which compound has the largest lattice energy?Pick the compound with the largest lattice energy by comparing ion charge products.

Molecular orbitals and magnetism

  • Which diatomic is paramagnetic?Use valence-electron parity and MO filling to pick the paramagnetic species among heteronuclear and homonuclear diatomics.
  • Which species has no net bond?Identify a diatomic species with zero bond order from its molecular orbital occupancy.

Comparar 8

Lewis structures and resonance

  • Best Lewis structure of the cyanate ionRank resonance contributors of the cyanate ion by formal charge and electronegativity.
  • Rank N–N bond lengthsRank N–N bond lengths across four nitrogen compounds from their Lewis structures and resonance.
  • Which atom is central in NOCl?Use formal charges to choose the atom connectivity of nitrosyl chloride.

Lattice energy and ionic bonding

  • Rank four salts by lattice energyRank lattice energies of four 1:1 ionic solids using ion charges first and ion sizes second.
  • Which lattice-energy comparison is correct?Compare lattice energies of salt pairs that differ in ion size or ion charge.

Molecular orbitals and magnetism

  • Rank N–O bond lengths in NO⁺, NO and NO⁻Rank N–O bond lengths of the nitrosyl cation, nitric oxide and the nitroxyl anion from their MO bond orders.

Shapes, hybridization and polarity

  • Rank O–X–O bond anglesRank the O–X–O angles of four oxyions from their Lewis structures and VSEPR domain counts.
  • Two molecules with the same shapeCompare molecular shapes of pairs with equal numbers of attached atoms but different lone-pair counts.

Calcular 9

Shapes, hybridization and polarity

  • sp² carbons in aspirinCount sp²-hybridized carbons in a drug molecule by counting electron domains at each carbon.

Lewis structures and resonance

  • C–O bond order in hydrogen carbonateFind the bond order of the resonance-delocalized C–O bonds in the hydrogen carbonate ion.
  • Lone pairs in the chlorate ionCount the lone pairs in the octet-rule Lewis structure of the chlorate ion from its valence-electron total.
  • Formal charge on nitrogen in fulminateComplete the octets of the fulminate ion on a given bonding skeleton and assign the formal charge of its central nitrogen.
  • Resonance contributors of oxalateCount the equivalent resonance contributors of the oxalate ion by combining its two independent carboxylate units.

Lattice energy and ionic bonding

  • How much larger is the lattice energy of CaO than KF?Estimate the ratio of two lattice energies from ion charges and radii with the Coulomb model.
  • Lattice energy of KF from a Born–Haber cycleCalculate the lattice energy of potassium fluoride from a Born–Haber cycle with given thermochemical data.

Molecular orbitals and magnetism

  • Unpaired electrons in N₂²⁻Apply Hund's rule in a molecular orbital diagram to count unpaired electrons in a diatomic anion.
  • Bond order of the dioxygenyl ionCalculate the bond order of O₂⁺ by filling its valence molecular orbitals.

Encontre o erro 3

Lewis structures and resonance

  • Does carbonate flip between structures?Identify the error in describing resonance as rapid switching between contributors.

Lattice energy and ionic bonding

  • Find the error in a NaBr Born–Haber cycleDiagnose a Born–Haber calculation for a bromide that omits the vaporization of liquid bromine.

Shapes, hybridization and polarity

  • Find the error: CO₂ and SO₂Diagnose a polarity prediction that ignores a lone pair on the central atom.

Interprete as evidências 3

Molecular orbitals and magnetism

  • Bond data for N₂, O₂ and F₂Interpret bond enthalpy and bond length data for three diatomics in terms of molecular orbital bond order.

Shapes, hybridization and polarity

  • Reading dipole moments of three triatomicsInterpret dipole-moment data to separate the effects of molecular shape and of unequal bond dipoles.

Lewis structures and resonance

  • Formal charges in nitric acidAssign the formal charge of every atom in a described Lewis structure of nitric acid.

Trabalhe de trás para frente 1

  • Identify the dication from its bond orderWork backwards from bond order and magnetism to the identity of a homonuclear diatomic dication.

Planeje uma síntese 1

  • Superoxide or peroxide?Choose a measurement that distinguishes superoxide from peroxide salts using their predicted magnetism.

Explique por quê 2

Shapes, hybridization and polarity

  • Why is NCl₃ polar but BCl₃ is not?Explain how a lone pair on the central atom makes a molecule polar when its bond dipoles would otherwise cancel.

Lattice energy and ionic bonding

  • Which lattice-energy statement is true?Judge comparative statements about lattice energy by applying the charge and size dependence of Coulomb attraction.

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