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Química Geral · Bonding and structure

Estruturas de Lewis e carga formal

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O que você vai aprender

  • Count the valence electrons of a molecule or ion from group numbers and its charge, and place them as bonds and lone pairs on a sensible skeleton.
  • Assign formal charges atom by atom and use them to choose between Lewis structures.
  • Recognise the octet exceptions: electron-deficient boron and aluminium, odd-electron molecules such as NO, and expanded octets on third-row and heavier atoms.
  • Draw resonance contributors with curved arrows that move only electrons, and use them to predict bond orders and bond lengths.
  • Describe Lewis acid–base adduct formation, such as BF₃ with NH₃ and AlCl₃ with chloride, as one lone pair filling an empty orbital.

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Count the valence electrons first

A Lewis structure must place exactly the right number of valence electrons. Add the group-number valence electrons of every atom (H 1, B 3, C 4, N 5, O 6, halogens 7), then add one electron for each negative charge or remove one for each positive charge. The nitrate ion, NO₃⁻, has 5 + 3 × 6 + 1 = 24; the ammonium ion, NH₄⁺, has 5 + 4 × 1 − 1 = 8. Core electrons never appear in a Lewis structure.

Build the skeleton, then complete the octets

The least electronegative atom other than hydrogen usually sits in the centre; hydrogen and fluorine make one bond and are always terminal. Join the atoms with single bonds (two electrons each), give the outer atoms lone pairs until each has an octet, and put any electrons left over on the central atom. If the central atom still has fewer than eight electrons, turn an outer-atom lone pair into a shared π pair. Hydrogen cyanide shows the result: H–C≡N with one lone pair on nitrogen uses all 10 valence electrons and gives carbon and nitrogen an octet each.

Formal charge

Formal charge = valence electrons − nonbonding electrons − number of bonds. It is bookkeeping that pretends every bond is shared equally, so it is not the real charge on an atom and not the oxidation state. The formal charges always add up to the charge of the species. When several valid structures exist, prefer the one with the fewest and smallest formal charges, with any negative formal charge on the more electronegative atom. Carbon monoxide is the classic surprise: in :C≡O: carbon owns 2 + 3 = 5 electrons, one more than its 4 valence electrons, so carbon is −1 and oxygen is +1.

When the octet rule bends

Boron and aluminium compounds such as BF₃ and monomeric AlCl₃ have only six electrons on the central atom; the empty orbital makes them Lewis acids. Molecules with an odd number of electrons, such as NO (11) and NO₂ (17), cannot give every atom an octet; the best Lewis structure of NO puts the unpaired electron on nitrogen, and both formal charges are zero. Atoms from the third row and below can hold more than eight electrons: sulfur in SF₆ has six bonds and twelve electrons. Second-row atoms (C, N, O, F) never exceed an octet.

Resonance and curved arrows

When one Lewis structure cannot show the bonding, draw several contributors that differ only in where π electrons and lone pairs sit. The atoms and σ bonds never move; moving a hydrogen gives a different compound (a tautomer), not a resonance form. A curved arrow starts at an electron pair (a lone pair or a bond) and ends where that pair goes. In acetate, an oxygen lone pair forms a new C=O π bond while the old C=O π pair moves onto the other oxygen. The real ion is a hybrid: both C–O bonds are identical, with a bond order of 1.5. In carbonate and nitrate, four bonding pairs are shared over three links, so each bond order is 4/3.

Lewis acids and bases

A Lewis base donates an electron pair and a Lewis acid accepts one. One curved arrow, from the lone pair of the base to the empty orbital of the acid, makes a new bond in which both electrons came from the base. Ammonia gives its lone pair to boron in BF₃; the adduct H₃N–BF₃ has nitrogen at +1 and boron at −1, and boron now has an octet. Chloride adds to AlCl₃ in the same way to give the tetrachloroaluminate ion, AlCl₄⁻, which is why AlCl₃ can pull chloride away from an alkyl or acyl chloride.

What to carry forward

Count the valence electrons, place them to give second-row atoms octets, then use formal charges to choose between structures. Expect boron, aluminium and odd-electron molecules to fall short of an octet and third-row atoms to exceed it. Resonance arrows move only electrons, and a Lewis acid–base reaction is one lone pair filling one empty orbital.

Problemas de prática (40)

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Desenhe o mecanismo 4

Octet exceptions and Lewis acids

  • Aluminium chloride pulls chloride from tert-butyl chlorideDraw how the Lewis acid AlCl₃ accepts a chlorine lone pair and then takes chloride from 2-chloro-2-methylpropane, giving the tetrachloroaluminate ion AlCl₄⁻ and the tert-butyl cation, and track the formal charges throughout.
  • Carbon monoxide binds to borane through carbonDraw the Lewis acid–base reaction in which the carbon lone pair of carbon monoxide fills the empty orbital of boron in BH₃, and track how the formal charges change.

Resonance and curved arrows

  • Ammonia and carbon dioxide give ammonium carbamateDraw ammonia adding to carbon dioxide as a Lewis base adding to an electron-poor carbon, the proton transfer to a second ammonia that gives ammonium carbamate, and the resonance contributor of the carbamate ion.
  • Nitric acid ionises; nitrate's three resonance contributorsDraw water removing the acidic proton of nitric acid, then use curved arrows to move from one resonance contributor of nitrate to the next until all three equivalent contributors have been drawn.

Preveja o produto 3

Octet exceptions and Lewis acids

  • BF₃ meets diethyl etherPredict the Lewis acid–base adduct formed when an oxygen lone pair fills boron's empty orbital, with its formal charges.

Counting electrons and skeletons

  • Lewis structure of hydrogen cyanidePlace multiple bonds and lone pairs so a small molecule uses its exact electron count and gives C and N octets.

Resonance and curved arrows

  • Follow the arrows in nitritePredict the resonance contributor that results from a given pair of curved arrows.

Classificar 7

Formal charge

  • Where is the charge in borohydride?Locate the formal charge in a tetrahedral anion of boron.

Octet exceptions and Lewis acids

  • Which central atom exceeds an octet?Recognise a central atom with more than eight electrons and connect it to its row of the periodic table.
  • Which central atom lacks an octet?Recognise an electron-deficient central atom in a neutral molecule.
  • Which species cannot give every atom an octet?Use valence electron parity to identify an odd-electron molecule among diatomic species.

Counting electrons and skeletons

  • Which atom is central in hypochlorous acid?Choose the skeleton of a triatomic molecule from bonding capacity and formal charge.
  • Same electron count as CO₂Use valence electron counts to find a species whose Lewis structure has the same pattern as carbon dioxide.

Resonance and curved arrows

  • Resonance forms or different species?Classify pairs of structures as resonance contributors, tautomers, isomers or different species.

Comparar 7

Resonance and curved arrows

  • Rank C–O bond lengthsRank carbon–oxygen bond lengths from bond orders derived from Lewis structures and resonance.
  • The second resonance contributor of formamidePick the more important charge-separated contributor of an amide and reject a tautomer offered as a resonance form.
  • Rank N–O bond ordersDerive and rank N–O bond orders across four nitrogen–oxygen ions from their Lewis structures and resonance.

Formal charge

  • Best Lewis structure of carbon dioxideChoose between valid electron arrangements of CO₂ using octets and formal charges.
  • Choose the skeleton of dinitrogen monoxideUse formal charges to decide between two possible atom connections for a triatomic molecule.
  • Sulfate structure with the fewest formal chargesApply the formal-charge criterion to choose among expanded-octet and octet structures of an oxyanion of sulfur.

Octet exceptions and Lewis acids

  • Best Lewis structure of nitric oxideChoose the best Lewis structure of an odd-electron molecule using formal charges and electronegativity.

Calcular 10

Counting electrons and skeletons

  • Lone pairs in the acetate ionCount lone pairs in an organic anion from its valence electron count and its bonds.
  • Lone pairs in phosgeneFind the number of lone pairs from the valence count and the number of bonding pairs.
  • Valence electrons in the sulfate ionCount the valence electrons of a polyatomic anion, including the electrons added by its charge.

Formal charge

  • Formal charge on the central nitrogen of azideAssign the formal charge of an atom with four bonds and no lone pair in a symmetric anion.
  • Formal charge on oxygen in hydroniumApply the formal-charge formula to an oxygen atom with three bonds and one lone pair.
  • Formal charge on sulfur in octet-only sulfateAssign the central-atom formal charge in an octet-only oxyanion and check the charge sum.
  • Formal charge on sulfur in thionyl chlorideBuild an octet-only Lewis structure with a lone pair on the central atom and find that atom's formal charge.

Resonance and curved arrows

  • N–O bond order in nitrateCalculate an average bond order from equivalent resonance contributors.

Octet exceptions and Lewis acids

  • Electrons around sulfur in SF₆Count the electrons around a central atom with an expanded octet.
  • Lone pairs on the central iodine of I₃⁻Place leftover electrons on a heavy central atom and recognise the resulting expanded octet.

Encontre o erro 3

Formal charge

  • Formal charge in carbon monoxideDistinguish formal charge from oxidation state and partial charge using carbon monoxide.

Counting electrons and skeletons

  • Find the error in a nitrite structureSpot a Lewis structure that has the right electron count and charge but breaks the octet limit for a second-row atom.

Resonance and curved arrows

  • A one-arrow resonance step for carbonateFind the missing arrow in a resonance conversion that would give a second-row atom ten electrons.

Interprete as evidências 1

  • Read the formal charges of ozoneRead lone pairs and bonds from a drawn Lewis structure and convert them into formal charges.

Trabalhe de trás para frente 2

Octet exceptions and Lewis acids

  • Which partners made ammonia borane?Work back from a Lewis adduct to the Lewis acid and Lewis base that formed it.

Counting electrons and skeletons

  • Identify the central atom of XF₄Work back from a Lewis structure's bonds and lone pairs to the group of the central atom.

Explique por quê 3

Formal charge

  • Why is nitrogen positive in ammonium?Explain a positive formal charge as an atom owning fewer electrons than its valence count.

Octet exceptions and Lewis acids

  • Why is BF₃ a Lewis acid but CF₄ is not?Explain Lewis acidity of a group-13 compound in terms of its incomplete octet.

Resonance and curved arrows

  • Why are ozone's two bonds identical?Explain equal measured bond lengths with a resonance hybrid rather than alternating structures.

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