Name: resonance in nitrite and carbonate ions
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 16 s
Channel: JoVE

La maggior parte delle molecole e degli ioni sono rappresentati da un'unica struttura di Lewis. Tuttavia, in particolari composti, alcuni elettroni sono delocalizzati su più legami o atomi piuttosto che localizzati su un legame o atomo specifico. Questi composti possono essere rappresentati accuratamente con più strutture di Lewis.
Si consideri la struttura di Lewis per il triossido di zolfo. I singoli legami tra ciascun ossigeno e gli atomi centrali di zolfo soddisfano l'ottetto per gli atomi di ossigeno. Tuttavia, per raggiungere un ottetto completo per lo zolfo, deve essere formato un legame aggiuntivo tra lo zolfo e uno degli atomi di ossigeno.
Queste strutture multiple di Lewis di triossido di zolfo sono chiamate strutture di risonanza o strutture contribuenti. Le posizioni spaziali di tutti gli atomi componenti rimangono le stesse; Tuttavia, gli elettroni di valenza sono distribuiti in modo diverso.
Queste frecce a due punte possono essere pensate come virgole e non devono essere confuse con le strutture di risonanza in equilibrio. In queste molecole, la struttura effettiva è la media ponderata o un ibrido delle sue strutture di risonanza.
Le strutture di risonanza sono identificate dalla "spinta degli elettroni", o dalla trasformazione di coppie solitarie in legami e viceversa, come indicato con frecce curve. In questo modo vengono mappate le aree delocalizzate.
Tale delocalizzazione si traduce in stabilizzazione per risonanza, cioè una molecola con un'energia potenziale inferiore a quella di qualsiasi struttura teorica non delocalizzata.
Se una struttura contribuente ha un'energia inferiore rispetto a un'altra, assomiglia più da vicino alla struttura molecolare effettiva. Alcune preferenze vengono utilizzate per stimare le energie relative di varie strutture contribuenti.
In primo luogo, le strutture in cui tutti gli atomi hanno gusci di valenza riempiti sono più stabili. In secondo luogo, le strutture con un numero maggiore di legami covalenti sono più stabili. In terzo luogo, le strutture che riducono al minimo gli oneri formali sono più stabili.
Infine, le strutture che trasportano cariche negative su atomi più elettronegativi e cariche positive su atomi meno elettronegativi sono più stabili.

risonanza e strutture ibride

Resonance and Hybrid Structures

Secondo la teoria della risonanza, se per una molecola, uno ione o un radicale si possono scrivere due o più strutture di Lewis con la stessa disposizione degli atomi, la distribuzione effettiva degli elettroni è una media di quella mostrata dalle varie strutture di Lewis.

Strutture di risonanza e ibridi di risonanza

La struttura di Lewis di un anione nitrito (NO2-) può in realtà essere disegnata in due modi diversi, distinti dalla posizione dei legami N–O e N=O.

<img alt="Figure1" src="/files/ftp_upload/11684/11684%20PT%20Figure%201.svg" style="text-align: center; height: 50px;">

Se gli ioni nitrito contengono un legame singolo e uno doppio, si prevede che le lunghezze dei due legami siano diverse. Un doppio legame tra due atomi è più corto (e più forte) di un legame singolo tra gli stessi due atomi. Tuttavia, gli esperimenti mostrano che entrambi i legami N–O in NO2- hanno la stessa forza e lunghezza e sono identici in tutte le altre proprietà. Non è possibile scrivere un’unica struttura di Lewis per NO2- in cui l’azoto ha un ottetto ed entrambi i legami sono equivalenti.

Viene invece utilizzato il concetto di risonanza: la distribuzione effettiva degli elettroni in ciascuno dei legami azoto-ossigeno in NO2- è la media di un doppio legame e di un legame singolo.

Le singole strutture di Lewis sono chiamate strutture di risonanza. La struttura elettronica effettiva di una molecola (la media delle forme di risonanza) è chiamata ibrido di risonanza delle singole forme di risonanza. Una freccia a doppia punta tra le strutture di Lewis indica che si tratta di forme di risonanza.

<img alt="Figure2" src="/files/ftp_upload/11684/11684%20PT%20Figure%202.svg" style="text-align: center; height: 50px;">

L'anione carbonato, CO32−, fornisce un secondo esempio di risonanza.

<img alt="Figure3" src="/files/ftp_upload/11684/11684%20PT%20Figure%203.svg" style="text-align: center; height: 75px;">

<ul style="list-style-type: disc; padding-left: 35px; margin: 0;">
	<li>Un atomo di ossigeno deve avere un doppio legame con il carbonio per completare l'ottetto sull'atomo centrale.</li>
	<li>Tutti gli atomi di ossigeno, tuttavia, sono equivalenti e il doppio legame potrebbe formarsi da uno qualsiasi dei tre atomi. Ciò dà origine a tre forme di risonanza dello ione carbonato.</li>
	<li>Poiché è possibile scrivere tre strutture di risonanza identiche, è noto che la disposizione effettiva degli elettroni nello ione carbonato è la media delle tre strutture.</li>
	<li>Quindi, gli esperimenti mostrano che tutti e tre i legami C–O sono equivalenti.</li>
</ul>

Una molecola descritta come un ibrido di risonanza non possiede mai una struttura elettronica descritta da nessuna delle due forme di risonanza. Non fluttua tra le forme di risonanza; piuttosto, la struttura elettronica effettiva è sempre la media di quella mostrata da tutte le forme di risonanza.

Questo testo è adattato da  <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, Chemistry 2e, Chapter 7.4 Formal Charges and Resonance</a>.

Risonanza e strutture ibride

Secondo la teoria della risonanza, se due o più strutture di Lewis con la stessa disposizione di atomi possono essere scritte per una molecola, uno ione o un radicale, ...

Education

Didattica

JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Covalent Bonding and Structure

Legame covalente e struttura

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

resonance in nitrite and carbonate ions

Resonance explains why some Lewis structures do not match the measured properties of a molecule or ion. When two or more Lewis structures have the same atom arrangement, the real electron distribution is an average of the structures shown.

The nitrite anion, NO2−, is one example. It can be drawn in two ways, with the N–O and N=O bonds in different places. If nitrite had one single bond and one double bond, the two bond lengths should be different, because a double bond is shorter and stronger than a single bond.

Experiments show that both N–O bonds in NO2− have the same length and strength. They are identical in all other properties as well. A single Lewis structure cannot show this and still give nitrogen an octet, so resonance is used to describe the ion.

The separate Lewis structures are called resonance structures. The true electronic structure is called a resonance hybrid, which is the average of the resonance forms. A double-headed arrow between Lewis structures shows that they are resonance forms. Nitrite does not switch back and forth between them. Its electron distribution is always the average of the forms.

The carbonate anion, CO3 2−, gives a second example. One oxygen must form a double bond to carbon to complete the octet on the central atom, but each oxygen atom is equivalent. That means the double bond can be placed with any one of the three oxygens, creating three resonance forms.

Because the three resonance structures are identical, the electron arrangement in carbonate is the average of all three. Experiments show that all three C–O bonds are exactly the same. A resonance hybrid never has the exact electronic structure of just one resonance form.

Resonance in Nitrite and Carbonate Ions

what is organic chemistry?

Carbon Compounds in Everyday Life

Organic chemistry focuses on carbon compounds called organic compounds. These compounds may come from living things or be made by chemists. Carbon is the main element in them, and it bonds with other carbon atoms as well as hydrogen, oxygen, nitrogen, and sulfur. Carbon can form up to four strong bonds. That ability lets carbon atoms build chains and rings of many sizes, shapes, and levels of complexity. Not every carbon-containing substance is classified as organic. Carbonates, cyanides, and...

electronic structure of atoms

Atomic Orbitals and Quantum Numbers

Atomic orbitals and quantum numbers describe how electrons are arranged around the nucleus. An atom has a small, positively charged nucleus with electrons surrounding it. The nucleus contains protons and neutrons, and the quantum mechanical model gives a better picture of this structure than older models. Protons carry a +1 charge and have a mass of 1.0073 amu. Neutrons have no charge and have a mass of 1.0087 amu. Electrons carry a −1 charge and have a mass of 0.00055 amu. The number of...

electron configurations

How Electrons Fill Atomic Orbitals

Electron filling in atoms follows a set order based on orbital energy. The Aufbau principle says each new electron enters the lowest-energy subshell available. The Pauli exclusion principle says no two electrons can share the same set of four quantum numbers. Hund’s rule of maximum multiplicity says electrons stay unpaired in equal-energy orbitals when possible. The order of filling follows the relative energies of the subshells. A common sequence is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.

chemical bonds

Chemical Bonds: Types and Strength

Chemical bonds help atoms reach a full valence shell like the nearest noble gas. The main bond types are ionic, covalent, and metallic. Bond energy and bond length also help explain how strong a bond is. An ionic bond forms from electrostatic attraction between cations and anions. In many cases, one atom transfers electrons to another atom, and the ions form from that transfer. Ionic bonds have no defined direction because the attraction spreads evenly through three-dimensional space. A...

polar covalent bonds

Electronegativity in Polar Covalent Bonds

Polar covalent bonds form when atoms share electrons unevenly. This happens in covalent bonds between atoms that have similar, but not identical, tendencies to attract electrons. Nonmetal atoms often make these bonds with other nonmetals. Hydrogen gas, H2, is a simple example of a covalent bond between two hydrogen atoms. A covalent bond begins when two atoms move close enough for their valence orbitals to overlap. In hydrogen, the 1s orbitals overlap and the single electrons on each atom...

lewis structures and formal charges

Lewis Structures, Bonds, and Formal Charge

Lewis structures show how atoms share electrons in covalent bonds and in polyatomic ions. Lewis symbols help draw these structures, and the periodic table helps predict how many valence electrons an atom has and how many bonds it can form to reach an octet. Group 18 elements, such as argon and helium, already have filled electron configurations. They rarely take part in chemical bonding. Atoms in group 17, such as bromine and iodine, need one more electron to reach an octet, so they can form...

vsepr theory

Molecular Shapes from Electron Pairs

Molecular shape can be predicted from the way electron pairs arrange around a central atom. Valence shell electron-pair repulsion theory, or VSEPR theory, uses a Lewis structure to count bonding pairs and lone electron pairs. The main idea is simple: electron pairs spread out as far as possible to reduce repulsion. In a molecule, the valence electrons around the central atom can be bonding pairs or lone pairs. Bonding pairs are found mainly between bonded atoms. Lone pairs stay on one atom and...

molecular geometry and dipole moments

Predicting Shape and Molecular Polarity

Molecular shape and molecular polarity can be predicted with VSEPR theory and dipole moments. Start by writing the Lewis structure of the molecule or polyatomic ion. Then count the electron groups around the central atom. Lone pairs and bonds both count as electron groups, and a single, double, or triple bond each counts as one region of electron density. Use that count to identify the electron-pair geometry. Next, use the number of lone pairs to determine the molecular structure. If more than...

resonance and hybrid structures

Resonance explains why some Lewis structures do not match the measured properties of a molecule or ion. When two or more Lewis structures have the same atom arrangement, the real electron distribution is an average of the structures shown. The nitrite anion, NO2−, is one example. It can be drawn in two ways, with the N–O and N=O bonds in different places. If nitrite had one single bond and one double bond, the two bond lengths should be different, because a double bond is shorter and stronger...

valence bond theory and hybridized orbitals

valence-bond-theory-and-hybridized-orbitals

Covalent Bond Overlap and Orbital Hybridization

Covalent bonds form when orbitals on two atoms overlap and their single electrons pair up. The bond is stronger when the overlap is greater. Maximum overlap happens when the orbitals meet along a direct line between the two nuclei. A sigma bond, or σ bond, is the kind of covalent bond found in a single bond in a Lewis structure. In a σ bond, electron density is concentrated along the internuclear axis. A pi bond, or π bond, forms when two p orbitals overlap side by side. In a π bond, the...

mo theory and covalent bonding

Bond Order and Molecular Orbitals

Molecular orbital theory explains how electrons are arranged in molecules. It treats electron regions in a molecule much like atomic orbitals in an atom. A molecular orbital is the region of space where a valence electron is most likely to be found. Molecular orbitals form through the linear combination of atomic orbitals, or LCAO. When atomic wave functions combine in phase, they create regions with high electron density. When they combine out of phase, they create nodes, or regions with no...

intermolecular forces and physical properties

intermolecular-forces-and-physical-properties

How Intermolecular Forces Affect Matter

Intermolecular forces are the attractions between molecules, and they help explain how matter behaves. These forces influence melting points, boiling points, and whether substances mix with one another. Water has a much higher boiling point than hexane, which shows that water has stronger intermolecular forces. Three main kinds of intermolecular interactions are covered here. They include ion-dipole forces, dipole-dipole interactions, and van der Waals forces, which include London dispersion...

solubility

Why Some Substances Dissolve in Water

Solution formation depends on how solutes and solvents interact. A solution is a homogeneous mixture made of a solvent, which is the larger part, and a solute, which is the smaller part. The physical state of a solution is usually the same as the solvent. Concentration is often described as dilute for a low amount of solute or concentrated for a high amount of solute. When a solute dissolves, its particles are surrounded by solvent species. This process is called solvation. If water is the...

introduction to functional groups

Functional Groups and Organic Molecules

Functional groups are groups of atoms with specific chemical properties that appear within organic molecules. They are sometimes shown as "R." These groups sit along the carbon backbone of macromolecules, which can form chains or rings of carbon atoms. Functional groups help functionalize a compound. That means they can give the molecule different physical and chemical properties. In organic chemistry, this change in group structure can strongly affect how a compound behaves. Common...

overview of advanced functional groups

Advanced Functional Groups in Organic Chemistry

Advanced functional groups are atom clusters in organic molecules that give those molecules specific chemical properties. They are sometimes shown as "R" in formulas. By changing the atoms attached to a carbon skeleton, a functional group can make a compound behave differently in physical and chemical reactions. This topic covers several major classes of organic compounds with advanced functional groups. Aromatic hydrocarbons include benzene. Phenol is another example and is also called...

key terms and concepts

Explore how resonance explains equal bond lengths in nitrite and carbonate ions, and learn the difference between resonance structures and a resonance hybrid.

Resonance in Nitrite and Carbonate Ions | JoVE K12

organic chemistry

JoVE Core Organic Chemistry spiega i concetti di base della stereochimica, delle reazioni e dei meccanismi usando video-lezioni concise e facili da capire. Inoltre, i video scientist-in-action dimostrano l'applicazione di tali concetti in esperimenti di ricerca classici e originali eseguiti al giorno d'oggi nei laboratori in tutto il mondo.

Resonance in Nitrite and Carbonate Ions

Description

Transcript

Tags