Name: covalent bond overlap and orbital hybridization
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 38 s
Channel: JoVE

La théorie des liaisons de valence propose qu’une liaison chimique résulte d’un chevauchement d’orbitales atomiques partiellement remplies, y compris celles autres que l’orbitale s sphérique.
Lorsqu’une liaison unique se forme entre deux orbitales non sphériques, les deux orbitales se chevauchent en tête-à-tête.
Le type de liaison covalente formée par le chevauchement direct des orbitales atomiques est appelé liaison sigma.
Les orbitales s et p qui se chevauchent pour former des liaisons covalentes ne peuvent pas produire les différentes formes moléculaires dans le modèle VSEPR. La théorie des liaisons de valence aide à expliquer cette géométrie moléculaire par l’hybridation, ou le mélange, des orbitales atomiques.
Certaines orbitales atomiques impliquées dans la liaison se recombinent pour former de nouvelles orbitales dont les formes sont un hybride des originaux. Le nombre initial d’orbitales atomiques et le nombre d’orbitales hybrides générées sont toujours les mêmes.
Les orbitales hybrides ont une forme différente de leurs orbitales atomiques constitutives, avec un lobe nettement plus grand que l’autre. Ainsi, la densité de probabilité électronique est fortement concentrée dans un lobe directionnel, ce qui conduit à un chevauchement plus efficace avec les orbitales d’autres atomes. Pour plus de clarté, ces orbitales sont souvent montrées sans les lobes mineurs.
Par exemple, à l’état excité du carbone, les orbitales un s et trois p qui contiennent les quatre électrons non appariés subissent une hybridation, produisant quatre orbitales hybrides équivalentes s-p-trois qui occupent les sommets d’un tétraèdre régulier.
Le mélange d’une orbitale s et de deux orbitales p à l’état excité du carbone génère trois orbitales hybrides s-p-deux équivalentes à géométrie plane trigonale.
De même, l’hybridation d’une orbitale s et d’une orbitale p peut créer deux orbitales sp orientées à 180 degrés l’une par rapport à l’autre. 
Le concept d’hybridation fournit également une explication de la formation de liaisons multiples. Le chevauchement latéral de deux orbitales p donne naissance à une liaison pi.
Cependant, une liaison pi ne peut être formée en liaisons doubles et triples que lorsqu’une liaison sigma existe déjà entre deux atomes. Parce que la liaison pi existe sur les côtés opposés de l’axe internucléaire, les liaisons pi sont incapables de tourner autour de cet axe.

théorie des liaisons de valence et orbitales hybridées

Valence Bond Theory and Hybridized Orbitals

Selon la théorie de la liaison de valence, une liaison covalente se forme lorsque : (1) un orbital sur un atome chevauche un orbital sur un deuxième atome, et (2) les électrons simples dans chaque orbital se combinent pour former une paire d'électrons. La force d'une liaison covalente dépend de l'étendue du chevauchement des orbitales impliquées. Le chevauchement maximal est possible lorsque les orbitales se chevauchent sur une ligne directe entre les deux noyaux.

Une liaison σ (liaison simple dans une structure de Lewis) est une liaison covalente dans laquelle la densité électronique est concentrée dans la région le long de l'axe interatomique. Une liaison π est une liaison covalente qui résulte du chevauchement côte à côte de deux orbitales p. Dans une liaison π, les régions de chevauchement orbital se trouvent sur des côtés opposés de l'axe interatomique, tandis qu'il y a un nœud (un plan où il n'y a aucune probabilité de trouver un électron) le long de l'axe. Toutes les liaisons simples sont des liaisons σ, tandis que les liaisons multiples sont constituées à la fois de liaisons σ et π.

Lorsque les atomes sont liés ensemble dans une molécule, les fonctions d'onde des orbitales atomiques peuvent se combiner pour produire de nouvelles descriptions mathématiques ayant des formes différentes. Ce processus est appelé hybridation et est mathématiquement accompli par la combinaison linéaire des orbitales atomiques. Les nouvelles orbitales résultantes sont appelées orbitales hybrides.

Les formes et orientations des orbitales hybrides, qui ne se forment que dans les atomes liés de manière covalente, sont différentes de celles des orbitales atomiques dans les atomes isolés. Le nombre d'orbitales hybrides est égal au nombre d'orbitales atomiques qui ont été combinées pour les générer. Toutes les orbitales dans un ensemble d'orbitales hybrides sont équivalentes en forme et en énergie, et leur orientation est prédite par la théorie VSEPR. Les orbitales hybrides se chevauchent pour former des liaisons σ, tandis que les orbitales non hybridées se chevauchent pour former des liaisons π.

Par exemple, dans l'état excité du carbone, les orbitales 2s et trois orbitales 2p subissent une hybridation produisant quatre orbitales hybrides sp3 dégénérées orientées tétraédriquement. Dans une molécule de méthane, l'orbitale 1s de chacun des quatre atomes d'hydrogène se chevauche avec l'une des quatre orbitales sp3 de l'atome de carbone pour former une liaison sigma (σ).

De même, le mélange d'un orbital 2s et de deux orbitales 2p du carbone génère trois orbitales hybrides sp2 équivalentes avec une géométrie plane trigonale, tandis que l'hybridation d'un orbital 2s et d'un orbital 2p crée deux orbitales sp orientées à 180° l'une de l'autre.

Pour les atomes qui ont des orbitales d dans leurs sous-couches de valence, l'hybridation de cinq orbitales atomiques de couche de valence (un s, trois p et une des orbitales d) donne cinq orbitales hybrides sp3d avec une géométrie bipyramidale trigonale. Un arrangement octaédrique de six orbitales hybrides est obtenu par le mélange de six orbitales atomiques de couche de valence (un s, trois p et deux des orbitales d), ce qui donne six orbitales hybrides sp3d2.

Ce texte a été adapté d';<a href="https://openstax.org/books/chemistry-2e/pages/8-1-valence-bond-theory">Openstax, Chemistry 2e, Section 8.1 Valence Bond Theory</a> and <a href="https://openstax.org/books/chemistry-2e/pages/8-2-hybrid-atomic-orbitals">Section 8.2 Hybrid Atomic Orbitals</a>.

Théorie des liaisons de valence et orbitales hybridées

Selon la théorie des liaisons de valence, une liaison covalente se produit lorsque : (1) une orbitale sur un atome chevauche une orbitale sur un deuxième atome, et (2) la ...

valence-bond-theory-and-hybridized-orbitals

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Organic Chemistry

Chimie organique

Unit 1

Unité 1

Covalent Bonding and Structure

Liaison covalente et structure

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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 overlap lies on opposite sides of the internuclear axis, and a node with no electron probability lies along the axis. Single bonds are σ bonds, while multiple bonds include both σ and π bonds.

When atoms join in a molecule, atomic orbital wave functions can combine to make new orbital shapes. This process is called hybridization, and it is described by the linear combination of atomic orbitals. The new orbitals are called hybrid orbitals. They form only in covalently bonded atoms, and their shapes and orientations differ from those of atomic orbitals in isolated atoms. The number of hybrid orbitals matches the number of atomic orbitals used to make them. Orbitals in one hybrid set have the same shape and energy, and their directions follow VSEPR theory. Hybrid orbitals form σ bonds, while unhybridized orbitals form π bonds.

Carbon shows this pattern clearly. In its excited state, one 2s orbital and three 2p orbitals hybridize to form four equivalent sp3 orbitals arranged tetrahedrally. In methane, each hydrogen 1s orbital overlaps with one carbon sp3 orbital to make a σ bond. One 2s orbital and two 2p orbitals can also mix to make three equivalent sp2 orbitals with trigonal planar geometry. When one 2s orbital mixes with one 2p orbital, two sp orbitals form and point 180° apart.

Atoms with d orbitals in the valence shell can form larger hybrid sets. Mixing five valence shell atomic orbitals, one s, three p, and one d, gives five sp3d hybrid orbitals with trigonal bipyramidal geometry. Mixing six valence shell atomic orbitals, one s, three p, and two d, gives six sp3d2 hybrid orbitals with octahedral geometry.

Covalent Bond Overlap and Orbital Hybridization

covalent-bond-overlap-and-orbital-hybridization

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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...

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Lewis Structures, Bonds, and Formal Charge

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valence bond theory and hybridized orbitals

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Functional Groups and Organic Molecules

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key terms and concepts

Learn how orbital overlap forms sigma and pi bonds, then see how hybridization creates sp3, sp2, sp, sp3d, and sp3d2 orbitals in molecules.

Covalent Bond Overlap and Orbital Hybridization | JoVE K12

organic chemistry

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Covalent Bond Overlap and Orbital Hybridization

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