Hybrid orbit

Chemical terminology

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synonym Track hybridization (term in the field of education) generally refers to the hybrid track

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Hybrid Orbital Theory was established in 1931 by Pauline (Pauling L) et al Valence bond theory It is still a modern valence bond theory in essence, but it enriches and develops modern valence bond theory in terms of bonding ability, molecular spatial configuration, etc.

Chinese name: hybrid orbital theory
Foreign name: Hybrid Orbital Theory
Alias: Hybrid orbit

Presenter: Pauling L
Proposed time: 1931
Applied discipline: Chemistry

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three Hybrid type
four judge

five limitations
six Experimental basis

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one In the process of bonding, due to the interaction between atoms, several different types of atomic orbitals with similar energy in the same molecule (i.e wave function ）, which can be linearly combined, redistribute energy and determine spatial direction to form a new atomic orbit with an equal number. This way of orbital recombination is called hybridization, and the new orbit formed after hybridization is called hybrid orbital.

Shape of electron cloud

two The value of the angle function of the hybrid orbital in a certain direction is much larger than that before hybridization, which is more conducive to the maximum overlap between atomic orbitals. Therefore, the hybrid orbital has stronger bonding ability than the original orbital (the orbital is bonded after hybridization).

three The hybrid orbits try to take the maximum angle distribution in space to minimize the mutual repulsion energy, so the bonds formed are more stable. Different types of hybrid orbitals have different angles, and the molecules formed after bonding have different spatial configurations.

four Only the electrons in different energy levels in the outermost electron layer can conduct orbital hybridization, and the two electrons in the first layer do not participate in the reaction.

five When the electrons in different energy levels undergo orbital hybridization, the electrons will transition from the lower energy layer to the higher energy layer, and the orbital energies of the electrons after hybridization are equal and higher than the energy of the original lower energy level but lower than the energy of the original higher energy level. Of course, there are several Atomic orbital Participate in hybridization, and several hybrid orbits will be generated after hybridization.

six When bonding hybrid orbitals, atomic orbitals should be satisfied Maximum overlap principle 。

The hybrid electronic orbit is more concentrated in angular distribution than the original one, so that it overlaps with the atomic orbits of other atoms to a greater extent when bonding covalent bond Stronger.

Type Introduction

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theoretical basis

Sp hybrid electron nephogram

According to the types of atomic orbitals involved in hybridization, orbital hybridization has two main types: sp and spd, which are divided into sp and sp two ，sp three ，dsp two ，sp three d，sp three d two ，d two sp three According to whether the energy of several hybrid orbits formed after hybridization is the same, the hybridization of orbits can be divided into equal and unequal hybridization.

Several atomic orbitals in an atom are redistributed to form equivalent orbitals. In the process of synthesizing molecules from atoms, the original atomic orbitals are further linearly combined into new atomic orbitals under the influence of surrounding atoms according to the bonding requirements of atoms. This linear combination of different atomic orbitals in an atom is called hybridization of atomic orbitals. The hybrid atomic orbitals are called hybrid orbitals. During hybridization, the number of orbits remains unchanged, and the distribution direction and distribution of orbits in space change. The hybrid orbitals obtained by combination generally form stronger σ Bond or arrange lone pairs of electrons instead of existing in the form of empty hybrid orbitals. In several hybrid orbitals of an atom, the hybridization involving all the orbitals of a single electron is called equal hybrid orbitals; Hybridization involving lone pair electrons is called unequal hybridization orbit.

Hybrid orbitals have the same properties as atomic orbitals such as s and p, and must satisfy orthogonality and normalization.

Sp3 hybrid electronic nephogram

The valence bond theory has made a strong demonstration of the nature and characteristics of covalent bonds, but it puts the basis of the discussion on the formation of a covalent bond by sharing a pair of electrons, and has encountered difficulties in explaining the number of valence bonds and molecular spatial structure of many molecules and atoms. For example, the valence electron of C atom is 2s two 2p two According to the law of electron arrangement, two s electrons are paired, only two p electrons are unpaired, and in many carbon containing compounds, C is 4 instead of 2, so it can be assumed that one s electron has been excited to the p orbital. So one s orbital and three p orbitals have unpaired electrons, which can form four covalent bonds, but the bonding direction and energy of s and p should be different. The experiment proves that: CH four In the molecule, four C-H covalent bonds are identical, the bond length is 109.3pm, and the bond angle is 109 ° 28 '. BCl 3、 BeCl 2、 PCl three Many other molecules have similar situations. In order to explain these contradictions, Pauling put forward the concept of hybrid orbits in 1928, which enriched and developed the valence bond theory. He based quantum mechanics (Quantum Mechenical) put forward that in the same atom, several atomic orbits of different types with similar energy can be superimposed and recombined with each other when bonding, and become new orbits with the same number and equal energy. This new orbit is called hybrid orbit. After one 2s electron in C atom is excited to 2p, one 2s orbital and three 2p orbitals are recombined into four sp three The hybrid orbitals form four identical C-H bonds with four H atoms. C is located at the center of the tetrahedron, and four H are located at the four top corners.

type

There are many kinds of hybrid orbits, such as Boron trichloride (BCl three ）B in molecule sp two Hybridization Orbits, that is, one s orbit and two p orbits are combined into three sp two Hybrid orbit, in Beryllium chloride (BeCl two ）With Sp hybridization Track, at transition metal In the compound, there are also sp with d orbitals three D and sp three d two Hybridization Track, etc. The above examples illustrate the properties of covalent single bond ethylene and acetylene Intramolecular double bond and Triple bond The formation of σ Keys and pi bond The concept of. Such as bonding two Nucleus The interconnects are called bond axes, which overlap atomic orbits into bonds in the way of "head to head" along the bond axis σ key 。 The way of overlapping atomic orbitals "side by side" along the bond axis is called π bond. For example, in ethylene (C two H four ）There are Carbon carbon double bond (C=C), 2p in the excited state of carbon atom x ，2p y And 2s Another 2p z The track does not participate in the hybridization and is perpendicular to the plane. Sp in carbon carbon double bond two The hybridization is shown below.

The three sp two Two of the hybrid orbitals are formed with two H atoms respectively σ Single bond , and 1 sp two The orbit is connected with the sp of another C two Track formation head to head σ The pz orbitals in the vertical direction form π bonds side by side. That is to say, carbon carbon double bond is composed of σ A bond is composed of a π bond, that is, two bonds in a double bond are not equal. pi bond The overlap of atomic orbitals is less than σ The π bond is unstable and easy to break, so it contains double bonds olefin Very easy to happen Addition reaction , such as ethylene (H two C=CH two ）And chlorine (Cl two ）Reaction formation Dichloroethylene (Cl—CH two —CH two —Cl)。

Acetylene molecule (C two H two ）Medium carbon Triple bond (HC ∨ CH), VSEPR (Valence Shell Electron Pair Repulsion) can judge the central atom hybrid model by the number of bonding electrons and the number of lone pair electrons. The number of bonding electrons is AB n The value of n in; Number of lone electrons: (number of valence electrons - number of bonding electrons)/2.

price Electron pair The total number is the sum of the two. If the total number of valence electron pairs is 2, it is sp hybrid (linear), and if it is 3, it is sp two Hybrid (plane triangle), when it is 4, it is sp three Hybrid (tetrahedron), 5-sp three D (triangular bipyramid), 6-sp three d two （ Octahedron ）。 The difference between the number of bonding electrons and the number of lone electrons makes the molecular geometry different.

The process of overlapping orbits is called hybridization of atomic orbits. The new atomic orbitals created by the superposition of atomic orbitals are called hybrid orbitals.

⑴ When forming molecules (mainly compounds), atomic orbitals with similar energy in the same molecule (generally the same Energy level group (atomic orbitals) are superimposed (hybridized) on each other to form a new set of atomic orbitals.

Large π bond formed by two P orbitals

(2) Hybrid track is more than the original track Bond energy With strong force, the formed chemical bond energy is large, making the generated molecule more stable. Because the shape of the orbital angle distribution map has changed after the bonding atomic orbital hybridization (the shape is large at one end and small at the other), the angular distribution of the hybrid orbital in some directions is much larger than that of the unhybridized p orbital and s orbital, and its big end can form greater overlap with the original orbital when bonding, Therefore, the hybrid orbitals have stronger bonding ability than the original atomic orbitals.

⑶ The hybrid tracks formed shall meet the minimum repulsion principle as far as possible（ Chemical bond The smaller the repulsion force, the more stable the system is). In order to meet the minimum repulsion principle, the included angle between hybrid tracks should reach the maximum.

⑷ The spatial configuration of molecules mainly depends on the σ key The skeleton formed by the hybrid orbital is σ Therefore, the type of hybrid orbital is related to the molecular spatial configuration.

Hybrid type

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(1) Sp hybridization

The hybridization of one ns orbital and one np orbital in the same molecule is called sp hybridization. The hybrid orbital is called sp hybrid orbital. Sp hybridization can and can only get two sp hybrid orbitals. Experiments show that the beryllium atom in gaseous BeCl2 is sp hybrid, and it is a linear covalent molecule. Be atom is located in the middle of two Cl atoms, Bond angle 180 °, the bond length and bond energy of the two Be Cl bonds are equal.

(2)sp two Hybridization

The hybridization of one ns orbital and two np orbitals in the same molecule is called sp two Hybridization. The hybrid orbital is called sp two Hybrid orbit. Gaseous Boron fluoride （BF three ）The boron atom in is sp two Hybrid, with a plane triangle structure. The B atom is located at the center of the triangle, and the three B-F bonds are equal, with the bond angle of 120 °.

(3)sp three Hybridization

The hybridization of one ns orbital and three np orbitals in the same molecule is called sp three Hybridized, the orbit formed after hybridization is called sp three Hybrid orbit. sp three Hybrid can and only can get four SPs three Hybrid orbit. CH four Intramolecular carbon atom It is the occurrence of sp three Hybridization, its structure is measured as Regular tetrahedron The four C-H bonds are identical, and the bond angle is 109 ° 28 ′. Such experimental results are difficult to explain by the electron pairing method, but hybrid orbital theory It is considered that the excited state C atom (2s one 2p three ）Sp can occur between the 2s orbital and the three 2p orbitals three Hybridize to form four sp with equal energy three Hybrid orbit.

(4)sp three D hybrid

Isomorphic hybrid into triangular bipyramid structure, such as PCl five

(5)sp three d two Hybridization

Isohybridized into regular octahedron structure, such as SF six

Note: The above are only common hybrid track types Coordination compound There are more hybrid types in.

The "head to head" mode is overlapped into σ The "side by side" mode overlaps the π bond.

For example, in ethylene (CH two = CH two ）There is carbon carbon double bond (C=C) in the molecule, and 2p in the excited state of carbon atom x ，2p y And 2s to form sp two Hybrid orbitals, these three orbitals have equal energy, are located in the same plane and form an angle of 120 ℃ with each other, and the other one is p z The track does not participate in the hybridization and is perpendicular to the plane. Sp in carbon carbon double bond two The hybridization is shown below.

acetylene Molecule (C two H two ）C atom with carbon carbon triple bond (HC ∨ CH), 2s and 2p in excited state x Orbitals form sp hybrid orbitals. These two sp hybrid orbitals with equal energy are on the same line, and one of them forms with H atom σ Single bond, another sp hybrid orbital forms the σ Key without participating in the hybrid p y And p z Then they are perpendicular to the x axis and perpendicular to each other, and they are side by side with the p of another C y ，p z Form π bond. That is, the carbon carbon triple bond is composed of a σ Bond and two π bonds. These two π bonds are different from σ The bonds and orbits overlap less and are unstable, so they are easy to be disconnected Alkyne It is also prone to addition reaction.

Hybrid orbitals are limited to the outermost electrons, while the two electrons in the first layer do not participate in the reaction, while there are many orbitals in other layers. The electrons will "jump" from the lower energy layer to the higher energy layer. The original lower energy layer is because the electrons move in the opposite direction. After the transition, the electrons will only move in one direction, so the energy will be higher. And the energy formed after the reaction is between the original S orbital and P orbital energy.

Molecular space morphology after several hybrid orbitals:

Sp hybrid: linear

sp two Hybridize: plane triangle (equihybridize to plane regular triangle)

sp three Hybridization: spatial tetrahedron (equi hybrid to regular tetrahedron)

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1. Judgment Central atom Of Lone electron pair number

2. Find out what is connected to the central atom Atomic number (i.e. formed σ key Number of)

3. If the sum of the two equals 2, then the central atom adopts SP hybridization; If equal to 3, the central atom adopts SP two Hybridization; If it is equal to 4, the central atom adopts SP three Hybridization.

For example, for ethylene, the carbon atom is the central atom, and the number of atoms connected to it is 3. At the same time, the four valence electrons of carbon are all bonded (3 σ Bond plus 1 π bond), so the logarithm of lone pair electrons is zero, so 0+3=3, SP two Hybridization; For example, for hydrogen oxide, the oxygen atom is the central atom, and the number of atoms connected to the oxygen atom is 2. At the same time, there are two pairs of lone electrons remaining in the oxygen atom, so 2+2=4 three Hybridization.

limitations

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To put it bluntly, the original hybrid concept of atomic orbits was completely artificial. To explain CH four --Such phenomena as tetrahedron. later Molecular orbital theory The hybridization of atomic orbitals is naturally explained - just a re linear combination of atomic orbitals with atoms. At the same time, the molecular orbital theory also shows that this combination (hybridization) has no practical significance and can sometimes cause confusion. For example, in the hybrid theory, CH four The eight bonding electrons in the same sp three Orbital energy level. In fact, they are divided into two different energy levels (both experiments and molecular orbital theory show this). However, due to the convenience of hybrid concepts, especially in organic chemistry, it is used to represent the geometric environment of an atom in a molecule. Today, hybrid orbits are only used to describe geometric shapes or environments.

In addition, overemphasizing the other "importance" of hybridization may cause unnecessary "detours" in learning chemistry in the future.

Experimental basis

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Experiments show that methane molecule CH four It is a regular tetrahedron configuration, with a bond angle of 109 ° 28 ', and four C-H bonds are identical. The base configuration of carbon atom C is 1s two 2s two 2p x one 2p y one There are only two unpaired electrons. According to the valence bond theory, the electron spins are antiparallel paired to form bonds. It can only form two C-H bonds and cannot explain CH four The fact that C generates four C-H keys. If a 2s electron is excited to 2p x On track, i.e

C(1s two 2s two 2p x one 2p y )— inspire —→C(1s two 2s one 2p x one 2p y 2p z one )

So there are four unpaired electrons, which can form four C-H bonds, thus explaining CH four The fact that C generates four C-H keys. But we already know that the bonding ability of the p orbital is greater than that of the s orbital, which means that three C-H bonds formed by three p orbitals are stronger than one C-H bond formed by one s orbital, making four C-H bonds different. And the bond angle of the four C-H bonds formed by three mutually perpendicular, spherical s orbitals and H is by no means 109 ° 28 '. Therefore, CH still cannot be explained four Molecular configuration. The only way to solve this problem is to eliminate the difference between p orbitals and s orbitals. For this reason, three p orbitals and one s orbitals must be mixed together to form a new atomic orbit. They point to four vertices of the tetrahedron respectively, and they are all single occupied orbitals. Then they pair with the 1s electron of H to form bonds, which can fully explain CH four Molecular geometry.

This process of mixing different atomic orbitals into new orbitals in an atom is called hybridization of atomic orbitals, and the new atomic orbitals obtained are called hybrid orbitals.

Then we will analyze this problem from the aspect of energy. The energy required for the 2s electron of C to be excited to the 2p orbit is 795kJ · mol -1 , and CH four The bond energy of C-H bond is 413kJ · mol -1 。 That is to say, the energy released from the formation of four C-H bonds can not only compensate for the energy required for electronic excitation, but also make CH four Energy that exists stably. CH four Molecular geometry can be explained and CH four The fact that molecular energy is low and stable provides an experimental basis for hybrid orbital theory.^[1]

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