Valence bond theory and the directionality and saturation of covalent bonds

German chemists W.
Heitler and F.
London used quantum mechanics to treat H ₂ molecules in 1927 and successfully explained the problem of bonding between two H atoms, which was later established by Pauling and others.
Modern valence bond theory
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1.
The chemical bond in the hydrogen molecule

Quantum mechanics calculations show that when two H atoms with a 1S ¹ electron configuration are close to each other, the 1s electrons of the two H form electron pairs with opposite spins, and the energy of the system is reduced
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If the 1s electrons of two H atoms form an electron pair with the same spin, the energy of the system increases

The chemical bond in the H ₂ molecule is the pairing of electrons with opposite spin directions to reduce the energy of the system
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When two H atoms with opposite spin directions approach, the atomic orbits of the two electrons can partially overlap, and the probability of electrons appearing between the two nuclei is large, forming a negatively charged region.

Figure 6-6 Schematic diagram of electron cloud overlap in H2 molecule

2.

Valence bond theory

The results of the processing of H ₂ molecules are extended to other molecules, forming a valence bond theory based on quantum mechanics (VB method for short)
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1) The formation of covalent bonds

If two atoms each have an unpaired electron, the orbital symmetry of the two single electrons can overlap each other.
The electrons are paired with opposite spins, the energy of the system is reduced, and a covalent bond is formed
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A pair of electrons forms a covalent bond

In the H ₂ O molecule, O and 2 H form 2 covalent bonds
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The electronic configuration of O is 2s ² 2p ⁴ , and the 2p orbital has 2 single electrons, each of which forms a bond with 1 H single electron

There are double bonds in the O ₂ molecule
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Because the 2p orbital of O has two single electrons, two covalent bonds are formed between two O atoms

There are triple bonds in the N ₂ molecule
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The electronic configuration of N is 2s ² 2p ³ , the 2p orbital has 3 single electrons, and 3 covalent bonds are formed between two N atoms

When forming a covalent bond, a single electron can be obtained by disassembling a pair of electrons
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For example, in the CH ₄ molecule, the valence electron configuration of C is 2s ² 2p ² , and there are only 2 single electrons; if one electron in the 2s orbital is excited to the 2p orbital, the excited C atom has 4 single electrons (2s ¹ 2p ³ ), as shown in Figure 6-7

Figure 6-7 Excitation of electrons in carbon atoms

When the PCl ₅ molecule is formed , one electron in the 3s orbital of P is excited to the 3d orbital, so that P has 5 single electrons and forms a covalent bond with 5 Cl to form a PCl ₅ molecule (Figure 6-8)
.

Figure 6-8 Excitation of electrons in phosphorus atoms

2) The covalent bond has directionality and saturation

The atomic orbital distribution has directionality.
In order to make the degree of orbital overlap large, the molecular energy is low.
When the orbitals overlap, they can only overlap in a specific direction, so the covalent bond formed has directionality
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In the HCl molecule, when the 1s orbital of H overlaps with the 3p orbital of Cl with a single electron, H and Cl must approach along the direction of the bond axis where the 3p orbital is located to ensure maximum overlap
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At the same time, in order to maintain the same symmetry, H must overlap from the positive direction of the 3p orbital of Cl
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As shown in Figure 6-9(a)
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In the Cl ₂ molecule, when the 3p orbitals of two Cl overlap, they must approach along the direction of the bond axis where the 3p orbitals are located to ensure maximum overlap
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At the same time, in order to maintain the same symmetry, the 3p orbitals of the two Cls must overlap in the positive direction
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As shown in Figure 6-9(b)
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Figure 6-9 The directionality of covalent bonds

Since the symmetry is not the same, the overlap shown in Figure 6-10 is invalid
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Figure 6-10 Invalid overlap with inconsistent symmetry

The number of covalent bonds is determined by the single electron number of the atom (including the single electron formed by excitation).
Due to the limitation of the single electron number of the atom, the number of covalent bonds formed is limited
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Therefore, the covalent bond is saturated
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For example, the electron configuration of the N valence layer is 2s ² 2p ³ , the valence layer has no d orbital, only 3 single electrons of the p orbital, and can only form 3 covalent bonds at most, such as the formation of NH ₃ , NCl _3, etc.
NCl5 is formed
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The element P of the same family as N, the electron configuration of the valence layer is 3s ² 3p ³ , and the valence layer has a 3d empty orbital.
It is possible to use 3 single electrons of the p orbital to form 3 covalent bond compounds, such as PH ₃ , PCl ₃ And so on; can also excite 3s electrons from 1 to 3d orbitals to form compounds with 5 covalent bonds, such as PCl ₅ and so on
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The P atom can form up to 5 covalent bonds, because its 2s and 2p orbital electrons are difficult to excite to the 3d orbital
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The electron pair shared by the covalent bond can also be provided by one of the two atoms forming the bond
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For example, for the triple bond in CO molecules, the third covalent bond is formed by O atoms providing electron pairs, and C atoms providing empty orbitals
.
This kind of covalent bond is called covalent coordination bond, or coordination bond for short
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