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The connecting line between the two nuclei that form a bond is called the bond axis.
According to the symmetry relationship between the bond axis and the bond orbital, covalent bonds are mainly divided into
.
Two key types: key and x key (uncommon delta key is not introduced in this chapter)
Rotate the bonding orbit through the key shaft at any angle, and the graphics and symbols remain unchanged, it is the σ key, and the key shaft of the σ key is the infinite multiple axis of the bonding orbit
.
The σ bond can be visualized as the "head-to-head" overlap of orbitals, such as those formed in both HCl and Cl 2 molecules
Rotate the bond orbit around the key axis by 180°, the graph is restored but the sign of the orbit is reversed, it is a π bond
.
The π bond can be visualized as the "side-by-side" overlap of orbitals
4) Key parameters
The characteristics of covalent bonds are often described by physical quantities such as bond energy, bond length, and bond angle.
These physical quantities are called bond parameters
.
The most important one is the bond angle, which determines the geometric configuration of the molecule
Figure 6-11 The overlap mode of the π bond and its relationship with the key axis
For diatomic molecules, the bond energy is the energy required for the dissociation of gaseous molecules into gaseous atoms, that is, the bond energy E is equal to the dissociation energy D of the molecule
.
ABK(g)=A(g)+B(g) △H=-D AB =E AB
But for polyatomic molecules, the bond energy is equal to the average dissociation energy of the molecule
.
For example, the dissociation of H2O molecules
H 2 O(g)=H(g)+HO(g) D 1 =△H 1
HO(g)=H(g)+O(g) D 2 =△H 2
Then the bond energy of the OH bond in the H 2 O molecule is
[Example 6-3] Calculate the bond energy of the NH bond in the NH 3 molecule from the following NH 3 dissociation reaction .
NH 3 (g)=H(g)+NH 2 (g) D 1 =435kJ·mol -1
NH 2 (g)=H(g)+NH(g) D 2 =377kJ·mol -1
NH(g)=H(g)+N(g) D 3 =314kJ·mol -1
Debonding energy is the average dissociation energy for gaseous molecules to break 1 mol of a certain chemical bond
.
There are 3 NH bonds in the NH 3 molecule, then the bond energy of the NH bond is the average of the dissociation energies of disconnecting the 3 NH bonds, namely
The bond length is the distance between the two atomic nuclei that form the bond in the molecule
.
Generally speaking, the shorter the bond length, the stronger the covalent bond and the greater the bond energy
It should be noted that the same bond has different bond length and bond energy in different compounds.
For example, the bond length and bond energy of the CH bond in CH 4 and CH 3 OH are different
.
The bond angle refers to the angle between the bond axis and the bond axis in the molecule
.
It is only in polyatomic molecules that the bond angle is involved, and the bond angle determines the geometric configuration of the molecule
.
For example, CO.
'S 2 molecule OCO bond angle is 180 °, determines the CO.
'S 2 molecular structure is linear; NO 2 molecule ONO bond angle of 134 °, determines the NO 2 molecular configuration is V-shaped; SO .
3 molecule The OSO bond angle is 120°, which determines that the SO 3 molecular configuration is equilateral triangle
.
Related Links: The Theory of Valence Bonds and the Directionality and Saturation of Covalent Bonds
