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6.
3.
1 Molecule polarity and dipole moment
1.
The polarity of the molecule
Cl 2 is a non-polar molecule because the two atoms that make up the molecule belong to the same element, that is, the difference in electronegativity between the two atoms is 0, and the covalent bond formed is non-polar
.
The HCl molecule is a polar molecule, because the electronegativity of the two atoms constituting the molecule is different, and the covalent bond formed is polar
From the examples of Cl 2 , HC1 and CO 2 molecules, it can be seen that polar molecules must be derived from the polarity of covalent bonds, while covalent bonds in non-polar molecules are not necessarily non-polar
.
Obviously, the polarity of the bond is related to the charge distribution of the two atoms forming the bond, and whether the molecule has polarity is related to the charge distribution in the molecule
.
Molecules where the center of gravity of positive charge and the center of gravity of negative charge coincide are non-polar molecules, such as H 2 , Cl 2 , CO 2 , SO 3 , BCl 3 , CH 4 , PF 5 , SF 6 ; the center of gravity of positive charge does not coincide with that of negative charge The molecules of are polar molecules, such as HCl, H 2 O, SO 2 , O 3 , NH 3 , CHCl 3 , SF 4
2.
Dipole moment
The polarity of polar molecules can be measured by the dipole moment
.
u=d·q
In the formula, d is the distance between the center of gravity of the positive charge and the center of gravity of the negative charge; q is the charge of the center of gravity of the positive charge and the center of gravity of the negative charge
.
The unit of the dipole moment u is represented by "Debye" (D), 1D=3.
The dipole moment is a vector.
The dipole moment of a diatomic molecule is the bond moment of a covalent bond, and the dipole moment of a polyatomic molecule is the vector sum of the covalent bond moments in the molecule
.
If the bond moments of the molecules with polar bonds cancel each other, that is, the vector sum of the bond moments is 0, the gravity center of the positive charge and the negative charge of the molecule coincide, such as CO 2 , SO 3 , BCl 3, etc.
The existence of lone electron pairs and the formation of delocalized π bonds sometimes also affect the dipole moment of molecules.
For example, the dipole moment of NH 3 molecules is larger and the dipole moment of NF 3 molecules is smaller
.
In the NH 3 molecule, the bonding electron pair is close to the side of the N atom, and the lone electron pair on the N atom has the same direction as the dipole moment of the NH bonding electron pair, which increases the dipole moment; in the NF 3 molecule, the bonding electron On the side close to the F atom, the dipole moment of the lone electron pair on the N atom is inconsistent with the NF bonding electron pair, which reduces the dipole moment, as shown in Figure 6-36
Figure 6-36 The influence of lone electrons on molecular dipole moment
The center of gravity of the positive charge and the center of gravity of the negative charge of polar molecules do not coincide, and there is always a dipole moment.
Therefore, the dipole moment of a polar molecule is called a permanent dipole, or an inherent dipole
Under the action of an external electric field, the center of gravity of the positive charge and the center of the negative charge of non-polar molecules no longer overlap, resulting in a dipole; similarly, under the action of an external electric field, the dipole moment of polar molecules will increase
.
The dipole produced under the action of an external electric field is called an induced dipole, which is represented by △ u , as shown in Figure 6-37
Figure 6-37 Induced dipole generation
For non-polar molecules and polar molecules, the relative position of the nucleus and electrons changes instantaneously due to motion, collision, etc.
, and the instantaneous positive and negative charge centers of gravity do not coincide to produce dipoles, which are called instant dipoles