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Bonds & Polar Molecules

Polarity of Bonds and Polar Molecules

Polarity of bonds and polar molecules are formed by the asymmetric charge distribution on chemical bonds like water, hydrogen chloride, ammonia, nitrobenzene, alcohol, etc. Polar and non-polar molecules are composed of positively charged nuclei and negatively charged electron particles distributed in space. The electronic arrangement of these particles is different for different polar or nonpolar molecules. The calculation of bond polarity for learning chemistry is given from the dipole moment chart for polar and non-polar molecules. Therefore, the dipole moment is an important tool for the structure determination of polar or nonpolar molecules. When the center of gravity of positive and negative charges coincide, the molecules are called non-polar. Hence the polarity or dipole moment of hydrogen, chlorine, carbon dioxide, methane, benzene, etc are zero.

On the other hand, hydrogen chloride, chloromethane, chloroform, nitrobenzene, water, ammonia, etc, are polar molecules but the polarity of the individual chemical bonding may be present or absent. The electric polarization of chemical substances arises when non-polar molecules are placed between positive and negative plates of the electric field. Therefore, the polarity of the molecule is quantitatively expressed by the term dipole moment which helps to determine the molecular size and shape, spatial arrangement, bond polarity, bond energy, and residue charge on the atoms.

The polarity of bonds in polar and nonpolar molecules like water and carbon dioxide in chemistry

Non-polar & Polar Chemical Bonds in Molecules

Binary homonuclear diatomic molecules are non-polar or polarity equal to zero because the bonding chemical elements possess the same electronegativity. Therefore, the absence of polarity in the homonuclear diatomic molecules like hydrogen, nitrogen, oxygen, chlorine, bromine, iodine, etc due to bonding electrons in these molecules shared equally by the two nucleus. These types of bonds are called non-polar chemical bonds.

According to the above rule heteronuclear diatomic molecules show polarity due to their electronegativity and ionization energy differences. But electronegativity difference not only the tools to determine the polarity of the molecule. Therefore, to decide the polarity of heteronuclear molecules need to learn the composition and geometry. Hence carbon dioxide, carbon disulfide, beryllium chloride phosphorus pentachloride, etc are non-polar but the bonding atoms of these molecules have different electronegativity and electron affinity.

Meaning of Polarity in Chemistry

When the center of gravity of the positive charge does not coincide with the center of gravity of the negative charge like a water molecule, polarity arises in the polar water molecule. Therefore, polar covalent molecules like Hydrogen chloride, water, ammonia, and chlorobenzene, etc show polarity in chemistry.

Beryllium chloride is linear with the bond angle 180° but the water molecule is non-liner with the bond angle 104°. The water molecule is formed by the overlap of two sp3-hybrid orbitals of an oxygen atom with two 1s-orbitals of the hydrogen atom. The excited-state electronic configuration of oxygen, 2s2 2px2 2py1 2pz1. Therefore, the polarization of water molecules arises from the two contributions, form the bond polarity or electronegativity difference and from the lone pairs.

Dipole Moment Calculation

Calculation of dipole moment and polarity given by the product of charge and distance of the separation or bond length. When +q amount of positive charge separates by -q of negative charge and l is the distance between two centers of the molecule,  dipole moment, µ = q × l. Therefore, perfectly non-polar substances show zero dipole moment or polarity and polar molecules have positive values. Therefore the calculation of polarity data quite helpful in the interpretation of the behavior of acid base solutions like pH scale, explanation of solubility, and the influence of solvents in the chemical kinetics rate of the chemical reactions.

The charge of an electron = 4.7 × 10-10 esu and the distance between the center is of order 10-8 cm. Hence the calculation of dipole moment order = 10-10 × 10-8 = 10-18 esu = 1 Debye. Therefore, 1Debye = 10-18 esu cm = 3.336 × 10-30 coulomb meter.

Dimension of the Dipole Moment

Unit of µ = unit of charge × unit of length. Therefore the CGS unit of dipole moment = esu × cm. According to Coulomb’s Law, F = q1q1/Dr2. Therefore, (esu)2 = dyne × cm2 = gm cm sec-2 × cm2
Hence, the dimensions of dipole moment(μ) M1/2 L5/2 T-1.

Question: P-F, S-F, Cl-F, and F-F which of the following chemical bonds shows the lowest polarity?
Answer: The electronegativity or electron affinity difference between the two fluorine atoms equal to zero. Therefore, the dipole moment of the fluorine among these molecules is zero.

Polarity Chart of Polar Molecules

In hydrochloric acid, due to the greater electronegativity or electron affinity of the chlorine atom, the bonding electron pair shifted towards the chlorine atom and molecules show polarity. Therefore, the chlorine atom acquires a small negative charge and the hydrogen atom acquires a small positive charge. Hence the dipole moment chart of these polar molecules suggested the polarity of the hydrogen chlorine chemical bonds.

Polar or nonpolar MoleculeDipole moment
Carbon dioxide0 Debye
Carbon disulfide0 Debye
Carbon tetrachloride0 Debye
Boron trichloride0 Debye
Water1.85 Debye
Hydrogen sulfide1.10 Debye
Ammonia1.46 Debye
Nitrogen trifluoride0.20 Debye

Polarity of Organic Hydrocarbons

All the saturated hydrocarbons of our environment like methane, ethane, propane, butane, etc are found to be non-polar or zero polarity. The non-polar character of methane easily understands from the symmetrical tetrahedral structure with the carbon atom at the center and four identical valencies making an angle of 109.5°. Therefore, the absence of polarity in methane molecules implies, the carbon-hydrogen bond moment balancing the net moment contributed by the remaining three carbon hydrogen bonding.

The experimental calculation of the bond moment or polarity of the alkyl group = 0.04 debye equal to the bond moment of the carbon-hydrogen bond. Not only alkyl groups but the radicals have also definite chemical bonding moments called group moments. Therefore, when a hydrogen atom in a different saturated hydrocarbon is replaced by a hydroxyl group to produces homologous alcohols, all of the latter should have the same polarity but the molecules are polar.