Polar Covalent bond

A polar bond is a chemical bond representing an intermediate between a purely covalent bond and an ionic bond. More precisely, a polar chemical bond exists between two atoms with an uneven distribution of shared electrons.

1.0What is Polar Covalent Bond?

A polar bond is a chemical bond between a purely covalent bond and an ionic bond. It occurs between two atoms with unevenly distributed electrons, resulting in the molecule having an electrical dipole moment, with one end slightly positive and the other slightly negative.

Electronegativity indeed plays a crucial role in determining the nature of covalent bonds. This dimensionless quantity denotes an atom's propensity to attract a shared pair of electrons towards itself. A polar chemical bond arises when there's a notable difference in electronegativities between the two atoms.

2.0Polarity of Bonds

The existence of a wholly ionic or covalent bond represents an ideal situation. In reality, no bond or compound is purely covalent or purely ionic. Even in a covalent bond between two hydrogen atoms, there is some ionic character.

Nonpolar Covalent Bonds

A covalent bond is formed when similar atoms, such as in H₂, O₂, Cl₂, N₂, or F₂, the shared pair of electrons are equally attracted by the two atoms. As a result, the electron pair is situated precisely between the two identical nuclei. The bond formed in such a case is called a Nonpolar Covalent Bond.

Polar Covalent Bonds

In a heteronuclear molecule such as HF, the shared electron pair between the two atoms gets displaced more towards the fluorine atom due to the significantly higher electronegativity of fluorine compared to hydrogen. Consequently, the resulting covalent bond in such a scenario is a polar covalent bond.

Dipole moment 

A molecule possesses a dipole moment as a result of polarization. The dipole moment is defined as the product of the magnitude of the charge and the distance between the centers of positive and negative charges. 

It is usually designated by the Greek letter 'µ'. Mathematically, it is expressed as follows:

Dipole moment (μ )= charge (Q)  ×  distance of separation(r)

Dipole moment is typically expressed in Debye units (D). The conversion factor is:

1 D=3.33564×10⁻³⁰C⋅m

where C is coulomb and m is meter.

3.0What determines the polarity of a bond?

The difference in electronegativity between the atoms primarily determines the bond polarity. Simultaneously, the molecular geometry and the distribution of electron density within the molecule also impact the overall molecular polarity. Here are the key factors:

• Electronegativity Difference: When there is a notable difference in electronegativity between the two bonded atoms, the bond becomes more polar. In such scenarios, one atom will attract the shared electrons more strongly, resulting in a partial negative charge on that atom and a partial positive charge on the other.
• Molecular Geometry: The shape of the molecule also affects the overall polarity. Even if individual bonds are polar, the molecular geometry can lead to an overall nonpolar molecule if the bond polarities cancel each other out due to symmetry.
• Bond Dipole Moments: The bond dipole moment, which is a vector quantity, depends on both the magnitude of the partial charges and the distance between them. The overall dipole moment of a molecule is the vector sum of its bond dipole moments.

• Presence of Lone Pairs: Lone pairs of electrons on the central atom can influence the polarity by affecting the molecular geometry and electron density distribution.

4.0Properties of polar covalent compounds

These properties are intrinsic to the nature of polar covalent bonds and their molecular interactions.

• Physical State: Often solid due to strong intermolecular forces.
• Melting and Boiling Points: Higher than nonpolar compounds due to stronger intermolecular attractions.
• Conductivity: Conduct electricity in solution because of ion mobility.
• Solubility: Highly soluble in polar solvents like water.

5.0Water is a Polar Covalent Molecule

• Water (H₂O), similar to hydrogen fluoride (HF), is a polar covalent molecule. Upon examining a diagram of water, it's clear that the two hydrogen atoms are not symmetrically arranged around the oxygen atom.
• This asymmetrical distribution and the unequal sharing of electrons result in the water molecule having two poles. Hydrogen atoms possess a partial positive charge, while oxygen atoms carry a partial negative charge. 
• Consequently, the water molecule is electrically polar, with a positive pole on the side with the hydrogen atoms and a negative pole on the side with the oxygen atom.

NOTE: Water molecules are like tiny magnets with positive and negative ends. This polarity is essential for many of water's distinctive properties, including its high surface tension, capability to dissolve numerous substances (which earns it the title of "universal solvent"), and relatively elevated boiling and melting points compared to molecules of similar size.

6.0Examples of Polar Covalent Bonds

Below are some instances of polar covalent bonds, accompanied by the disparity in electronegativity between the two bonded atoms.

• Methanol (CH₃OH) is a polar molecule due to the presence of the hydroxyl group, which creates an asymmetrical shape and a significant dipole moment. The -OH side is different from the other three -H sides, contributing to the molecule's overall polarity and resulting in properties like high solubility in water and relatively high boiling and melting points.
• Hydrogen Bromide (HBr): Hydrogen bromide (HBr) has a polar covalent bond due to the difference in electronegativity between hydrogen (H) and bromine (Br). Although this difference is not extremely high, it is sufficient to create a partial negative charge on the bromine atom (δ-) and a partial positive charge on the hydrogen atom (δ+). The electron pair in the bond is more attracted to the bromine, resulting in polarization.
• Ammonia (NH₃): Ammonia is a polar molecule with nitrogen (N) as the central atom bonded to three hydrogen (H) atoms. Nitrogen is more electronegative than hydrogen, causing the electrons to be more concentrated around the nitrogen atom. This results in a partial negative charge on the nitrogen and partial positive charges on the hydrogen atoms.
• Sulfur dioxide (SO₂) is a polar molecule due to its bent molecular shape and the significant difference in electronegativity between sulfur and oxygen.The dipole moments of the individual S-O bonds do not cancel out, resulting in a net dipole moment and making the molecule polar.This polarity influences its solubility in water, intermolecular interactions, and physical properties.