Bond Polarity - April 13

Covalent bonds are formed by the sharing of electrons between atoms. In covalent bonds, the bonding electrons are shared between the nuclei of the atoms.

However, all covalent bonds are not the same. Their character depends on the kind and nature of the atoms joined together.

There are 2 types of covalent bonds.

1) Non-Polar Covalent Bonds

A non-polar covalent bond between atoms of the same or similar elements, where the sharing of electrons is equal. These are usually diatomic molecules, where an element cannot stably exist on it’s own, so it bonds to another atom of the same element to exist in nature.

Ex: O₂ and H₂ These molecules share the electrons equally, with neither atom using it more than the other.

Non-polar covalent bonds are also called “perfect“ non-polar bonds.

2) Polar Covalent Bonds

A polar covalent bond is a bond where two atoms of different elements share the bonding electrons unequally. In these cases, unequal sharing refers to the electron spending more time in the vicinity of one atom than another.

This unequal sharing is determined by comparing electronegativity, or the ability of an atom to draw electrons to itself. Electronegativity increases from left to right on the periodic table, and decreases from top to bottom. Based on this periodic trend, the most electronegative element is fluorine, since it has the ability to pick up electrons easily, and hold on to them strongly. An element such as cesium has a low electronegativity.

Due to the unequal sharing, the electrons will spend more time around the atom with the highest electronegativity in the bond, and less time around the atom with the least electronegativity. This results in one atom acquiring a slightly negative charge, while the other acquires a slight positive charge. There are no cases so extreme that one atom entirely possesses the electron and forms an ion, because that is a permanent charge. The charges that the atoms acquire are always less than a whole positive or a whole negative.

Ex: Hydrogen Fluoride (HF) This represents an electrostatic map of the Hydrogen Fluoride molecule. The electron distribution varies according to the colours of the rainbow. The most electron-rich region is on the right; the most electron-poor region is on the left.

How to Calculate Polarity

A polar molecule will always contain polar bonds, but can also contain some non-polar bonds. A non-polar molecule can have only polar bonds, which can cancel each other out to become non-polar in special cases.

In a polar molecule, one end of the molecule is slightly negative, and one end is slightly positive. This creates electrically charged regions or poles. When a molecule has two poles it is known as a dipole.

By examining the electronegativity values of the atoms and calculating the difference between them, the type of bond and polarity can be determined.

The difference in electronegativity between atoms of a molecule is calculated by subtracting the electronegativity value of the terminal atom from the central atom. This equation is written as ΔΣN, or “change in value of electronegativity“.

Examples:

N + H → 3.04 - 2.20 = 0.84 →ΔΣN = 0.4 → polar covalent molecule

F + F →3.98 - 3.98 = 0 → ΔΣN = 0 → perfect non-polar molecule

Ca + O → 3.44 - 1.00 = 2.44 → ΔΣN = 2.44 →ionic molecule

It is also important to note that electronegativity values are scalar, so they do not have unit, and cannot be negative, even if a larger value is subtracted from a smaller one.