Medium

Polarity of Covalent Bonds

Simple Explanation

A covalent bond is nonpolar when both atoms share electrons equally (identical or very similar electronegativity), and polar when one atom pulls the shared electrons closer to itself, giving it a slightly negative charge (δ−) and the other atom a slightly positive charge (δ+).

Why Do We Need It?

Polarity is what gives molecules like water their unusual properties — high boiling point, ability to dissolve salts, surface tension — and is the reason 'like dissolves like': polar substances mix well with other polar substances, and nonpolar substances mix well with other nonpolar substances.

Formula

Electronegativity Difference

ΔEN = |EN₁ − EN₂|

How unequally two bonded atoms share their electrons — the bigger the difference, the more one-sided the bond.

ΔEN: — electronegativity difference between the two bonded atoms (no units — the Pauling scale is a relative scale)
EN₁, EN₂: — the Pauling electronegativity values of each atom in the bond

When to use it: Whenever you need to predict what type of bond forms between two atoms: roughly ΔEN < 0.4 is nonpolar covalent, 0.4–1.7 is polar covalent, and ΔEN > 1.7 is ionic.

Worked Example

Determine whether an H–F bond is polar

Hydrogen has an electronegativity of 2.20; fluorine has 3.98. Is the H–F bond polar or nonpolar, and which atom is δ−?

Why Does This Work?

When one atom is more electronegative, the shared electron cloud spends more time closer to that atom than to the other — this uneven distribution of negative charge creates a small negative region around the more electronegative atom and a small positive region around the less electronegative one, called a dipole.

Real-Life Example

Why water beads up on a waxed car

Water forms round beads on a freshly waxed car surface instead of spreading out flat.

Water's polar O–H bonds make water molecules attract each other strongly, while wax is made of nonpolar molecules that water's polar molecules are not attracted to — so instead of spreading across the nonpolar wax, water molecules cling to each other and minimize contact with it, forming beads.

Practice

In a C=O bond, oxygen (EN 3.44) is more electronegative than carbon (EN 2.55). Which atom carries the δ− partial charge?

Medium

Common mistake

Confusing a polar bond with a polar molecule — a molecule can have polar bonds but still be nonpolar overall if its shape makes the individual bond dipoles cancel out (like CO₂, which has two polar C=O bonds but is a linear, nonpolar molecule overall).

Quick Review

Polar bonds form when bonded atoms have significantly different electronegativities.
The more electronegative atom becomes δ−; the other becomes δ+.
A molecule's overall polarity also depends on its shape, not just its individual bonds.