A 100% ionic bond or a 100% covalent bond represents an ideal situation. However, in reality, no bond is completely ionic or completely covalent. Even in covalent bonds (e.g., $H_2$), there is always some ionic character.
What is a Non-Polar Covalent Bond?
When a covalent bond is formed between two similar atoms, the shared pair of electrons is equally attracted by both nuclei.
The electron pair is exactly midway between the two nuclei.
Examples: $H_2$, $O_2$, $N_2$, $Cl_2$, $F_2$
Such a bond is called a non-polar covalent bond.
What is a Polar Covalent Bond?
When covalent bonds form between different atoms, their electronegativities differ.
The shared pair of electrons is displaced towards the more electronegative atom.
Example: Hydrogen chloride ($HCl$)
Electronegativity of $Cl = 3.0$
Electronegativity of $H = 2.1$
The bonding pair shifts towards chlorine.
As a result:
$Cl$ acquires a partial negative charge ($\delta^-$)
$H$ acquires a partial positive charge ($\delta^+$)
This creates two poles in the molecule, making it a polar molecule. The bond is called a polar covalent bond.
What Determines the Degree of Polarity?
The magnitude of electronegativity difference reflects the degree of polarity.
Greater the difference in electronegativity → greater charge separation → stronger polarity.
Examples of polar molecules: $BrCl$, $H_2O$, $HF$, $HCl$
