Why are water molecules polar, and how do hydrogen bonds form between them

Sourav PanNovember 7, 2024

Answered

Water molecules exhibit polarity due to the arrangement of their atoms and the differences in electronegativity between oxygen and hydrogen. Each water molecule (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms, forming a bent shape. This geometry is crucial because it leads to an uneven distribution of charge within the molecule.

Why Water Molecules Are Polar

1. Electronegativity Difference: Oxygen is significantly more electronegative than hydrogen, with values of approximately 3.5 for oxygen and 2.1 for hydrogen on the Pauling scale. This difference (about 1.4) means that the shared electrons in the O-H bonds are pulled closer to the oxygen atom, creating a partial negative charge (δ−δ−) on the oxygen and a partial positive charge (δ+δ+) on the hydrogen atoms.
2. Molecular Geometry: The bent shape of the water molecule (approximately 104.5 degrees) prevents the charges from cancelling out, leading to a net dipole moment. This results in one end of the molecule being more negative (the oxygen side) and the other end being more positive (the hydrogen side), thus making water a polar molecule.

Formation of Hydrogen Bonds

Hydrogen bonds are weak attractions that occur between water molecules due to their polarity:

• Attraction Between Charges: The partially positive hydrogen atom of one water molecule is attracted to the partially negative oxygen atom of another water molecule. This creates a hydrogen bond, which is significantly weaker than covalent bonds but plays a crucial role in determining the properties of water.
• Hydrogen Bonding Dynamics: Each water molecule can form up to four hydrogen bonds with neighboring molecules—two through its hydrogen atoms and two through its lone pairs of electrons on oxygen. This network of hydrogen bonds contributes to many unique properties of water, such as high surface tension, boiling point, and cohesive behavior