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A water molecule (H₂O) can form a maximum of four hydrogen bonds. This is because of the molecular structure of water and the presence of polar covalent bonds within the molecule.
Each water molecule contains two hydrogen atoms covalently bonded to a highly electronegative oxygen atom. Due to oxygen’s high electronegativity, the O–H bonds are polar, meaning that oxygen acquires a partial negative charge (δ⁻), while hydrogen atoms carry partial positive charges (δ⁺). This polarity makes water molecules strongly attracted to each other through hydrogen bonding.
Two hydrogen atoms in water can each form one hydrogen bond with the oxygen atoms of neighboring water molecules.
Oxygen atom in water has two lone pairs of electrons, and each lone pair can form one hydrogen bond with a hydrogen atom from another molecule.
Thus, in total, one H₂O molecule can act as a hydrogen bond donor twice (via its hydrogens) and as a hydrogen bond acceptor twice (via its lone pairs), giving a maximum of 4 hydrogen bonds.
Why is this important?
The ability of water molecules to form up to four hydrogen bonds makes water unique compared to many other substances. These hydrogen bonds are responsible for:
High boiling point and melting point compared to other hydrides.
High surface tension and capillarity.
Ice being less dense than liquid water (open hexagonal structure in ice due to hydrogen bonding).
Excellent solvent properties for polar and ionic compounds.
Why not the other options?
2 or 3 bonds: Possible in some cases, but the maximum theoretical hydrogen bonds are 4.
None: Incorrect, as hydrogen bonding is the most significant intermolecular force in water.
Key fact:
This extensive hydrogen bonding network is the reason why life on Earth depends so heavily on water’s unique physical and chemical properties.
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