A water molecule consists of one oxygen atom and two hydrogen atoms bonded together by covalent bonds.
Covalent bonds form when atoms share electrons.
However, in water, the sharing is unequal because oxygen is more electronegative than hydrogen.
Definition
Electronegativity
Electronegativity is an atom's ability to attract electrons in a bond.
Unequal Sharing Creates Polarity
Because oxygen pulls electrons closer:
The oxygen atom develops a partial negative charge (δ−).
The hydrogen atoms develop partial positive charges (δ+).
This uneven charge distribution makes water a polar molecule, with a positive end (hydrogen side) and a negative end (oxygen side).
Polarity Drives Hydrogen Bond Formation
This polarity is what allows hydrogen bonds to form between water molecules:
The δ+ hydrogen of one molecule is electrostatically attracted to the δ− oxygen of a neighbouring molecule.
Because opposite charges attract, these polar regions create weak intermolecular attractions.
These attractions are hydrogen bonds.
Note
Although a single hydrogen bond is weak, water molecules are small and numerous.
This allows for a dense network of these bonds.
Tip
Polarity from unequal electron sharing → partial charges → electrostatic attraction between molecules → hydrogen bonding.
Analogy
Think of a game of tug-of-war between two players of unequal strength.
The stronger player pulls the rope closer to their side.
Drawing Water Molecules in Exams
To show the structure and polarity of water:
Draw the Atoms:
Place one oxygen atom (O) in the centre.
Add two hydrogen atoms (H) on either side, at an angle of approximately 104.5°, reflecting the molecule's bent shape.
Show the Bonds:
Draw a single covalent bond (-) between each hydrogen atom and the oxygen atom, representing shared electrons.
Indicate Partial Charges:
Label the oxygen atom with a partial negative charge (δ⁻) because it is more electronegative and pulls shared electrons closer.
Label each hydrogen atom with a partial positive charge (δ⁺) because electrons are drawn away from them.
Show Hydrogen Bonding Between Molecules:
Draw a second water molecule next to the first, also labelled with δ⁺ hydrogens and a δ⁻ oxygen.
Line up a δ⁺ hydrogen of one molecule with the δ⁻ oxygen of the other, because opposite charges attract.
Mark the hydrogen bond between them as a dashed or dotted line, to distinguish it from the solid covalent bonds.
Exam technique
Always include partial charges (δ+ and δ−) when drawing water molecules to show polarity.
When a question asks about hydrogen bonding, draw at least two molecules and show each hydrogen bond as a dashed line between a δ+ hydrogen and a δ− oxygen.
Active recall
Why does oxygen have a partial negative charge in water?
What does it mean for a molecule to be polar?
How does polarity lead to hydrogen bonding between water molecules?
Draw a water molecule showing its bent shape and partial charges.
Draw two water molecules and show a hydrogen bond between them, with the partial charges labelled.
What is a hydrogen bond and between which atoms does it form in water?
