Build a Water Molecule

Why it matters

Imagine snapping two hydrogen atoms onto an oxygen atom like Lego bricks. The result isn't just a pile of atoms, but a distinct molecule with its own identity.

Predict first

How many hydrogen atoms are needed to bond with one oxygen atom to form a stable water molecule?

Oxygen has a valency of 2, meaning it needs two electrons to complete its outer shell. Each hydrogen provides one, so two hydrogens are required.

• One hydrogen atom (HO)
• Two hydrogen atoms (H₂O) — correct
• Three hydrogen atoms (H₃O)
• Four hydrogen atoms (H₄O)

What you do

Build water: oxygen needs 2 more electrons and each hydrogen needs 1. Share pairs until every atom's outer shell is full — two single bonds, one bent little molecule.

Check yourself

Why is water (H₂O) considered a compound and not a mixture?

In a compound, elements are chemically combined in fixed proportions. In a mixture, they retain their individual properties and can be separated physically.

What is the molecular formula of water?

Two hydrogens bonded to one oxygen: H₂O. H₂O₂ is hydrogen peroxide — a different compound with different properties.

How does oxygen hold on to its two hydrogens in a water molecule?

Oxygen shares one electron pair with each hydrogen: two shared pairs, two covalent bonds, one stable H₂O.

• It can be separated by filtering
• Hydrogen and oxygen are chemically bonded in a fixed ratio — correct
• It is a liquid at room temperature
• HO
• H₂O — correct
• H₃O
• H₂O₂
• Each O–H link is a shared pair of electrons — a covalent bond — correct
• The atoms are stuck together by trapped air
• Oxygen absorbs the hydrogen atoms so they vanish