How Many Atoms?

Why it matters

A single ball of gas, a pair holding hands, a ring of three — molecules come in different sizes! The number of atoms in one molecule is called its atomicity.

Predict first

How many atoms are joined together in one molecule of ozone (O₃)?

The small number after the symbol tells the atomicity. O₃ means three oxygen atoms in one molecule, so ozone is triatomic.

• 1 atom
• 2 atoms
• 3 atoms — correct
• 4 atoms

What you do

Drag each substance into the bin that matches its atomicity — the number of atoms in one of its molecules.

Check yourself

What does the 'atomicity' of a substance tell you?

Atomicity is simply how many atoms are combined in a single molecule — 1 for helium, 2 for oxygen (O₂), 3 for ozone (O₃), and so on.

Noble gases like helium, neon and argon exist as single atoms. What is their atomicity?

Noble gases are so stable that their atoms do not join with others. Each molecule is a single atom, so they are monoatomic.

Oxygen (O₂) and ozone (O₃) are both made only of oxygen. How do they differ?

Both contain only oxygen atoms, but a molecule of O₂ has two atoms while a molecule of O₃ has three. Same element, different atomicity.

• The number of atoms present in one molecule of the substance — correct
• The mass of one atom
• The number of molecules in a container
• The colour of the substance
• Monoatomic (1 atom) — correct
• Diatomic (2 atoms)
• Triatomic (3 atoms)
• Polyatomic (many atoms)
• They have different atomicity — O₂ is diatomic and O₃ is triatomic — correct
• They are made of different elements
• O₂ is a compound and O₃ is an element
• They have exactly the same number of atoms