Counting Atoms
Monatomic, diatomic, and polyatomic molecules
What this lesson covers
Why it matters
Imagine a lonely helium atom drifting through space, completely content on its own. Now picture two oxygen atoms holding hands tightly, refusing to let go.
Predict first
Why does oxygen exist as O₂ while helium stays as He?
Helium already has a full, stable outer shell, so it is happy alone — it stays monatomic (He). Oxygen's outer shell is not full, so two atoms pair up and share electrons to become stable, which is why oxygen exists as diatomic O₂.
- Oxygen atoms are too unstable alone and need a partner to survive — correct
- Helium is too heavy to bond with other atoms
- Oxygen is a compound made of two different elements
- Helium atoms repel each other because they have the same charge
What you do
Build a single nitrogen atom — 7 protons, 7 neutrons and 7 electrons (mass number 14). In nature two of them lock together as N₂ — an atomicity of 2.
Check yourself
What is the atomicity of a Sulphur molecule (S₈)?
The subscript 8 indicates eight sulphur atoms are bonded together in one molecule.
Which of these is a triatomic molecule?
Ozone consists of three oxygen atoms bonded together, making it triatomic.
- 1
- 2
- 8 — correct
- 16
- Ozone (O₃) — correct
- Chlorine (Cl₂)
- Neon (Ne)
- Phosphorus (P₄)