Elements & compounds
The idea
Roughly 118 kinds of atoms exist, yet the world holds millions of different substances. The trick is combination. A substance built from only one kind of atom — gold, oxygen, carbon — is an element; it cannot be broken into anything simpler by ordinary chemistry. A compound is built from two or more kinds of atoms chemically bonded in a fixed ratio, like H₂O or CO₂. You already know how to count atoms in a formula, so you can spot the difference: one capital-letter element symbol means element, two or more means compound.
Think of elements as letters and compounds as words: a tiny alphabet spells an enormous vocabulary, and changing one letter changes the word completely. The classic mistake is treating a compound like a mixture of its elements. Water is not hydrogen and oxygen stirred together — hydrogen burns and oxygen feeds fire, yet water puts fires out. Bonding builds a genuinely new substance with its own properties, and the recipe is fixed: H₂O is water, while H₂O₂ is peroxide, a different substance entirely.
Worked example
Decide whether each of these is an element, a compound, or neither: the oxygen gas O₂ in a diver's tank, the carbon dioxide CO₂ you breathe out, and the air in your classroom. Give a reason for each.
- O₂: each molecule contains two atoms, but both are oxygen atoms — only one kind of atom is present, so this is an element. Being a molecule does not stop something from being an element.
- CO₂: carbon and oxygen atoms are bonded in a fixed 1 to 2 ratio, so this is a compound. Its properties belong to neither parent — carbon is a black solid and oxygen feeds flames, yet CO₂ is an invisible gas that smothers them.
- Air: it contains nitrogen, oxygen, CO₂, and water vapor mingling freely without bonds between them, and the proportions vary from place to place — so it is neither; it is a mixture.
- Check each verdict with two questions: how many kinds of atoms are present, and are they actually bonded in a fixed recipe? Element needs one kind; compound needs bonds plus a fixed recipe; a mixture fails the bonding test.
Answer. O₂ is an element, CO₂ is a compound, and air is neither — it is a mixture of unbonded gases.
Check your understanding
- Why can a substance made of two-atom molecules still count as an element, and what would have to change for it to become a compound?
- How is water fundamentally different from a balloon filled with both hydrogen gas and oxygen gas?
- What does the fixed ratio in a compound's formula guarantee about every sample of that compound, anywhere in the world?
- How would you sort a tray of mystery substances into elements, compounds, and mixtures if you could see their particles?
Build the foundations first
Elements & compounds builds on these concepts. If any feel shaky, start there.