To classify a sample, ask how many different kinds of atom are present and whether they are chemically joined. One kind of atom means an element, and two or more kinds chemically bonded in a fixed ratio means a compound. Different substances simply mixed together make a mixture.
This closes the route through atoms, elements and isotopes. It uses the proton number idea from reading proton and nucleon numbers: atoms of one element all share a proton number.
What is the test?
- Count the different element symbols in the formula, or different types of atom in the particle picture.
- One symbol only: element, even if the particles are joined pairs or larger groups.
- Two or more symbols in one formula: compound.
- More than one separate substance mixed together with no fixed ratio: mixture.
| Sample | Different atoms | Class |
|---|---|---|
| Fe | 1 (iron) | Element |
| O₂ | 1 (oxygen) | Element |
| O₃ | 1 (oxygen) | Element |
| H₂O | 2 (hydrogen, oxygen) | Compound |
| NaCl | 2 (sodium, chlorine) | Compound |
| Air | several substances together | Mixture |
Worked example
Two samples are described. Classify each.
Sample A: every particle is made of two joined atoms, and both atoms have proton number 17.
Sample B: every particle is made of one atom with proton number 6 and two atoms with proton number 8, all joined.
Sample A, step 1: both atoms have proton number 17, so they are chlorine atoms. Only one kind of atom.
Sample A, step 2: classification is element, written Cl₂.
Sample B, step 1: proton number 6 is carbon and proton number 8 is oxygen. Two kinds of atom, chemically joined in a fixed ratio of 1 carbon to 2 oxygen.
Sample B, step 2: classification is compound, written CO₂.
Answer: A is an element (Cl₂). B is a compound (CO₂).
The mistake to watch for
The typical slip is to count atoms instead of kinds of atom.
Mistaken answer: “Cl₂ is a compound because it has two atoms joined together.”
The student equated “joined” with “compound”, ignoring that both atoms are the same.
The correction is to look at the symbols. A compound needs at least two different symbols. Joined atoms of the same element, such as Cl₂ or O₃, are still an element.
Another slip is to say that a compound is just a mixture of its elements. A compound has a fixed ratio and different properties from the elements in it, and needs a chemical change to separate.
Check yourself
1. Classify each as element or compound: Ne, NH₃, N₂, HCl.
Show answer
Ne: one symbol, element. NH₃: nitrogen and hydrogen, compound. N₂: one symbol, element. HCl: hydrogen and chlorine, compound.
2. A sample of sea water contains water, dissolved salt and other substances in no fixed proportion. Is it an element, a compound or a mixture?
Show answer
It has several substances with no fixed ratio and they can be separated by physical methods, so it is a mixture.
3. Why is carbon dioxide a compound even though it contains carbon and oxygen, both of which are elements?
Show answer
The carbon and oxygen atoms are chemically bonded in a fixed ratio of 1 to 2. The compound has its own properties and cannot be separated into carbon and oxygen by physical methods. Only a chemical change can do that.
Where this leads next
With element, compound and mixture sorted, bonding explains why atoms join the way they do in bonding and structure. Practise classification in the mixed practice set, then use the equation balance reasoning trainer to see how formulas show atom counts.
Sorting formulas is quick once the test is automatic, and building that habit is what teachers do in online one-to-one Chemistry tuition.