Lesson focus: elementA substance made of only one type of atom, such as oxygen or iron./compoundA substance made of two or more different elements chemically joined together, such as water (hydrogen and oxygen)./mixtureTwo or more substances mixed together but not chemically joined, like sand and salt. and molecular/non-molecular answer different questions.
Classify particle representations using atomThe tiny particle that everything is made of; each element has its own kind of atom. types, bonding and particle grouping.
Build on earlier learning
ElementsUse element identity and symbols to classify substances.
You will be successful when you can…
- Distinguish element, compound and mixture.
- Define molecule as a discrete bonded group.
- Recognise molecular elements.
- Recognise non-molecular ionic compounds.
Words that carry the science
- element
- Pure substance containing one type of atom.
- compound
- Different elements chemically bonded in fixed ratio.
- mixture
- Two or more substances together without chemical bonding between them.
- molecule
- Discrete group of covalently bonded atoms.
- lattice
- Repeating three-dimensional arrangement of particles.
Classify with two independent questions
01First question: how many substances and atom types?
O₂ is an elementA substance made of only one type of atom, such as oxygen or iron.; H₂O is a compound; a sample containing both is a mixtureTwo or more substances mixed together but not chemically joined, like sand and salt.. Chemical bonds and fixed ratio distinguish compoundA substance made of two or more different elements chemically joined together, such as water (hydrogen and oxygen).. Useful definitions: a moleculeTwo or more atoms joined together, such as a water molecule (H₂O). is two or more atoms joined together. An elementA substance made of only one type of atom, such as oxygen or iron. has only one type of atom. A compoundA substance made of two or more different elements chemically joined together, such as water (hydrogen and oxygen). has two or more different types of atom joined together. So O₂ is a moleculeTwo or more atoms joined together, such as a water molecule (H₂O). of an element (only oxygen atoms), but H₂O is a molecule of a compound (hydrogen and oxygen atoms).
02Second question: discrete molecules?
O₂ is a molecule of an element; H₂O is a molecule of a compound. “Molecule” does not automatically mean compound.
03Some compounds are lattices
NaCl is a compound with a repeating ionic latticeA regular repeating pattern of particles in a solid, like oranges stacked in a box., not separate NaCl molecules.
Atoms join to make molecules
In a reaction the bonds in the reactants break and the atoms join up in new ways. No atoms are made or destroyed, so the same atoms appear on both sides.
Controls
Hydrogen, oxygen and chlorine are elements made of molecules of two atoms (H₂, O₂, Cl₂). Water, carbon dioxide and hydrogen chloride are compounds.
Classify four samples
Classify O₂, H₂O, O₂ + H₂O, and NaCl.
- O₂: one atom type, discrete molecules → molecular element.
- H₂O: two bonded atom types → molecular compound.
- O₂ + H₂O: more than one substance → mixture.
- NaCl: two elements in fixed ratio → non-molecular compound lattice.
How to make the reasoning visible: Apply element/compound/mixture first, then molecular/non-molecular.
Two-axis classification
Classify each row using both criteria.
| Sample | Element/compound/mixture | Particle structure | Reason |
|---|---|---|---|
| N₂ | Element | Molecules | One atom type in bonded pairs |
| CO₂ | Compound | Molecules | Two atom types bonded |
| Air | Mixture | Several particle types | Gases not chemically joined |
| MgO | Compound | Ionic lattice | Fixed Mg:O ratio |
Can an element contain molecules?
Why is NaCl a compound but not molecular?
Use the table. Why is N₂ an element, but CO₂ a compound?
Check your understanding
Answer from memory, then check against the suggested answer.
What is element?
What is compound?
What is mixture?
What is a molecule?
Is air an element, a compound or a mixture?
A diagram shows identical red-blue bonded pairs only. Classify it.
Why can filtration separate sand from water but not split water into elements?
Iron and sulfur powders are mixed. Then they are heated and make iron sulfide. Which is the mixture and which is the compound? Give a test.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
O₂ is a molecule but not a compound. H₂O is both a molecule and a compound. Answer the parts to explain why.
What is a molecule?
(1)How many types of atom are in O₂? So is it an element or a compound?
(1)Why is H₂O a molecule?
(1)How many types of atom are in H₂O? So why is it a compound?
(1)Total for this question: 4 marks
Command word: Use the definitions in section 01 for each word.