Grade 8
Matter, atoms, molecules, chemical elements
Distinguish pure substances, elements, compounds and mixtures by the particles they contain. Connect atoms to molecules and chemical formulas.
IntuitionIntuition: particles and change
A copper wire is made of copper atoms; the oxygen in air is mostly O₂ molecules; water contains H₂O molecules. The formula tells which atoms occur and in what ratio.
SchoolSchool: models and rules
Definition: Key idea
An element contains one kind of atom; a compound has different elements chemically combined in a fixed ratio; a mixture combines substances physically in variable proportions.
Atoms are the smallest units that retain an element’s chemical identity. Molecules are groups of atoms joined by covalent bonds; not every substance consists of separate molecules—ionic solids form extended lattices.
| Type | Composition | Example |
|---|---|---|
| Element (simple substance) | Only one kind of atom | Cu, O₂, Ne |
| Compound | Several elements, fixed ratio | H₂O, CO₂, NaCl |
| Mixture | Substances mixed, ratio varies | Air, salt water |
Example: Worked example
How do oxygen and water differ?
Solution
Both are pure substances, but O₂ is an elemental molecule containing only oxygen atoms; H₂O is a compound with hydrogen and oxygen in a fixed 2:1 atom ratio.
Example: Second example
Classify: oxygen gas O₂, pure water H₂O, sea water.
Solution
O₂ is an element (one atom type); H₂O is a compound (two elements, fixed 2:1 ratio); sea water is a mixture (water, salts and gases in variable proportions).
A pure substance has a uniform composition. Air is a mixture: its main components retain their own identities and can be separated physically, unlike hydrogen and oxygen in water.
A chemical symbol names an element, while a formula describes a particle or compound: Cu is the element and also the metal sample; O₂ is the oxygen molecule; CO₂ is carbon dioxide.
Example: Calculation / application example
How many atoms of each element are in one glucose molecule, C₆H₁₂O₆?
Solution
The subscripts give the atom count in one molecule: 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms (total 24 atoms).