Atoms, elements, compounds and mixtures

Chemistry · Atomic structure and the periodic table. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

All substances are made of atoms. An element contains only one type of atom and has its own symbol, such as Na for sodium and K for potassium.

A compound contains two or more elements chemically combined in fixed proportions. Its properties are different from those of its elements, and it can be split up only by a chemical reaction.

A mixture contains two or more substances that are not chemically joined. Each substance keeps its own properties, so a mixture can be separated by physical methods. Reactions are shown by equations. A balanced symbol equation has the same number of each type of atom on both sides.

Rules to remember

Filtration: separates an insoluble solid from a liquid.
Crystallisation: gets a soluble solid out of its solution.
Simple distillation: collects the solvent from a solution, e.g. pure water from salt solution.
Fractional distillation: separates liquids that mix and have different boiling points.
Paper chromatography: separates dissolved substances, e.g. the dyes in an ink.
State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
To balance an equation, change the large numbers in front of the formulae, never the small numbers inside them.

Common mistake

Students change a formula to make an equation balance, for example writing MgO₂. The formula of a substance is fixed, so only put numbers in front of formulae.

Worked example

Balance the equation for methane burning: CH₄ + O₂ → CO₂ + H₂O.

Carbon: 1 atom on each side, so carbon is already balanced.
Hydrogen: 4 on the left, 2 on the right, so write 2H₂O to give 4.
Oxygen: the right now has 2 + 2 = 4 atoms, so write 2O₂ on the left to give 4.

Answer: CH₄ + 2O₂ → CO₂ + 2H₂O

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