Atoms, Elements and Compounds Notes

AQA GCSE Chemistry: Revision notes

Key facts

  • An atom is the smallest part of an element that can exist.
  • Elements contain one type of atom; compounds are elements chemically combined; mixtures are not chemically combined.
  • Compounds can only be separated into their elements by a chemical reaction, never by physical means.
  • Learn the names and symbols of the first 20 elements, plus Groups 1 and 7.
  • Choose the right separation technique: filtration, crystallisation, simple distillation, fractional distillation or chromatography.
1234567012341H2He3Li4Be5B6C7N8O9F10Ne11Na12Mg13Al14Si15P16S17Cl18Ar19K20Ca
  • Metals
  • Non-metals
  • Semi-metals
The first 20 elements: you need their names and symbols.

Atoms and chemical symbols

An atom is the smallest part of an element that can exist. Each element has its own one- or two-letter chemical symbol.

Every substance is made of atoms. Each element has its own chemical symbol, such as Na\ce{Na} for sodium, Cl\ce{Cl} for chlorine and O\ce{O} for oxygen. You need the names and symbols of the first 20 elements, the elements in Groups 1 and 7, and the others named in the specification.

Symbols are used in formulae and equations. You can name a compound from its formula: NaCl\ce{NaCl} is sodium chloride and COX2\ce{CO2} is carbon dioxide.

1234567012341H2He3Li4Be5B6C7N8O9F10Ne11Na12Mg13Al14Si15P16S17Cl18Ar19K20Ca
  • Metals
  • Non-metals
  • Semi-metals
Sodium, chlorine and oxygen are ringed.

Which of these is the correct symbol for sodium?

Elements, compounds and mixtures

An element is one type of atom. A compound has elements chemically combined. A mixture is not chemically combined.

  • An element contains only one type of atom.
  • A compound forms when two or more elements are chemically combined: new bonds form, and the properties are completely different from the elements.
  • A mixture contains two or more elements or compounds that are not chemically combined. Each keeps its own properties.

Compounds can only be separated into elements by a chemical reaction.

  1. 1

    Mixture

    iron and sulfur are not bonded; a magnet removes the iron

  2. 2

    Heat strongly

    a chemical reaction takes place

  3. 3

    Compound: iron sulfide

    new bonds formed; a magnet no longer works

Iron and sulfur: a mixture first, then a compound once they react.

Element

Made of:
One type of atom
Properties:
Its own
Separated by:
Cannot be broken down chemically
Example:
OX2\ce{O2}

Compound

Made of:
Elements chemically combined
Properties:
Different from its elements
Separated by:
A chemical reaction
Example:
COX2\ce{CO2}

Mixture

Made of:
Substances not combined
Properties:
Each keeps its own
Separated by:
Physical methods
Example:
Salt and water

Sodium chloride is made of sodium and chlorine that are chemically combined. What is it?

Naming compounds and equations

A formula shows the elements present and the ratio of their atoms. You must write word equations and balanced symbol equations.

A chemical formula shows the elements and the ratio of atoms of each. You should write word equations and balanced symbol equations for reactions in the specification, using correct symbols and formulae.

Example: magnesium + oxygen → magnesium oxide, or 2 Mg+OX2→2 MgO\ce{2Mg + O2 -> 2MgO}.

  1. 1

    Word equation

    magnesium + oxygen → magnesium oxide

  2. 2

    Symbols

    Mg + O₂ → MgO (oxygen is O₂)

  3. 3

    Count atoms

    2 oxygen atoms on the left, 1 on the right

  4. 4

    Balance

    2Mg + O₂ → 2MgO

Writing a balanced equation for magnesium burning.

Water

Name:
Water
Formula:
HX2O\ce{H2O}

Carbon dioxide

Name:
Carbon dioxide
Formula:
COX2\ce{CO2}

Sodium chloride

Name:
Sodium chloride
Formula:
NaCl\ce{NaCl}

Magnesium oxide

Name:
Magnesium oxide
Formula:
MgO\ce{MgO}

Which is the correct balanced equation for magnesium reacting with oxygen?

Separating mixtures

The substances in a mixture are not chemically bonded, so physical methods based on differences in properties can separate them.

  • Filtration separates an insoluble solid from a liquid.
  • Crystallisation separates a soluble solid from a solution by evaporating the solvent.
  • Simple distillation separates a solvent from a solution.
  • Fractional distillation separates liquids with different boiling points.
  • Chromatography separates substances by how they distribute between a stationary phase and a mobile phase.
  1. 1

    Mix with water

    salt dissolves; sand does not

  2. 2

    Filtration

    insoluble sand stays in the filter paper

  3. 3

    Crystallisation

    evaporate the solvent from the filtrate to leave salt crystals

Example: separating sand from a salt solution.

Filtration

Separates:
Insoluble solid from liquid
Based on:
Solubility

Crystallisation

Separates:
Soluble solid from solution
Based on:
Evaporating solvent

Distillation

Separates:
Solvent, or liquids
Based on:
Boiling point

Chromatography

Separates:
Several substances
Based on:
Stationary and mobile phase

Which method would you use to separate sand from water?

Try an exam question

A mixture contains sand, salt and water. Describe how you could obtain a sample of dry salt from the mixture.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.