GCSE Chemistry 1

• All things are made of atoms
• A substance made from only 1 atom is called an element
• Atoms of each element are represented by a chemical symbol, for example oxygen is represented by an 'O', or Sodium is represented by 'Na'.
• Atoms have a nucleus in the centre made up of protons and neutrons.
• The protons and neutrons are surrounded by electrons.
• The relative electrical charges are:

Protons: +1 Charge

Neutrons: 0 Charge

Electrons: -1 Charge

• In an atom, the number of protons is equal to the number of electrons, and therefore each atom has a neutral charge (they cancel each other out).
• All atoms of the same element have the same amount of protons.
• The number of protons in an element is called the atomic number.
• The sum of the protons and neutrons equal the element's mass number.

• Electrons occupy different 'shells' - each electron is in a particular shell.
• Electrons occupy the innermost shells.
• The shells can hold a specific amount of electrons:

1st Shell: 2 electrons

2nd Shell: 8 elctrons

3rd Shell: 8 electrons

4th Shell: 18 electrons

(no need for further knowledge for GCSE)

On the next page it is shown how electron shells are drawn for GCSE.

Electron Shell Drawing - Potassium ('K'):

• When elements react, they form a compound.
• This reaction involves giving, taking, or sharing electrons to form ions or molecules.
• Ions are compounds that are a mix of metals and non-metals.
• Molecules are compounds that are made of non-metals.
• Molecules are held together by covalent bonds.
• Metals lose electrons to become positive ions.
• Non-metals gain electrons to become negative ions.
• These chemical reactions are represented by equations, such as:

CH₄+ 2 O₂ CO₂+ 2 H₂O

• No atoms are lost or created during a chemical reaction

• Limestone is mainly composed of Calcium Carbonate (CaCO3)
• Calcium Carbonate can be decomposed by thermal decomposition (heating)
• The carbonates of zinc, magnesium, copper, sodium and calcium can be decomposed using the same method
• Calcium oxide reacts with water to produce calcium hydroxide - an alkali used to neutralise acids
• Calcium hydroxide then reacts with calcium dioxide to produce Calcium Carbonate
• Limestone is damaged by acid rain
• Limestone can be heated with clay to produce cement
• Cement can be mixed with sand to make mortar, and aggregate and sand to produce concrete.