Atoms, elements and compounds

• All substances are made of atoms.

• An atom is the smallest part of an element that can exist and still have the properties of that element.

Elements

• An element is a substance made up of only one type of atom.

• Each element is represented by a unique chemical symbol.

  • Eg: O = Oxygen, Na = Sodium, H = Hydrogen, Cl = Chlorine.

• There are approximately 100 different elements.

• Different elements have dfferent properties, for example:

  • Metal elements like silver (Ag), iron (Fe) and copper (Cu) are shiny and solid.

  • None metal elements like oxygen (O), fluorine (F) and nitrogen (N) are gases at room temperature.

• All elements are organised on the periodic table (see The modern periodic table for more information).

Compounds

• Compounds are formed from two or more different elements that are chemically combined.

• The elements in a compound are combined in fixed proportions. This means the ratio of atoms of each element in a specific compound is always the same (e.g., water is always H₂O, never H₃O or HO).

• Compounds can be represented by formulae using the symbols of the atoms they contain (e.g., H₂O, NaCl, CO₂).

• Crucially: Compounds can only be separated back into their elements by chemical reactions, not physical methods (like filtering or heating).

Chemical reactions

• Chemical reactions are processes where atoms are rearranged to form one or more new substances.

• Chemical reactions often involve a detectable energy change (release or absorption of heat, light, etc.).

• Chemical reactions can be shown using equations.

• Word Equations: Use the names of the substances.

  • Example: Sodium + Chlorine → Sodium Chloride

• Symbol Equations: Use the chemical formulae of the substances.

  • Example: Na + Cl₂ → NaCl

• Half-equations show you what happens to one specific species (an atom, ion, or molecule) during a reaction.