Atoms, Elements and Compounds

• All substances are made of atoms
• An atom is the smallest part of an element that can exist
• An element is a substance that contains only one sort of atom (There are about 100 different elements)
• Elements are displayed in the periodic table
• The atoms of each element are represented by a different chemical symbol (e.g. sodium = Na)
• Most substances are compounds
• Compounds contain two or more elements - they are chemically combined in fixed proportions
• Compounds are represented by a chemical formula
• A chemical formula is a combination of numbers and chemical symbols
• Why do scientists use chemical formulae?

  To show the different elements in a compound - To show how many atoms of each element one molecule of the compound contains
• Water contains 2 hydrogen (H) atoms and 1 oxygen (O) atom , so its chemical formula would be H₂O
• Sulfuric acid contains 2 hydrogen (H) atoms , 1 sulfur (S) atom, and 4 oxygen (O) atoms, so its chemical formula would be H₂SO₄