Science, half yearly

• Elements


  Pure substances that consist of only one type of atom
• Elements


  • Cannot be broken down into simpler substances by chemical means
  • Each element has a unique atomic number (number of protons in its nucleus)
• Compounds


  Substances made from two or more different elements that are chemically bonded together
• Compounds


  • Water (H₂O)
  • Carbon Dioxide (CO₂)
  • Sodium Chloride (NaCl)
• Compounds


  • Have properties different from their component elements
  • Can be broken down into simpler substances by chemical means (e.g., electrolysis of water)
• Molecules


  Groups of two or more atoms held together by chemical bonds. They can be elements or compounds.
• Molecules


  • Oxygen Molecule (O₂)
  • Water Molecule (H₂O)
• Molecules


  • Molecules of an element: Consist of only one type of atom (e.g., O₂, N₂)
  • Molecules of a compound: Consist of different types of atoms (e.g., H₂O, CO₂)
• Periodic Table: A table where all elements are organized by their atomic number
• Chemical Bonds: Forces that hold atoms together in molecules and compounds (e.g., covalent bonds in water, ionic bonds in salt)