chemical reactions and compounds

• Stable elements, whose atoms have full outer electron shells, are found as single atoms. Most elements have atoms with incomplete outer electron shells, and these atoms form chemical bonds with other atoms in order to gain full outer shells and become stable. These chemical bonds can be broken or made in chemical reactions to produce different combinations of elements that we call compounds.
• Reactants


  Starting chemicals in a chemical reaction
• Products


  Chemicals made in a chemical reaction
• Chemical reaction


  1. Chemical bonds in the reactants are broken

  2. New chemical bonds are made to form the products

  3. Atoms themselves remain unchanged, only the bonding changes
• The mass of the reactants is equal to the mass of the products. This is called conservation of mass.
• As the chemical reaction progresses, there will be observable and/or measurable changes, such as change in colour, temperature, flames, production of gas, production of solid precipitate, etc.
• Following the chemical reaction, the chemical and physical properties of the products will be different to those of the reactants.
• Elemental compounds


  Substances that consist of one type of element chemically bonded together
• Compounds


  Substances that consist of two or more elements chemically bonded together
• Chemical formula


  Representation of elements and compounds with a specific, fixed ratio of each type of atom
• Changing the ratio of atoms in a chemical formula changes the chemical. It describes a substance with different chemical and physical properties.
• Word equation


  Chemical reaction described using the names of the reactants and products
• Symbol equation


  Chemical reaction described using the chemical formulae of the reactants and products
• Balancing a symbol equation

  1. Ensuring the number of atoms of each element is the same before and after the chemical reaction

  2. Using a tally table to record the number of atoms of each element involved
• The mass of the reactants is always equal to the mass of the products (in a chemical reaction).
• New chemical bonds between atoms, and different physical and chemical properties.
• Balancing the symbol equation: CH4 + O2 → CO2 + H2O


  CH4 + 2O2 → CO2 + 2H2O