2. When elements are chemically combined, they form compounds having properties that are different from those of the uncombined elements.

3. Compounds A compound is a pure substance made of two or more elements chemically combined in a set ratio. A compound may be represented by a chemical formula which shows the elements in the compound and their ratios.

4. Chemical Formulas A subscript, the number below the symbol, tells how many of that specific atom is present in the compound. In the chemical formula for carbon dioxide, CO 2, the subscript for oxygen tells you that the ratio of carbon to oxygen is 1 to 2. If there is no number after the element’s symbol, the number 1 is understood.

5. Chemical Formulas & Valence Electrons 4 valence electrons C :: 6 valence electrons O ::.. 6 valence electrons O ::.. Carbon & oxygen form a covalent bond – they share electrons. The ratio of carbon to oxygen is 1 to 2.

6. Chemical Equations A chemical equation is a way to show a chemical reaction using symbols instead of words. All chemical equations use formulas to represent the substances involved in the reaction.

7. Structure of an Equation

8. The principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products. All the atoms present at the start of the reaction are present at the end.

9. Conservation of Mass

10. Balancing Chemical Equations To describe a chemical reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation. An equation is balanced when it accurately shows conservation of mass.

12. H2H2 O2O2 + Reactants H2OH2O Products Write the equation – H2H2 O2O2 +H2OH2O Count the atoms – 2221

13. Use coefficients to balance atoms – H2H2 O2O2 +H2OH2O 2 4222 H2H2 O2O2 +H2OH2O 2 4224 2 Look back and check!

14. H2H2 O2O2 + Reactants H2OH2O Products