1. Chemical Formulas

2. Molecules and Compounds 4 A molecule is two or more atoms that are bonded together. –H 2, O 2, N 2, H 2 O, CO 2 4 A compound is two or more elements chemically combined (bonded) in a set ratio. –H 2 0, HCl, C 6 H 12 O 6

3. Chemical Formula 4 The type of element is expressed by the chemical symbol of the element. 4 A Chemical Formula tells which elements make up a compound and how many atoms of each element are needed to make a single molecule. –It shows the exact ratio of the elements NaCl Symbol for Sodium Symbol for Chlorine

4. Chemical Formula 4 The more metallic element is written first in a chemical formula. NaCl Sodium Metal Chlorine Nonmetal MgO Magnesium Metal Oxygen Nonmetal CO Carbon Nonmetal Oxygen Nonmetal Although Carbon is a nonmetal, it is to the left of Oxygen on the Periodic Table which makes it more metallic than Oxygen.

5. Chemical Formula 4 The subscript tells you the number of atoms that are bonded together to make one molecule. MgCl 2 Subscript The subscript 2 tells us that there are two Chlorine ions involved in bonds to make a molecule of Magnesium Chloride. There is no subscript on the Magnesium ion. This is because there is only one Magnesium ion in this molecule. Subscripts of 1 are not written out. This formula tells us that one molecule of Magnesium Chloride is composed of 1 Magnesium bonded to 2 Chlorines.

6. Chemical Formula  What do the following chemical formulas tell you? NaCl Li 2 S Fe 2 O 3 = 1 Sodium bonded to 1 Chlorine = 2 Lithiums bonded to 1 Sulfur = 2 Irons bonded to 3 Oxygens

7. Chemical Formula 4 Subscripts are extremely important in a chemical formula because they determine the compound. CO 2 CO 2 is the formula for Carbon Dioxide. A molecule with 1 Carbon bonded to 2 Oxygens. Carbon Dioxide is the gas you exhale all day long. CO CO is the formula for Carbon Monoxide. A molecule with 1 Carbon bonded to 1 Oxygen. Carbon Monoxide is a deadly gas in your car’s exhaust. When you change the subscript, you change the number of atoms involved in bonds to make the molecule and therefore you change the compound. CO 2 and CO are two different compounds.

8. Chemical Formula 4 There is another number that you may see written in front of a chemical formula. 4 This number is called the Coefficient. 4 The Coefficient is used to tell how many molecules of the compound are present. 3MgCl 2 Coefficient This tells us there are three molecules of Magnesium Chloride (MgCl 2 ). Each molecule has 1 magnesium bonded to 2 chlorines.

9. Chemical Formula 4 Coefficients only tell the number of molecules of a compound. CO 2 2CO 2 3CO 2 = 1 molecule of Carbon Dioxide = 2 molecules of Carbon Dioxide = 3 molecules of Carbon Dioxide Changing the Coefficient doesn’t change the compound. All the formulas above are carbon dioxide. The coefficient only changed the amount of the compound.

10. Chemical Formula 4 Interpret the following chemical formulas. CaCl 2 2N 2 3Al 2 S 3 = 1 molecule of Calcium Chloride (CaCl 2 ) The molecule has 1 Calcium bonded to 2 Chlorines. = 2 molecules of Nitrogen Gas (N 2 ) Each molecule has 2 Nitrogen atoms bonded to each other. = 3 molecules of Aluminum Sulfide (Al 2 S 3 ) Each molecule has 2 Aluminum atoms bonded to 3 Sulfurs.