1. ELEMENTS Pure substance Can’t be separated physically or chemically 92 naturally occurring elements (only 91 found on Earth) Each has a unique name and symbol Examples: gold, hydrogen, mercury and oxygen

2. PERIODIC TABLE Organizes elements into rows (periods) and columns (families or groups) Organized according to physical and chemical properties Mendeleev first conceived of the idea

3. COMPOUND Two or more elements chemically combined Can be broken down Much of the matter of the universe exists as compounds Examples: water, salt, sugar and aspirin About 10 million known compounds About 100,000 new compounds developed and discovered each year

4. LAW OF DEFINITE PROPORTIONS Any given compound is ALWAYS composed of the same elements in the same proportion by mass. The mass of the compound = mass of each element in compound Compounds with the same mass proportions (percent mass) must be the same compound

5. Try It What is the mass proportion or mass % of hydrogen and oxygen in water? 2.016 / 18.015 x 100% = 11.19% 15.999 / 18.015 x 100% = 88.81%

6. Try It If you had a liquid that weighed 5.0 grams and.30 grams of this mass is hydrogen, is it water?.30 / 5.0 x 100% = 6% hydrogen but water is 11.19% hydrogen, so it’s not water

7. LAW OF MULTIPLE PROPORTIONS When different compounds contain the same elements, different masses of one element combine with the same relative mass of the other element in a ratio of small whole numbers. In other words, the same elements can combine in more than one way. Mass ratios can be used to determine the formulas for each compound.

8. Try It What are the mass proportions for the mystery compound of hydrogen and oxygen?.30 / 5.0 x 100% = 6.0% 4.7 / 5.0 x 100% = 94% So what is the mystery liquid?

9. Try It What is the mass ratio of oxygen to hydrogen for water and peroxide? H 2 0: 88.81/11.19 = 7.937 15.999/2.016 = 7.936 H x 0 x : 94/6.0 = 16 16 / 7.936 = 2 which means the 2 nd compound has 2X more oxygen than water, so the mystery compound must be H 2 O 2.