1. Elements combine to form compounds. Section 2.1D

2. Compounds have different properties from the elements that make them. There are just over 100 elements, but there are millions of different substances. Most substances are compounds: substances made of atoms of two or more different atoms. Just as the 26 letters in the alphabet can form thousands of words, the elements in the periodic table can form millions of compounds.

3. Compounds have different properties than the elements that make them. Chemical bonds hold the atoms together and the bonds help determine the properties of a compound. Properties of a compound depend not only on which atoms the compound contains, but also on how the atoms are arranged. Ex: methane, ethane, propane

4. Compounds have different properties from the elements that make them. The properties of compounds are often very different from the properties of the elements that make them. Example: water is made of 2 hydrogen atoms bonded to 1 oxygen atom At room temp. both are colorless, odorless gases; water is a liquid

5. Atoms combine in predictable numbers. A given compound always contains atoms of elements in a specific ratio. Ex. Ammonia = 3 atoms of hydrogen, 1 atom of nitrogen – ratio of 3 to 1 or 3:1 Ex. Hydrazoic acid = one hydrogen atom, three nitrogen atoms – ration of 3:1

6. Chemical Formulas Use the chemical symbols to represent the atoms of the elements and their rations in a compound Ex. CO 2 : one atom of carbon attached by chemical bonds to two atoms of oxygen Write the symbols for each element side by side. Use a subscript to indicate how many of each atom. If there is only one atom in the compound, there is no subscript. H 2 O HCl NH 3 CH 4 C 3 H 8

7. Same Elements, Different Compounds Nitrogen monoxide, NO Nitrogen dioxide, NO 2 Dinitrogen monoxide, N 2 O

8. Same Elements, Different Compounds Water, H 2 O Hydrogen peroxide, H 2 O 2