 History:  Around 450 B.C, a Greek Philosopher thought that all matter was proposed of 4 “elements”- air, earth, fire and water. This idea lasted for 2000 years

 Now scientists know that all matter in the universe is made of more than 100 different substances, still called elements.  Element: a pure substance that can’t be broken down into any other substances by chemical or physical means

 ELEMENTS ARE THE SIMPLEST SUBSTANCES.  Examples of elements- look at the period table of ELEMENTS

 Atoms are particles of elements.  Atoms are the basic particles from which all elements are made.  Different elements have different properties because their atoms are different.

 When atoms combine they form a CHEMICAL BOND (a force of attraction between 2 atoms)  Atoms combine to form larger particles called MOLECULES (groups of 2 or more atoms held together by chemical bonds)  Example: molecule of water

 A compound is a pure substance made of 2 or more elements chemically combined in a set ratio.  Chemical formula: shows the elements in the compound and the ratio of atoms.  Example: CO 2

 The number 2 below the symbol of oxygen tells you that the ratio of carbon to oxygen is 1 to 2. (that number 2 is also a subscript).  When elements are chemically combined, they form compounds having properties that are different from uncombined elements.

 Why is the ratio of different atoms in a compound important?

 Compounds made of the same elements but having different ratios of atoms are different compounds.  CO 2 and CO have two different ratios of the same elements, and both are different compounds with different properties.

 A mixture is made up of 2 or more substances – elements,compounds, or both – that are together in the same place but are not chemically combined  Each substance in a mixture keeps its individual properties  The parts of a mixture are not combined in a set ratio

 There are 2 kinds of mixtures:  Heterogeneous  Homogeneous

 In this mixture, you can see the different parts  Example: salad.

 Mixed very evenly, you can’t see the different parts  A solution is an exampled of a homogeneous mixture

 A compound can be difficult to separate into its elements.  A mixture is usually easy to separate into its components  Separation requires a series of laboratory techniques like shown in figure 10 on page 14.

 What elements make up carbon dioxide?  What holds atoms together in molecules?  How can oxygen be both an element and a molecule?

 Carbon and oxygen  Chemical bonds  An oxygen molecule is formed from 2 atoms of oxygen

 Workbook: 9-11,15, 16 (questions 4-7)  Section 1 Assessment questions 2 and 3.