 Objectives: ◦ Explain  The law of conservation of mass  The law of definite proportions  The law of multiple proportions ◦ Summarize the 5 points of Dalton’s atomic theory ◦ Explain the relationship between Dalton’s theory and the law of conservation of mass, the law of definite proportions, and the law of multiple proportions.

 Democritus (400 BC) – particle theory of matter ◦ Called basic particles atomos (from Greek indivisible)  Aristotle – believed matter was continuous, not particulate – this opinion was accepted for almost 2000 years  Neither view supported by evidence until 18 th century Thinkers, not doers!

 Late 17 th century – generally accepted that: ◦ Elements can’t be broken down further by ordinary means ◦ Elements combine to form compounds, which have different physical & chemical properties than the elements that form them  Controversy – whether elements always combine in the same ratio when forming a particular compound

 1790s – emphasis on quantitative analysis of chemical reactions led to discovery of several basic laws ◦ Law of conservation of mass ◦ Law of definite proportions – Whether you have 2 crystals of NaCl or 200 grams, it is always 39.34% Na and 60.66% Cl by mass. ◦ Law of multiple proportions – when two or more compounds are made up of the same two elements, the second element always appears in a ratio of small whole numbers when compared from the first compound to the next  Ex. CO & CO 2 – say both contain 1.0g of C – in CO 1.33g of oxygen combine with the 1.0g of C; in CO g of oxygen combine with the 1.0 g of C.  Basically, this law just says you will never find CO 1.5

 1808 – John Dalton proposed an explanation for each of those three laws 1. All matter is composed of atoms 2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. 3. Atoms cannot be subdivided, created, or destroyed. 4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5. In chemical rxns, atoms are combined, separated, or rearranged.

 Law of conservation of mass explained by point #3 and #5.  Law of multiple proportions explained by #4.  Law of definite proportions explained also by #4. A given compound is always made up of the same combination of atoms.

 Dalton turned Democritus’ idea into a theory that could be, and was, tested by experiment  Not all aspects of Dalton’s theory have proven to be correct ◦ Atoms can be divided into smaller particles (the law of conservation of mass still holds true in reactions though) ◦ A given element can have atoms with different masses (we’ll talk more about this in section 3)  Has been modified, but the ideas 1) that matter is composed of atoms and 2) atoms differ between elements - remain unchanged.

 Contributors to atomic theory ◦ Democritus ◦ Dalton  Points of Dalton’s theory ◦ Which ones have been modified?  Laws explained based on Dalton