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.

Section 1 – The Atom: From Philosophical Idea to Scientific Theory

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 18th century

Early theories/thinkers

Thinkers, not doers!

Late 17th 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

Foundations of Atomic Theory

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 & CO2 – say both contain 1.0g of C – in CO 1.33g of

oxygen combine with the 1.0g of C; in CO2 2.66g of oxygen combine with the 1.0 g of C.

Basically, this law just says you will never find CO1.5

Foundations (cont.)

1808 – John Dalton proposed an explanation for each of those three laws1. All matter is composed of atoms2. 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.

Dalton’s Atomic Theory

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’s Theory (cont.)

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.

Modern Atomic Theory

Contributors to atomic theory◦ Democritus◦ Dalton

Points of Dalton’s theory◦ Which ones have been modified?

Laws explained based on Dalton

What’s the Big Idea?