1. Chapter 4: Atoms and Elements

2.  Recognize that all matter is composed of atoms.  Explain the early ideas that led to the current understanding of the atom.

3.  Atoms compose matter.  The properties of atoms determine the properties of matter.  An atom is the smallest identifiable unit of an element.  An element is a substance that cannot be broken down into simpler substances.

4.  There are about 91 different elements found in nature, and consequently about 91 different kinds of atoms.  Scientists have succeeded in making about 20 synthetic elements (not found in nature).  The exact number of naturally occurring elements is controversial because some elements previously considered only synthetic may actually occur in nature in very small quantities.

6.  Many ancient scholars believed matter was composed of such things as earth, water, air, and fire.  Many believed matter could be endlessly divided into smaller and smaller pieces.

7.  Democritus (460–370 B.C.E. ) suggested that if you divide matter into smaller and smaller pieces, you end up with tiny, indestructible particles.  Everything else is empty space.

8.  Democritus called these particles “atomos”  meaning “indivisible”  Democritus is the first person on record to have postulated that matter is composed of atoms.

9.  Aristotle (384–322 B.C.) disagreed with Democritus because he did not believe empty space could exist.  Believed everything was composed of fire, air, earth, and water.

11.  Law of Conservation of Mass:  Antoine Lavoisier (1743-1794)  In a chemical reaction, matter is neither created nor destroyed.

12.  Joseph Proust (1754-1826): a French chemist who made observations on the composition of compounds.  In 1797, found that the elements composing a compound always occurred in fixed proportions.

13.  Law of Definite Proportions:  All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements.

14. 2 H : 1 O

15.  John Dalton (1766-1844): an English chemist who offered convincing evidence in support of Democritus.  In 1804, published his law of multiple proportions.

16.  Law of Multiple Proportions:  When two elements (A and B) form two different compounds, the masses of element B that combine with 1 g of element A can be expressed as a ratio of small whole numbers.

18. 1 g Carbon 2.66 g Oxygen1.33 g Oxygen 2.66 g Oxygen = 1.00 2.00 Consistently getting small whole number ratios supports the idea that there is are indivisible atoms that make up the compounds.

19.  In 1808 (over 2000 years after Democritus) John Dalton formalized a theory of atoms that gained broad acceptance.

20.  Dalton’s atomic theory has five parts: 1. Each element is composed of tiny, indestructible particles called atoms. 2. All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements.

21. 3. Atoms combine in simple, whole-number ratios to form compounds. 4. Atoms cannot be subdivided, created, or destroyed 5. In chemical reactions, atoms are combined, separated, or rearranged.

23.  Compare and contrast the atomic theories of Democritus and Dalton. Mark an X under each name if a statement in the table applies to that person’s theory.