Chapter 4: Atoms and Elements

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

 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.

 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.

 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.

 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.

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

 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.

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

 Joseph Proust ( ): 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.

 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.

2 H : 1 O

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

 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.

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

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

 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.

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.

 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.