3-1: The Atom: From Philosophical Idea to Scientific Theory

1. 3-1: The Atom: From Philosophical Idea to Scientific Theory Objectives: Explain the law of conservation of mass, the law of definite proportions, and the law of multiple proportions Summarize the five essential points of Dalton’s atomic theory Explain the relationship between Dalton’s atomic theory and the law of conservation of mass, the law of definite proportions, and the law of multiple proportions

2. Think Pair Share • Opinion vs. Theory • Which of the following is an opinion and which is a theory? • 1) Smoking at an early age may make it more difficult to quit smoking later. • 2) Young people should not smoke. • Opinion: an unsupported idea • Theory: uses reason to explain observations and experimental data

3. The idea of the atom has been around since the time of the ancient Greeks • Democritus: • Coined the term “atom” from the Greek term meaning “indivisible” • Believed that matter cannot be divided • Aristotle: • Ideas succeeded Democritus • Did not believe in atoms • Thought matter was continuous  Idea was accepted for nearly 2000 years! • Neither Democritus nor Aristotle’s ideas were supported by scientific evidences

4. Foundations of Atomic Theory • With the invention of the balance, matter was studied quantitatively (numerically; with numbers) • This lead to the discovery of several basic chemistry laws: • Law of conservation of mass • Law of definite proportions • Law of multiple proportions

5. Law of Conservation of Mass: law which states that mass is neither created nor destroyed during ordinary chemical reactions or physical changes • Discovered by French chemist Antoine Lavoisier who found that the mass after a chemical change equaled the mass before the chemical change • Law of Definite Proportions: law which states that a compound always contains the same elements in the same ratio by mass • Discovered by French chemist, Joseph Proust • Law of Multiple Proportions: law which states that elements can combine in different ways to form different substances, whose mass ratios are small, whole-number multiples of each other

6. Dalton’s Atomic Theory • Atoms and molecules faithfully obey certain sets of laws that help the study of chemistry • John Dalton • English schoolteacher • Proposed an explanation for the law of conservation of mass, law of definite proportions, and the law of multiple proportions (1808) • His theory can be summed up by the following: • All matter is composed of extremely small particles called atoms. • Atoms of a given element are the same in size, mass, and other properties; Atoms of different elements have differ in size, mass, and other properties • Atoms cannot be subdivided, created, or destroyed. • Atoms of different elements combine in simple whole-number ratios to form chemical compounds. • In chemical reactions, atoms are combined, separated, or rearranged. • Dalton was able to turn Democritus's idea into a scientific theory

7. New Atomic Theory • Not all of Dalton’s theories were correct! • Some of Dalton’s theories have been changed: • Atoms can be subdivided into smaller particles • Can break down atom into neutrons, protons, and electrons • A given element can have atoms with different masses • Some elements have isotopes (atoms of the same element that have different masses) • Important concepts that remain unchanged: • All matter is made up of atoms • Atoms of any one element differ in properties from atoms of another element

8. Using your notes, create a Venn diagram showing the differences and similarities between Dalton’s atomic theory & the Modern atomic theory