The Atom: From Philosophical Idea to Scientific Theory Section 3.1

Defining the Atom The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”) He believed that atoms were indivisible and indestructible His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy

Foundations of the Atomic Theory Better equipment for the study of matter became available in the 1790s This lead to the discovery of several basic laws There was no real explanation for the laws until Dalton proposed his atomic theory

Law of Conservation of Mass Law of conservation of mass: mass is neither created nor destroyed during ordinary chemical reactions or physical changes

Law of Definite Proportions Law of definite proportions: chemical compounds contain the same elements in exactly the same proportions by mass, regardless of the size of the sample

Law of Multiple Proportions Law of multiple proportions: if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers

Visual for Law of Multiple Proportions

Dalton John Dalton was a schoolteacher in the early 1800s He proposed an explanation for the three laws He said that elements were composed of atoms and only whole numbers of atoms could combine to form compounds

Dalton’s Atomic Theory 1) all matter is composed of extremely small particles called 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

Continued 4) atoms of different elements combine in simple whole-number ratios to form chemical compounds 5) in chemical reactions, atoms are combined, separated, or rearranged

Modern Atomic Theory Dalton’s theory was scientific, but not entirely correct We now know that atoms are divisible into smaller particles A given element can have atoms with different masses Dalton’s model is called the “Solid Sphere” model.