EARLY MODELS OF THE ATOM

Objectives: 1. Define the term atom. 2. List the postulates of Dalton's atomic

theory.Key Terms: Atom, law of constant position, atomic theory

of matter

ATOMS - A BRIEF HISTORY

Atoms are the smallest particle of an element retaining all of its properties. Though, invisible to the eye and microscope (limited images are possible with the tunneling microscope), you could theorize that an atom of gold would have the same luster and texture (look and feel) as a solid gold bar. The belief in atoms as the basic building block of matter dates back o the Greek philosophers (450 BC) when Democritus first proposed the existence of atomos (small particles of matter). Unfortunately Democritus' views were not accepted until the early 1700s.

ATOMIC THEORY - TIMELINE

1700 - Lavoisier - The Law of Conservation of matter - Matter can not be created nor destroyed.

1799 - Proust - Law of Constant Composition - Given compounds always contain the same proportion (ratios) of elements.

1803 - Dalton - Atomic Theory of Matter Elements are composed of atoms (Smallest particle

retaining the identity of the element) Atoms in an element are identical but different from

other elements Atoms can be joined or separated in chemical reactions

but cannot be altered to form different atoms Atoms can be mixed or chemically combined to form

compounds that always have the same ratios and kinds of atoms

ATOMIC THEORY – TIMELINE (PART 2)

Late 1800s - Henri Becquerel accidentally discovers radioactivity (emission of particles from the nucleus an the atom)

1897 - J.J. Thompson used a CRT to discover the moving part of an atom was negatively charged

1916 - Robert Millikan determines the size and charge of an electron

Early 1900s - Ernest Rutherford aims an alpha beam at gold foil to determine the arrangement of subatomic particles