Download Ch. 4.1 Defining the Atom

Ch. 4.1 Defining the Atom
Early Models of the Atom
Atom—the smallest particle of an element
that retains its identity in a chemical
reaction.
 Democritus—460 B.C. – 370 B.C.,
believed that atoms were indivisible and
indestructible…his ideas did not explain
chemical behavior.

Dalton’s Atomic Theory
John Dalton (1766-1844)—used
experimental methods to change
Democritus’ ideas into a scientific theory of
atoms…
 1. All substances are composed of tiny
indivisible particles called atoms.
 2. Atoms of the same element are
identical and different from other elements.

3. Atoms of different elements can
physically mix, or can chemically combine
in whole-number ratios to form
compounds.
 4. Chemical reactions occur when atoms
are separated, joined, or rearranged.
Atoms of one element are never changed
into another element by a chemical
reaction.

Sizing up the Atom
Very small. Average size is 5 x 10-11m to 2
x 10-10m.
 Individual atoms are observable with
scanning tunneling electron microscopes.
