Unit 2:
Atomic Theory
and Structure
Early Models of the Atom
(c. 400 B.C.) Democritus
proposed that “atomos”
are indivisible.
Atom: smallest particle of
matter that retains its properties.
(1803) Dalton’s experiments
developed a scientific theory
of atoms.
Dalton’s Atomic Theory
(4 conclusions)
Dalton’s Atomic Theory
1) All matter is composed of
tiny, indivisible atoms.
2) In size, mass, & prop’s
atoms of the same
element are identical,
atoms of different
element are different
Dalton’s Atomic Theory
3) Atoms chemically
combine, separate,
or rearrange.
4) Atoms combine in simple
whole-number ratios to
form compounds.
(Law of Definite Proportions)
(Law of Conservation of Mass)
(1904) J.J. Thomson
Cathode Ray Tube Experiment
Cathode Ray Tube
(1904) J.J. Thomson
Cathode Ray Tube Experiment
 cathode rays deflect toward a positive plate
WHY?
negatively charged
(1904) J.J. Thomson
Cathode Ray Tube Experiment
Conclusion:
 atoms contain negatively charged
particles called electrons
Thomson’s
Plum Pudding Model
atoms are + charged
material with – charged
electrons evenly
distributed
(1911) Ernest Rutherford
Gold Foil Experiment
(expected)
1908 Nobel Prize for
work with radiation
(+ alpha particles)
(1911) Ernest Rutherford
Gold Foil Experiment
1/10,000 + alpha particles deflect from the gold
foil
+
(1911) Ernest Rutherford
Gold Foil Experiment
Conclusion:
 atoms are mostly empty space filled with e–
 the + charge and mass are in a
tiny central nucleus
nucleus:
tiny, massive, core
of atoms containing
+ protons and
(neutral) neutrons
the atomic nucleus
(1 trillionth of the atom)
17.7 The Atomic Nucleus
3 subatomic particles:
≈0
4.2
Quick Quiz!
1. The ancient Greek philosopher Democritus is
credited with suggesting that ____.
A. atoms react in whole number ratios
B. atoms can change into other elements
C. all matter is made of indivisible atoms
D. atoms are stupid anyway
Quick Quiz.
2. Dalton's atomic theory improved earlier ideas
about atoms by ___.
A. teaching that all matter is composed of tiny
particles called atoms
B. theorizing that all atoms of the same
element are identical
C. using experimental methods to establish a
scientific theory
D. not relating atoms to chemical change
Quick Quiz.
3. Which of the following is NOT part of Dalton’s
atomic theory?
A. All elements are made of atoms.
B. Atoms of the same element are identical.
C. Different elements can mix or combine in
simple whole-number ratios called compounds.
D. Atoms can be changed into other atoms
through chemical reactions.
Quick Quiz.
4. Which of the following is TRUE about
subatomic particles?
A. protons and neutrons are positively charged
B. electrons have practically no mass
C. protons and electrons are in the nucleus
D. neutrons and electrons have no charge
Quick Quiz.
5. Most of the volume of the atom is occupied
by the ________________.
A. electrons
B. protons
C. neutrons
D. protons and neutrons