Transcript History of Atomic Theories

5.1 p. 168-176
Theories of the Atom
Atomic theory: the study of the nature of atoms and how
they combine to form all types of matter
Ancient Greek Philosophers
(450 B.C.) p. 170
Empedocles
Everything that existed was thought to be
composed of four elements.
(i) Water
(iii) Air
(ii) Fire
(iv) Earth
***Democritus
(Greek Philosopher, 440 B.C.)
Democritus proposed…
all matter can be divided into smaller pieces until a
single indivisible particle is reached This particle is
called the Atom (meaning -“cannot be cut”)
Democritus proposed that atoms are…
of different sizes
in constant motion
separated by empty space
Aristotle
(Ancient Greek Philosopher, 350 B.C.)
Accepted Empedocles’
atomic model theory
that all matter was one
of four different forms;
earth, fire, air or water.
The substances were
thought to have specific
qualities: dry, wet, cold,
and hot
This model was accepted
for almost 2000 years.
Note: The Greek
philosophers did NOT
test their ideas with
experiments. They were
thought of as great
“thinkers”, but not
scientists.
***John Dalton
(English Scientist, 1800’s) p.171
Dalton used the following to explain the nature of matter
1. Atoms are tiny indestructible particles that cannot be
broken down (indivisible)
2. Atoms of an element are identical (e.g. H2 gas are all
H atoms)
3. Atoms of different elements have different properties
4. Atoms of different elements combine with other atoms
to form molecules (e.g. carbon + oxygen  carbon dioxide)
Dalton developed a theory based on
these experiments which explain…
1. The Law of Conservation of Mass
 The total mass of the reactants = the total
mass of the products
2. The Law of Constant Composition
 A compound always contains the same
elements in the same proportions by mass.
Model: Billiard Balls
Theory
Model
Analogy
(Featureless
Sphere)
(Billiard Balls)
 Indivisible atoms
***J.J. Thompson
(English Physicist, 1904)
Thompson discovered electrons (negatively
charged particles)
Thompson theorized (i.e. gathered
experimental evidence)…
Atoms contain negatively charged electrons
Since atoms are neutral, the rest of the atom
is positive
Negatively charged electrons are evenly
distributed throughout the atom
Model: The Raisin Bun Model
Theory
Model
Analogy
(Uniform
Charge
Distribution)
(Raisin Bun)
 Electrons
embedded within
a positive sphere
 Net charge of
zero
***Ernest Rutherford
(Nuclear Physicist, 1911)
Rutherford’s Gold Foil Experiment
 Positive particles were fired at a thin
sheet of gold foil
Rutherford predicted that the particles
would pass directly through the metal foil
The Gold Foil Experiment
The Results…
 Most particles passed through in a direct path,
however, some deflected and bounced back.
“Its like shooting a bullet at a
piece of tissue paper and
having the bullet bounce off”
The Gold Foil Experiment
Conclusion…
 Atoms are mostly empty space with a tiny
central nucleus which contains almost all of
the total mass of the atom and is positively
charged. He called these positively charged
particles protons. Protons have a mass nearly
2000x greater than electrons.
 The nucleus is surrounded mostly by empty
space, containing rapidly moving negative
charges called electrons.
Model: The Nuclear Model
Theory
Model
Analogy
(Nuclear
Model)
(Beehive)
 Small positive
nucleus
surrounded by
negative electrons
***James Chadwick
(Nuclear Physicist, 1932)
Chadwick discovered particles in the nucleus
of an atom possessing no electrical charge
(neutral). He called them neutrons. The
neutrons make up the remaining mass of the
nucleus.
A neutral atom has an equal # of protons &
electrons.
***Neils Bohr
(Danish Physicist, 1920)
Suggested
electrons moved
around the nucleus
in a definite “orbit”
arranged in
“shells”.
Model: The Planetary Model
Theory
 Explains
periodic law
 Electrons are
quantized in
energy levels
Model
Analogy
(Planetary
Model)
(Planets
Orbiting
Around the
Sun)
Neutrons + Protons are
in the nucleus
Electrons in orbits
around the nucleus
Protons = Positive Charge
Electrons = Negative Charge
Neutrons = Zero Charge
None
Homework
Read section 5.1 and supplement
your notes
Complete p. 175 #2, 5, 6
p. 177 #1-3,6, 8,11