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Chapter 3
The History of the
Atom
I. The Scientists and their Discoveries
A.Democritus
1. Date = 400 B.C
2. Discovery = Theorized the
smallest unit of an
element
3. Importance = gave the term
and definition of atom
Democritus
ATOM
Smallest
particle
B. John Dalton
1. Date = 1803
2. Discovery =
a. Every element is made of
atoms
b. atoms of same element
exactly the same
c. Atoms of different elements
join together to form
molecules
3. Importance = proposed the
ATOMIC THEORY
Atomic Theory =
1. everything is made up of
atoms
2. atoms make up molecules
3. atoms of the same
element are the same
John Dalton
Atomic Theory
Molecules
made of
atoms
C. J.J. Thomson
1. Date = 1898
2. Discovery =
a. discovered electrons
b. electrons are negative
3. Importance =
atoms have both negative and
positive particles
J.J. Thomson
“Plum Pudding”
model
Electrons are
negative
D. Ernest Rutherford
1. Date = 1911
2. Discovery =
a. discovered nucleus
b. did a “gold foil” experiment
c. atoms mostly made of space
3. Importance =
a. nucleus is very tiny, and
positively charged
b. electrons outside of nucleus
Discovered
Ernest
Rutherford nucleus
Electrons
outside
Gold Foil
Experiment nucleus
E. Niels Bohr
1. Date = 1913
2. Discovery =
a. Electrons are in energy levels
b. s, p, d, f orbitals
c. 1st = 2 electrons;
2nd = 8 electrons;
3rd = 18 electrons
4th = 32 electrons
3. Importance =
electrons orbit around nucleus in
a certain pathway
This is
the
Bohr
Model
Niels Bohr
Electrons in
energy levels
F. Schrödinger
1. Date = present
2. Discovery = electrons are
impossible to find ( a
moving fan)
3. Importance =
a. Atoms are neutral
b. atoms must have the same
number of protons and
electrons
Schrödinger
Atoms
have no
charge
Same
number of
protons and
electrons
II. The Mass of Atom
A. Protons
1. are positively charge
2. have a mass of 1.67 X 10-27 kg
3. Found in the nucleus
B. Neutrons
1. Neutral
2. have a mass of 1.67 X 10-27 kg
3. Found in the nucleus
C. Electrons
1. are negatively charged
2. Have a mass of 9.11 X 10-31 kg
= 0 atom mass
3. Found orbiting outside the nucleus, so
their mass does not effect the
atom’s mass!
Section Question
1. Which scientist gave us the word
atom?
2. Which scientist made up the
atomic theory?
3. Every element is made of atoms,
atoms of the same element are
exactly the same, different atoms
make up molecules. These
statements make up the what?
4. Which scientist discovered the
electrons?
5. Which scientist discovered the
nucleus?
6. Which scientist discovered that
electrons where in energy
levels?
7. Which scientist said atoms where
neutral because they had equal
number of protons and electrons?
8. What is the mass of protons?
9. What is the mass of neutrons?
10. What is the mass of
electrons?
11. Which part of the atom has
the least amount of mass?