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Chapter 4:
Structure of the Atom
4.1 Early theories and
4.2 Defining the atom

Historical Background:

Models of the Atom: -see reference chart

(On Pg. 10 of your packet)
4.1 Early theories and
4.2 Defining the atom
Models of the Atom
Date
100 BC
scientist
Democritus/Greeks
discovery________________
concept of the atom
1770
Antoine Lavosier
Law of conservation of mass
1800
Joseph Proust
Law of definite proportions
1803
John Dalton
Law of multiple proportions
Atomic Model I
1880
William Crookes
Cathode Rays (electrons)
1885
Goldstein
Canal Rays (protons)
1900
J.J. Thomson
Plum Pudding Model
Electron
Atomic Model II
1909
Ernest Rutherford
nucleus of atom
Atomic Model III
1913
Niels Bohr
Planetary Model
Atomic Model IV
1920Present
Schroedinger/Planck/
Modern or Wave Model
DeBroglie/Einstein/etc. Atomic Model V
4.1 Early theories and
4.2 Defining the atom

1. Atomists and Democritus



Greeks approx 2,500 years ago
Matter was made up of atoms “atomos” or
“Indivisible” particles
Seashell experiment—broken into smaller &
smaller pieces
4.1 Early theories and
4.2 Defining the atom

2. John Dalton



1766-1844; returned to theory of atoms
Atoms are like billiard balls (solid spheres)
which cannot be broken down further
4 major postulates




1) All elements are composed of atoms
2) Atoms of the same element are identical
3) Atoms can physically mix or chemically combine
in simple whole number ratios
4) Reactions occur when atoms separate, join, or
rearrange
4.1 Early theories and
4.2 Defining the atom

3. William Crookes



developed Crookes tube (CRT) in 1870’s
first evidence for existence of electrons
because you could “see” electrons flow and
confirm their existence.
tube is precursor to today’s TV picture tubes
4.1 Early theories and
4.2 Defining the atom

4. J.J. Thomson



discovered electron in 1897
discovered positively charged particles
surrounded by electrons
found the ratio of the charge of an electron to
its mass to be 1/1837
4.1 Early theories and
4.2 Defining the atom

4. J.J. Thomson
4.1 Early theories and
4.2 Defining the atom

4. J.J. Thomson


cathode ray tube experiments – advancement
of Crookes tube
“plum-pudding model”
4.1 Early theories and
4.2 Defining the atom

5. Ernest Rutherford

Discovered nucleus (dense core of atom) in
1911
Gold foil experiments

Quote from E.R.’s Lab Notebook


“It is about as incredible as if you had fired a
15-inch shell at a piece of tissue paper and it came
back and hit you.” -ER
4.1 Early theories and
4.2 Defining the atom
Video Clip: Rutherford Gold Foil Experiment
4.1 Early theories and
4.2 Defining the atom

6. Robert Milliken


Oil drop experiment
determined the charge and mass of an
electron
Video Clip: Milliken Oil Drop Experiment
4.1 Early theories and
4.2 Defining the atom

7. James Chadwick

discovered the neutron (no charge, but same
mass as proton)

Neutrons help disperse the strong repulsion
of positive charges
 Relative Sizes
 Nucleus diameter = 10-5 nm
 Atom diameter = 10-1 nm
 Nucleus = basketball --> Atom = 6
miles wide!
4.1 Early theories and
4.2 Defining the atom

8. Niels Bohr


improved on Rutherford’s work
“planetary model”- positive center is
surrounded by electrons in defined orbits
circling the center
4.1 Early theories and
4.2 Defining the atom

defined the following:

energy level – the location where an electron
is found at a set distance from the nucleus
dependent on the amount of energy it has

ground state – the typical energy level where
an electron is found; lowest energy

excited state – an energy level higher than the
ground state for an electron; temporary
condition
4.1 Early theories and
4.2 Defining the atom

9. Quantum Mechanical Model



Erwin Schroedinger; Mathematical model
Electron locations are based on probability
Electrons are not particles, but waves!

http://phet.colorado.edu/en/simulation/hydrogenatom
Defined:

Orbital – region where an electron is likely to
be found 90% of the time
4.3 How atoms differ

Atoms – vocabulary and classifications

Atom – the smallest particle of matter that
retains its properties.

can “see” individual atoms with a scanning tunneling
microscope.
4.3 How atoms differ

Subatomic particles – the component
parts of an atom: proton, neutron, and
electron
4.3 How atoms differ

Ion- atom with the same number of
protons but a different number of
electrons.

If the atom has a (+) charge it has fewer
electrons than protons, If the atom has a (-)
charge it has more electrons than protons.
4.3 How atoms differ
Subatomic
Particle
Mass and Abbreviation
Charge
Location
Discoverer
Proton
1 amu, p+
+1
Nucleus
None
Neutron
1 amu, n
0
Nucleus
Chadwick in
1932
Electron
Almost zero, e-
-1
Electron
cloud
Thomson
4.3 How atoms differ





Calculations involving Subatomic
Particles:
atomic number = # of protons
mass number = # of protons + # of neutrons
(neutral atom): # of protons = # of electrons
(charged ion): charge = #p+ - #e-
4.3 How atoms differ

Isotopes and Calculations:

Isotope – atoms of the same element with
different numbers of neutrons

Atomic mass – weighted average of the
masses of all the isotopes of an element
4.3 How atoms differ

Isotope (Isotopic Notation)
Mass
Atomic
#
Atomic Symbol
#
Example: Uranium-238
Z
A
X
4.3 How atoms differ

Isotope Problems:


Multiply the mass number of the isotope by the
decimal value of the percent for that isotope
Add the relative masses of all of the isotopes
to get the atomic mass of the element
4.3 How atoms differ

Example:

If 90% of the Beryllium in the world has a mass number
of 9 and only 10% has a mass number of 10, what is the
atomic mass of Beryllium?
4.4 Unstable Nuclei and
Radioactive Decay

Vocabulary

Radioactivity-the spontaneous emission of
radiation from substances

Nuclear reactions- changes in an atom’s
nucleus

Radiation-rays and/or particles emitted from
radioactive material
4.4 Unstable Nuclei and
Radioactive Decay

Types of Radiation
Alpha radiation -stream of high energy alpha
particles
 alpha particles consist of 2 protons and 2
neutrons and are identical to helium-4 nucleus.

symbol
4He 2+

not much penetrating power, travel a few
centimeters, stopped by paper, no health
hazard
2
4.4 Unstable Nuclei and
Radioactive Decay

mass number decreases by 4 atomic number by 2

alpha decay:
226
Ra 
88

Example: Uranium-238
222Ra
86
+
4
He
2
4.4 Unstable Nuclei and
Radioactive Decay

Beta radiation

high speed electrons


To form beta radiation a neutron splits into a proton and an
electron
The proton stays in nucleus and the electron propels out at
high speed.

Symbol 0e- 0e-

100 times more penetrating then alpha, pass
through clothing to damage skin
-1
-1
0B
-1
4.4 Unstable Nuclei and
Radioactive Decay

Beta decay:
131I

131
53

Example: Astatine-220
54
Xe +
0B
-1
4.4 Unstable Nuclei and
Radioactive Decay

Gamma radiation


similar to X rays
doesn’t consist of particles

symbol:
0

0




penetrates deeply into solid material, body
tissue, stopped by Pb or concrete, dangerous
usually emitted with alpha and beta radiation
no mass or electrical charge
emission of gamma rays by themselves cannot
result in the formation of a new atom
4.4 Unstable Nuclei and
Radioactive Decay

Practice:

What is the alpha decay of plutonium-250?
4.4 Unstable Nuclei and
Radioactive Decay

2. What is the beta decay of Carbon-14?
4.1 Early theories and
4.2 Defining the atom
Models of the Atom
Date
100 BC
scientist
Democritus/Greeks
discovery________________
concept of the atom
1770
Antoine Lavosier
Law of conservation of mass
1800
Joseph Proust
Law of definite proportions
1803
John Dalton
Law of multiple proportions
Atomic Model I
1880
William Crookes
Cathode Rays (electrons)
1885
Goldstein
Canal Rays (protons)
1900
J.J. Thomson
Plum Pudding Model
Electron
Atomic Model II
1909
Ernest Rutherford
nucleus of atom
Atomic Model III
1913
Niels Bohr
Planetary Model
Atomic Model IV
1920Present
Schroedinger/Planck/
Modern or Wave Model
DeBroglie/Einstein/etc. Atomic Model V
End of Chapter 4!