Models of the Atom
Ms Fedor
Arts and College Preparatory Academy
ChemCatalyst
The model shown here is a tiny cube of gold.
1. What do you think a scientific model is?
2. The spheres in this model represent atoms.
What do you think atoms are?
Agenda
Objectives: describe the structure of an atom, understand
how atomic theory developed over time
Collect Assignments
2. Notes: Atomic Structure
3. Activity: Atomic Pudding
HW:
 Students who have completed quiz 2: study and
schedule Quiz 3
 Students who have completed Quiz 3: work on
organizing your notebook, finishing your
preplanning for your element project
1.
What I Need From You
 Supplements
for Quiz 2
◦ Mole practice problems
◦ Chemical formulas/reactions
 Activities:
◦ A New Language
◦ Copper Cycle Lab
◦ What Goes Around Comes Around
Earliest Ideas About Atoms
Democritus – 460 BCE
 Philosopher
 All matter is made up
of indivisible particles
 Atoms means
“indivisible”
 No experimental
evidence to support
idea
Democritus
Greek banknote with Democritus and drawing of atom
Where did Democritus get his ideas for
small, indestructible, indivisible atoms?
Atoms are small…
new wedding rings
old wedding ring
EVIDENCE: Old wedding rings get worn
away slowly, and get thinner and thinner,
but you never see the gold atoms on your
finger, so gold atoms must be very small!
Atoms are indestructible…
EVIDENCE: The Earth has been around a
long time. Even though mountains get
washed away, the rocks continue to exist.
New plants grow where old plants die.
Atoms are indivisible…
EVIDENCE: None.
This was a HYPOTHESIS made by
Democritus to explain nature as he saw
it.
Aristotle had other ideas
Greek banknote and coin picturing Aristotle
ARISTOTLE: Famous Greek philosopher, born 384
BCE. He was a student of Plato (another famous
philosopher) and the teacher of Alexander the Great,
who later conquered the world.
Aristotle’s idea of matter
Aristotle did not believe that
Democritus’s idea of atoms
was correct.
He believed all matter was made
from four elements:
Earth
Air
Fire
Water
Aristotle’s idea of matter
Aristotle was more
famous than
Democritus, so people
believed him, even
though he was wrong.
So Democritus’s idea
of atomos (atoms) was
lost for nearly 2000
years until John Dalton
brought it back in
1803.
Then in 1803…
John Dalton, an English chemistry teacher, made
several conclusions about atoms from his
experiments:
1. All elements are composed of tiny particles called
atoms, and the atoms of different elements are
different from one another.
2. Atoms of different elements combine to form
new substances in whole number ratios.
3. Chemical reactions occur when atoms are
rearranged so that they attach to different atoms.
Dalton’s Chemical Symbols
We use different symbols today.
Overarching Question
These ideas formed the basis of atomic theory.
Dalton was wrong about atoms being indivisible. So if
atoms aren’t indivisible, then where does our modern
idea of the atom come from?
Activity: Atomic Pudding
Objectives:
• I can interpret the results of each major experiment in
the development of atomic theory
• I understand and can describe the structure of an atom,
and am able to explain the differences between atoms of
different elements
1.
2.
3.
Examine and compare the different
models
Use new evidence to determine the
correct sequence of the models
Identify the parts of the atom
Atoms are the smallest units of an
element
Sulfur (S)
Average Atomic Mass and Reactivity
: a simplified representation of
something more complex, that facilitates
understanding certain aspects of a real object
or process
the smallest unit of an element that
retains the chemical properties of that
element.
ChemCatalyst
Which model represents Dalton’s idea that
atoms are indivisible?
Which model suggests that there are negative
particles in an atom?
Atomic Theory
 All
matter is made of atoms
first proposal of the atom, 442 BC
Solid sphere model, 1803
Plum pudding model, 1897
nuclear model, 1911
Solar system model, 1913
cloud model, quantum model, 1927-1932
How did Thomson know to run an
electrical current through matter?
Michael Faraday, English
chemist, born 1791.
In 1834 he learned that
matter would interact with
electricity.
He realized that electricity
had to me made up of
particles that could be
counted, but he did not
know what they were.
(They were electrons.)
ChemCatalyst
What are the three particles that make
up an atom?
 What is the charge of each particle?

J.J. Thomson
1856-1940

Discovered electron 1897
◦ Cathode Ray Experiment:
electric charges make
materials lose negatively
charged particles.
◦ Atoms contain negatively
charged particles

Plum Pudding model 1904
◦ Electrons in a soup of
positive charges
More About Charge…
Opposite charges attract, alike charges repel
(push away).
Cathode Ray Experiment
Thomson’s “Plum pudding” atom
– electron
ENGLISH PLUM PUDDING
– raisin
RULE: Matter is NEUTRAL, so for every electron
(one negative charge) they must also have a
proton (one positive charge) to balance the math to
zero charge (“neutral”).
Ernest Rutherford
1871-1937

Gold foil experiment:
◦ Shot alpha particles
(positively charged
particles) at gold atoms
◦ Most went through, but
some bounced back

Nuclear model 1911
◦ An atom’s mass is mostly in
the nucleus
◦ The nucleus has a positive
charge
◦ Electrons in fixed orbit
Rutherford’s Gold Foil Experiment
Rutherford gold foil experiment
Rutherford expected the a
particles to pass straight
through, like this…
But what he got was most
a particles went straight
through, but a few were
deflected, like this…
Problem with Rutherford’s Model
But there also was a problem
with Rutherford’s nuclear atom!
To prevent the negative (-)
electrons from being attracted
to the positive (+) nucleus, the
electrons would have to orbit
the nucleus like the Earth orbits
the sun.
Why don’t the electrons crash into the
nucleus?
Niels Bohr
1885-1962

Each element emits specific
set of wavelengths of light
(colors)
◦ Wavelength = amount of
energy
◦ Electrons of those elements
have different energy levels

Solar system Model 1913
◦ Nucleus surrounded by
orbiting electrons at different
energy levels
◦ Electrons have definite orbits
Bohr’s Quantized Energy Levels
Bohr’s new atomic model had quantized energy
levels, meaning the electrons could only move
by jumping between levels (numbered n = 1, n
= 2,
n = 3, etc.). They absorb energy when they
jump out, and emit (send out) energy when
they jump in.
Ernest Rutherford
1871-1937

Alpha particles can knock
positively charged particles
out of the nucleus.
◦ Nucleus contains positively
charged particles called
protons

Proton model 1918
◦ Central nucleus made of
protons
◦ Tiny orbiting electrons
(negatively charged)
◦ Most atom is empty space
ChemCatalyst
 Who
discovered the electron?
 What did Bohr’s research change
about the atom?
Ernst Schrödinger 1887-1961
Werner Heisenberg 1901-1976
Quantum Mechanical
Model 1926
◦ Electrons are in
probability zones called
“orbitals”, not orbits and
the location cannot be
pinpointed (Heisenberg
uncertainty principle)
Cloud/Quantum mechanical model
James Chadwick
1891-1974
 Discovered
Neutron
1932
 Neutrons hold
positively charged
nucleus together
Atomic Theory
JJ
Thomson
Democratus
400 BC
1803
John
Dalton
1904
Niels
Bohr
1910
Ernest
Rutherford
1913
1926
Schroedinger
/ Heisenberg
The Complete Modern Atom
An atom is mostly empty space with a nucleus
containing protons and neutrons which contain
most of the mass of an atom.
Electrons are in energy levels around the
nucleus, and are very tiny.
Electrons jump between levels, emitting and
absorbing energy as they jump. We often refer
to these energy levels as “orbitals”, but real
orbitals are much more complicated…
Simple atomic model
Includes:
1. Rutherford’s discovery of the proton
2. Chadwick’s discovery of the neutron
3. Bohr’s discovery of electrons in orbits
Regions of an atom
Electron
cloud:
Nucleus:
•center of the
atom
•Makes up most
of its mass
•Mostly empty
space
•Take up most
of the radius
of the atom
Subatomic Particles
Nucleus
Electron
Cloud
Protons (+)
electrons (-)
Neutrons
(Neutral)
Atomic Particles
* amu = atomic mass units
Particle
Charge
mass
Relative
mass
proton
+1
1 amu
neutron
0
1.67 x 10-24
g
1.67 x 10-24
g
electron
-1
9.11 x 10-28
g
1/1840 amu
1 amu
A neutral atom has no net charge: number of
protons is equal to the number of electrons.
#P +
#e
=
0
Images of atoms
Scanning tunneling
electron
microscope 1981
Bombards surface
with electrical
current (electrons)
 Read changes in
current, translate
into image
