Atomic Theory
• Observation vs. Inference
• Square = Model/Theory Activity
• Black Boxes
– Hypotheses & Theory
• History of Atomic Structure
– Explain to a classmate and to one other outside of
class. Must Initial.
• Notes not used with outside person.
Ancient…Greek
• philosopher Democritus – “Atoms”
• Greek word atomos, which means “uncut” or
“indivisible.”
Aristotle did not think
there was a limit to the
division of matter.
Dalton (1803)
• all matter is made up of individual particles
called atoms, which cannot be divided.
Dalton’s Theory
• All elements are composed of atoms.
• All atoms of the same element have the same mass, and atoms of different
elements have different masses.
• Compounds contain atoms of more than one element.
• In a particular compound, atoms of different elements always combine in the
same way
Thomson (1904)
• Observation
Thomson (1904)
Thomson’s Experiments
In his experiments, Joseph John Thomson used a
sealed tube containing a very small amount of
gas.
Thomson (1904)
Thomson’s Experiments
In his experiments, Joseph John Thomson used a
sealed tube containing a very small amount of
gas.
Evidence for Subatomic Particles
• Thomson hypothesized that the beam was a stream of
charged particles that interacted with the air in the tube
and caused the air to glow.
• Thomson observed that the beam was repelled by the
negatively charged plate and attracted by the positively
charged plate.
•the particles had about
1/2000 the mass of a hydrogen
atom, the lightest atom.
Thomson revised Dalton’s model to
account for these subatomic particles.
• “plum pudding” model
Rutherford’s Atomic Theory
Alpha particles
The Gold Foil Experiment
Deflected
particle
Undeflected
particle
Gold atoms
Slit
Beam of alpha
particles
Alpha particles
Screen
Source of alpha
particles
Nucleus
Discovery of the Nucleus
• Thomson’s model did
not explain all of the
evidence from
Rutherford's
experiment.
Rutherford proposed a
new model.
• The nucleus of the atom is a
dense, positively charged mass
located in the center of the
atom.
Assessment Questions
1. Dalton’s theory did not include which of the following
points?
a.
b.
c.
d.
All elements are composed of atoms.
Most of an atom’s mass is in its nucleus.
Compounds contain atoms of more than one element.
In a specific compound, atoms of different elements always
combine in the same way.
Assessment Questions
1. Dalton’s theory did not include which of the following
points?
a.
b.
c.
d.
All elements are composed of atoms.
Most of an atom’s mass is in its nucleus.
Compounds contain atoms of more than one element.
In a specific compound, atoms of different elements always
combine in the same way.
ANS:
B
Assessment Questions
2. J. J. Thomson’s experiments provided the first evidence
of
a.
b.
c.
d.
atoms.
a nucleus.
subatomic particles.
elements.
Assessment Questions
2. J. J. Thomson’s experiments provided the first evidence
of
a.
b.
c.
d.
atoms.
a nucleus.
subatomic particles.
elements.
ANS:
C
Assessment Questions
1. The concept of an atom as a small particle of matter
that cannot be divided was proposed by the ancient
Greek philosopher, Democritus.
True
False
Assessment Questions
1. The concept of an atom as a small particle of matter
that cannot be divided was proposed by the ancient
Greek philosopher, Democritus.
True
False
ANS:
T
Atomic Theory Flip Book
•
Purpose: Create a flip-book to show how the Atomic Theory has changed over time. Worth 35 points.
•
Your flip-book must have the following:
Title page (creatively done)
One page for each of the following 7 names
–
–
–
–
–
–
–
Democritus
Aristotle
Dalton
Thomson
Rutherford
Bohr
Schrodinger & Heisenberg
Each page needs to have the following information: (4 pts for each scientist, 1pt if colored)
–
–
–
–
–
Full name(s)
Approximate time in history
Major points of contribution to the understanding of the atom and a brief description of experiments (if any)
Sketch or diagram of the scientists atomic model
One additional interesting fact about the person or model
How to Make a Flip-Book
•
Stack 4 sheets of paper, and place each sheet one higher than the previous sheet in the stack.
•
Bring the bottom of all 4 sheets upward and align the edges so that all of the layers are the same distance
apart.
•
When all the tabs are an equal distance apart, fold the papers and crease well.
•
Open the papers and glue them together all the center fold.
Protons, electrons, and neutrons
can be distinguished by mass,
charge, and location in an atom.
Atomic Structure
• No tool to “see”
inside an atom
• Indirect observations
Practice
The atomic number of aluminum is 13. An atom of
aluminum that has a mass number of 27 has 13
protons and 14 neutrons
More Practice
How many P+? E-? N0?
Bean Bag Isotopes….Activity
Isotopes
With most elements, it is hard to notice any differences in
the physical or chemical properties of their isotopes.
Hydrogen is an exception...
Hydrogen-1 has no neutrons. (Almost all hydrogen is hydrogen-1.)
Hydrogen-2 has one neutron
Hydrogen-3 has two neutrons.
..Because a hydrogen-1 atom has only one proton, adding a
neutron doubles its mass.
Isotopes
Water that contains hydrogen-2 atoms in place of
hydrogen-1 atoms is called heavy water. Hydrogen-2
atoms have twice the mass of hydrogen-1 atoms, so the
properties of heavy water are different from the
properties of ordinary water.
Assessment Questions
1. In which way do isotopes of an element differ?
a.
b.
c.
d.
number of electrons in the atom
number of protons in the atom
number of neutrons in the atom
net charge of the atom
Assessment Questions
1. In which way do isotopes of an element differ?
a.
b.
c.
d.
number of electrons in the atom
number of protons in the atom
number of neutrons in the atom
net charge of the atom
ANS:
C
Assessment Questions
1. Of the three subatomic particles that form the atom,
the one with the smallest mass is the neutron.
True
False
Assessment Questions
1. Of the three subatomic particles that form the atom,
the one with the smallest mass is the neutron.
True
False
ANS:
F, electron
4.3
Bohr’s Model of the Atom
What can happen to electrons when atoms gain or
lose energy?
Bohr’s Model of the Atom
Bohr’s model focused on the electrons and their
arrangement.
In Bohr’s model, electrons move with constant speed
in fixed orbits around the nucleus, like planets around
a sun.
Each electron in an atom has a specific amount of
energy.
Bohr’s Model of the Atom
Energy Levels
When an atom gains or loses energy, the energy of an
electron can change.
• The possible energies that electrons in an atom can have are
called energy levels.
• An electron cannot exist between energy levels.
Bohr’s Model of the Atom
An electron in an atom can move from one energy
level to another when the atom gains or loses
energy.
Electron
Electrons gain or
lose energy when
they move
between fixed
energy levels
Nucleus
Bohr Model
Bohr’s Model of the Atom
An analogy for energy levels of electrons is a staircase.
Bohr’s Model of the Atom
An electron may move up or down two or more
energy levels if it gains or loses the right amount of
energy.
The size of the jump between energy levels
determines the amount of energy gained or lost.
No two elements have the same set of energy levels.
Bohr’s Model of the Atom
Evidence for Energy Levels
Scientists can measure the energy gained when
electrons absorb energy and move to a higher
energy level or measure the energy released
when the electron moves to a lower energy level.
Light is a form of energy that can be observed.
Electron Cloud Model
What model do scientists use to describe how
electrons behave in atoms?
An electron cloud is a visual model of the most likely
locations for electrons in an atom.
Scientists use the electron cloud model to describe
the possible locations of electrons around the
nucleus.
Bohr’s  Electron Cloud Model
Today, scientists use probability when trying to predict
the locations and motions of electrons in atoms.
An electron cloud is a visual model of the most likely
locations for electrons in an atom.
Electron Cloud Model
The electron cloud model replaced Bohr's vision of
electrons moving in predictable paths.
The nucleus
contains protons
and neutrons
electrons
Electron Cloud Model
Electron Cloud Model
Electron Cloud Model
What model do scientists use to describe how
electrons behave in atoms?
An orbital is a region of space around the nucleus
where an electron is likely to be found.
The electron cloud represents all the orbitals in an
atom.
An electron cloud is a good approximation of how
electrons behave in their orbitals.
Electron Cloud Model
For an analogy to the concept of an orbital, imagine a
map of your school:
1 dot every 10 minutes between 0730 and 1400
Electron Cloud Model
The level in which an electron has the least
energy—the lowest energy level—has only one
orbital. Higher energy levels have more than one
orbital.
Electron Configurations
The most stable electron configuration is the one in
which the electrons are in orbitals with the lowest
possible energies.
Assessment Questions
1. According to Bohr’s model of the atom, which of the
following can happen when an atom gains energy?
a.
b.
c.
d.
An atom returns to its ground state.
A neutron can be changed into a proton.
A proton can move to a higher energy level.
An electron can move to a higher energy level.
Assessment Questions
1. According to Bohr’s model of the atom, which of the
following can happen when an atom gains energy?
a.
b.
c.
d.
An atom returns to its ground state.
A neutron can be changed into a proton.
A proton can move to a higher energy level.
An electron can move to a higher energy level.
ANS:
D
Assessment Questions
2. How does the modern atomic theory describe the
location of electrons in an atom?
a. Electrons move randomly in space around the nucleus.
b. Electrons can be described as a cloud based on probable
locations.
c. Electrons orbit the nucleus in the same way that planets
orbit the sun.
d. Electrons move in a spiral pattern if increasing distance from
the nucleus.
Assessment Questions
2. How does the modern atomic theory describe the
location of electrons in an atom?
a. Electrons move randomly in space around the nucleus.
b. Electrons can be described as a cloud based on probable
locations.
c. Electrons orbit the nucleus in the same way that planets
orbit the sun.
d. Electrons move in a spiral pattern if increasing distance from
the nucleus.
ANS:
B
Assessment Questions
3. What is meant when an atom is said to be in its ground
state?
a.
b.
c.
d.
There is no net charge on the atom.
The number of protons equals the number of neutrons.
The atom’s electrons all have the lowest possible energies.
It is the isotope with the least number of neutrons.
Assessment Questions
3. What is meant when an atom is said to be in its ground
state?
a.
b.
c.
d.
There is no net charge on the atom.
The number of protons equals the number of neutrons.
The atom’s electrons all have the lowest possible energies.
It is the isotope with the least number of neutrons.
ANS:
C