Download Atomic Models and the Development of the Atom

Name:_______________________________________ Date:______________ Period:__________
Your Notes
POGIL: Atomic Models and the Development of the Atom
Why?
Our understanding of the physical world has grown at an incredible rate in the last 200 years.
The key to the advances made in chemistry has been our growing knowledge about atoms. In
this unit we will look at some of the early historical discoveries that helped us build up a
useful 'working model' of the atom. Where did the idea of atoms originate? What is the
evidence that allows us to conclude that atoms exist? How have our models of the atom
evolved over time? Let’s take a tour through a history of scientific discovery and find
answers to these questions…
Model 2: The Early Years
Democritus was the first to believe the idea of the atom. He was a philosophers and believed
that the material world must be made of tiny indivisible particles called “atomos” His ideas
did agree with later scientific theory, but did not explain chemical behavior, and was not based
on the scientific method – but just philosophy Aristotle, who was more widely respected at the
time, rejected their beliefs and the “atomic” view faded.
Dalton came up with the theory that described atoms in postulates, meaning,
a set of concepts or theories proposed by scientists. Dalton used
experimentation to develop his postulates through testing gases. He first
differentiated atoms via atomic weight. His data was flawed, however, but he
is known as the father of atomic theory due to his immense strides in this
topic. His postulates are listed on the next page:
Name:_______________________________________ Date:______________ Period:__________
Your Notes
Indicates that the postulate is still held as true today.
Key Content Questions:
1. Even though new models were discovered after Dalton created his principles, which
of these principles are not held true today?
Model 3: The Scientists - JJ Thomson
Using a cathode ray tube, JJ Thomson discovered the electron. A cathode ray tube is
composed of a cathode or negative electrode, and an anode or positive electrode inside of an
evacuated glass container. (Electrodes are metal plates with positive or negative charges on
the surface).
(a) Thomson filled the CRT tube with various
gases and saw the same cathode rays
appear. Thomson found that when different
cathodes were used, the electron beam
generated was the same, thus he concluded
that electrons are a fundamental and universal
constituent of matter.
(b) Thomson put a magnetic field around the
cathode ray tube and the ray bent.
(c) Thomson put an electric field in the cathode
ray tube. It bent towards the positive plate.
Name:_______________________________________ Date:______________ Period:__________
Your Notes
J. J. Thomson had discovered electrons (although he called them “corpuscles”). He was
unable to determine the mass or charge of an electron separately, but he found the ratio of the
electron’s charge to its mass (charge-to-mass ratio) by designing an experiment using
magnetism to move the electrons in the opposite direction caused by the electric field:
e
= 1.759 x 108
m
Charge-to-Mass Ratio
C/g
e = magnitude of the electron charge in Coulombs (C) and m = mass of the electron in
grams
Thomson believed that the electrons were like
plums embedded in a positively charged
“pudding,” thus it was called the “plum
pudding” model.
These observations changed the way that atoms were
portrayed since it was now determined that atoms could
be further broken down.
Watch this video: : http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%25::100%25::/sites/dl/free/00725126
44/117354/01_Cathode_Ray_Tube.swf::Cathode%20Ray%20Tube
Key Content Questions:
2. When the magnet bent the cathode ray, this meant that the ray itself was
a. charged
b. uncharged
3. When Thomson put an electric field in the cathode ray tube, which charged plate did
the cathode ray bend? a. positive b. negative
4. What did Thomson determine the charge of an electron to be? a. positive b. negative
5. What was the significance of the fact that Thomson saw the same cathode rays for any
gas with which he filled the tube?
6. Beside the discovery of the electron, what else did Thomson discover?
Your Notes
Name:_______________________________________ Date:______________ Period:__________
Read this!
In 1886 (before Thomson) Eugen Goldstein noted that cathode-ray tubes with a perforated
cathode emit a glow from the end of the tube near the cathode. Goldstein concluded that in
addition to the electrons (cathode rays) that travel from the negatively charged cathode
toward the positively charged anode, another ray travels in the opposite direction, from the
anode toward the cathode. Because these rays pass through the holes, or channels, in the
cathode, Goldstein called them canal rays. These canal rays are now known as protons.
Protons are subatomic particles that carry a charge equal to but opposite that of an electron.
Model 4: The Scientists- Rutherford
In 1911, the model of the atom changed once again after Ernest Rutherford
discovered the nucleus. Two of Ernest Rutherford's students, Hans Geiger
and Ernest Marsden, were doing an experiment at Manchester University
with radiation. They were using the dense, positively charged particles (called
alpha particles) as 'bullets' to fire at a very thin piece of gold foil. They
expected the particles to barge their way straight through the gold atoms
unobstructed by the diffused positive charge spread throughout the atom as
Thomson described in his model. However, they got a big surprise.
Rutherford observed some particles that went straight through, but he also noticed some
deflections back towards him and off to the side. Rutherford said: "It was quite the most
incredible event that has ever happened to me in my life. It was almost as incredible as if you
fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
Rutherford Concluded That….
▫ The atom is mostly empty space
▫ All the positive charge, and almost
all the mass, is concentrated in a
small area in the center. He called
this a “nucleus”
▫ The nucleus is composed of protons
and neutrons (they make the
nucleus!)
▫ The electrons distributed around the
nucleus, and occupy most of the volume
This means that Thomson’s model was no longer
correct, and that protons were found in a dense area
of the atom called the nucleus. Electrons, on the
other hand, are found in the empty space around the
atom.
Rutherford’s Model of the atom
Your Notes
Name:_______________________________________ Date:______________ Period:__________
Key Content Questions:
7. What did Rutherford expect to happen in his experiment?
8. What actually happened? What did this mean about the make-up of the atom?
9. What part of the atom is Rutherford credited for discovering?
Video: http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
Model 5 The Scientists- Millikan
An experiment performed by Robert Millikan in 1909
determined the size of the charge on an electron. He also
determined that there was a smallest 'unit' charge, or that
charge is 'quantized'. He received the Nobel Prize for his
work. We're going to explain that experiment here, and show
how Millikan was able to determine the size of a charge on a
single electron.
What Millikan did was to put a charge on a tiny drop of oil,
and measure how strong an applied electric field had to be in
order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop,
and he could calculate the force of gravity on one drop, he could then determine the electric
charge that the drop must have. By varying the charge on different drops, he noticed that the
charge was always a multiple of -1.6 x 10 -19 C, the charge on a single electron. This meant that
it was electrons carrying this unit charge.
VIDEO: http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%::100%::/sites/dl/free/0072512644/117354/02_Millikan_Oil_
Drop.swf::Milikan%20Oil%20Drop
Key Content Questions:
10. What was Millikan’s main contribution to atomic theory? Give the value for this
discovery.
11. What was the experiment he used to do make this discovery?
Name:_______________________________________ Date:______________ Period:__________
Your Notes
Model 6 The Scientists-Bohr
In 1913, Niels Bohr came up with a theory that explained why electrons do not spiral inward
(as they should based upon classical physics and particle
movement). He stated that electrons follow two rules.
1.
Electrons orbit at specific distances from the
nucleus. In other words, all electrons can be found at
a certain distance from the nucleus in an orbital.
2. Atoms radiate (give off) energy when an electron
jumps from a higher-energy orbit to a lower-energy
orbit, and absorbs (take in) energy when the electron
gets boosted from a low-energy orbit to a highenergy orbit.
This model is known as the planetary model since the electrons
are orbiting around the nucleus much like the planets orbit
around the sun. The first orbital only holds two electrons.
Orbital two can hold up to eight electrons, orbital three can
hold up to eighteen electrons, and orbital four and higher can
hold up to thirty-two electrons. (2-8-18)
Key Content Questions:
12. What are the circles around the nucleus called? What particle resides in them?
13. How can electrons move from one orbital to another?
Model 7: Today’s Model…Quantum Mechanical Model
Fifteen years passed before another model of the atom was put into the mix. The
next model, the quantum mechanical model, is a mathematical model that predicts
the probability of electron locations and paths within an electron cloud. This model
resulted from scientists such as de Broglie, Schrödinger, and Heisenberg.
•
1925 - Louis de Broglie suggests postulated that if light can act as a particle,
then a particle might have wave properties
•
In 1926, Schrodinger further explained the nature of electrons in an atom by
stating that the exact location of an electron cannot be stated; therefore, it is
more accurate to view the electrons in regions called electron clouds; electron
clouds are places where the electrons are likely to be found. Did extensive
work on the Wave formula  Schrodinger equation
Your Notes Name:_______________________________________ Date:______________ Period:__________
•
The Heisenberg uncertainty principle states that it is impossible to know
both the momentum and the position of a particle at the same time.

James Chadwick realized that the atomic mass of most elements was
double the number of protons. He discovered the neutron in 1932
These scientists started to look at electrons acting as both particles and waves,
and determined that there can be no certainty in finding the position of an
electron if you know the momentum at which it travels. This led to the idea of
electrons existing in an electron cloud moving as both a particle and a wave at the same
time. Hence came the quantum mechanical model
.
Quantum Mechanical Model:
Key Content Questions:
14. List all the scientists, their discovering, and year of their discovering for the Quantum
Mechanical Model (use the picture above).
15. In the Quantum Mechanical model, how do electrons travel? Where are they located?