DO NOT WRITE ON – CLASS SET
Example 1
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Example 2
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DEMOCRITUS'S ATOMIC THEORY
This is the Greek philosopher Democritus who began the search for a description of matter more than
2400 years ago. He asked: Could matter be divided into smaller and smaller pieces forever, or was
there a limit to the number of times a piece of matter could be divided?
His theory: Matter could not be divided into smaller and smaller pieces forever,
eventually the smallest possible piece would be obtained. This piece would be
indivisible. He named the smallest piece of matter “atomos,” meaning “indivisible” or
“uncuttable.”
To Democritus, atoms were small, hard particles that were all made of the same material but were
different shapes and sizes. Atoms were infinite in number, always moving and capable of joining
together.
*Democritus did no actual experiments or collecting of evidence, he just theorized.
DALTON'S ATOMIC THEORY
John Dalton (1766-1844) was such a brilliant youth that he became an English school teacher when
barely 12 years old.
Dalton came up with his Atomic Theory as a result of his research into gases. He
Billiard Ball Model
discovered that certain gases could only be combined in certain proportions
(even if two different compounds shared the same common element or group of
elements). Through deductive reasoning and experimentation, he made an
interesting discovery. His findings led him to hypothesize that elements combine at
the atomic level in fixed ratios. This ratio would naturally differ in compounds due
to
the unique atomic weights of the elements being combined.
This was a revolutionary idea but further experimentation by himself and others confirmed his theory.
The findings became the basis of Dalton’s Atomic Laws or Model. These laws focus on five basic
theorems.
1. All matter is made of atoms.
2. Atoms cannot be subdivided, made or destroyed.
3. All atoms of the same element are identical; different elements have atoms with different
properties.
4. Chemical reactions occur when atoms are rearranged.
5. Compounds are combinations of atoms in simple, whole number ratios.
*Remember we did an experiment about putting an alka seltzer tablet in a bottle with water to measure
the mass before the chemical reaction and after. We learned that the mass did not change from before and
after the reaction, what law is this called…? (if you are reading this quietly go up & tell your teacher the
answer) ;)
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THOMSON'S ATOMIC THEORY
Plum-Pudding Model
Before Thomson did his work at the end of the nineteenth century, the concept of the
atom was similar to a small solid billiard ball that could not be divided into smaller
parts.
Thomson's atomic theory proposed a model of the atom which is known as the “plum
pudding” model (or Christmas pudding, or chocolate chip cookie model). This new model
suggested that an atom had both positive and negative parts, and that those parts were distributed
evenly throughout the atom.
In the year 1897, Joseph John Thomson (1856–1940) completely changed the view of an atom by
discovering the electron. Thomson’s atomic theory suggested that the atom is not indivisible: it was
made of smaller pieces– electrons and protons.

An atom consists of a sphere of positive charge with negatively charged electron
embedded in it.

The positive and the negative charges in an atom are equal in magnitude, due to which an
atom is electrically neutral. It has no overall negative or positive charge.
J.J. Thomson designed an
experiment using a Cathode-Ray
Tube. You will see in the figure to the
right that Thomson used electricity to
force a stream of particles to be
ejected from the metal plate
(Cathode) and he observed that this
stream of particles was attracted a
positive plate. This evidence led him
to the inference that the particles
were negatively charged due to the
rule of “opposites attract.” He called
each negative particle an “electron.”
Old televisions are made up of
thousands of tiny cathode ray tubes
that sit behind the glass screen.
*What experiments/labs have we done in class that prove that J.J. Thomson was right in showing that
atoms have parts with positive and negative charges? _______ (go whisper the answer to your teacher) ;)
Was J.J. Thomson’s atomic model right? Read more to find out!
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ERNEST RUTHERFORD and “THE GOLD FOIL EXPERIMENT”
In 1911, Ernest Rutherford performed an experiment to test the “plum pudding” model. He fired
energetic alpha particles (overall positively charged) at a thin sheet of gold foil, and measured the
angles (deflection) of the particles as they came out the other side. From this evidence (his
observations of deflected alpha particles) he made an inference about the structure of the foil. To
understand how this works, imagine shooting a rifle at a mound of loose snow: one expects some
bullets to emerge from the opposite side with a slight deflection and a bit of energy loss depending on
how regularly the pile is packed. One can deduce (infer) something about the internal structure of the
mound if we know the difference between the initial trajectory (before it hits the pile) and final
trajectory (after it emerges from the pile) of the bullet. If the mound were made of loose, powdery
snow, the bullets would be deflected very little; if the bullets were deflected wildly, we might guess
that there was a brick of hard material inside.
Rutherford expected all of the particles to be deflected just a bit as they passed through the plum
pudding. He found that most of the alpha particles he shot at the foil were not deflected at all. They
passed through the foil and emerged undisturbed. Occasionally, however, particles were scattered at
huge angles. While most of the alpha particles passed through the foil, a few of them bounced back
directly. Imagine if something like this happened at our mound of snow. We shoot bullets at the pile
for days, and every round passes straight through, unperturbed – then a bullet hits the snow, reflects
back, and splinters the gun’s stock! Rutherford’s evidence led him to infer that most of the foil was
made of empty space, but had extremely small, dense lumps of matter inside. No other model
accounted for the occasional wide angle scattering of the alpha particle. With this experiment,
Rutherford discovered the nucleus.
Rutherford’s Atomic Model
The gold-foil experiment
He almost had it all figured out, but wait…?
Why don’t those negative electrons fall into the ohhh-so yummy positive nucleus? Why is the
atom STABLE and not constantly falling part due to the attraction of opposite charges?
READ MORE…
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NEILS BOHR 1885-1965
In 1913 Niels Bohr came to work in the laboratory of Ernest Rutherford. Rutherford,
who had recently developed a planetary model of the atom, asked Bohr to work on it
because there were some problems with the model: According to the physics of the
time, Rutherford's planetary atom should have an extremely short lifetime.
Bohr thought about the problem, but
also worked on a second problem.
When energized, hydrogen only gives
off certain colors of light.
Didn’t we do this as a class? Does
every element emit (give off) a
different set of lines in its emission spectrum? Why?
(Whisper the answer to your teacher… ;)
He quickly realized that the two problems were connected, and after some thought came up with a new model:
the Bohr model of the atom. Bohr's model of the atom revolutionized atomic physics.
The Bohr model consists of four principles:
1. Electrons occupy only certain orbits around
the nucleus. These orbits are stable and
called "stationary" orbits.
2. Each orbit has an energy associated with it.
For example the orbit closest to the nucleus
has an energy of E1, the next closest has an
energy of E2 and so on.
3. Light is emitted when an electron drops
from a higher orbit to a lower orbit and
absorbed when it jumps from a lower to higher orbit.
4. The energy and frequency of light emitted or absorbed is given by the difference between the two orbits’
energies.
With these conditions Bohr was able to explain the stability of atoms as well as the
emission spectrum of hydrogen. According to Bohr's model only certain orbits were
allowed which means only certain energies are possible. This understanding of the
energies of the orbits naturally led to the explanation of the hydrogen atom spectrum.
Bohr's model was so successful that he immediately received world-wide fame.
Unfortunately, Bohr's model worked only for hydrogen. What-ohhh?
Bohr’s Atomic Model
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Problems with Bohr’s Model….
1) Spectra of Large atoms:
a. It could NOT accurately calculate the spectral lines of larger atoms.
2) Relative Spectra Intensity
a. Bohr's model could not explain why the intensity of the spectral lines were NOT all equal.
3) Hyperfine spectral lines
a. With better equipment and careful observation, it was found that there were previously
undiscovered spectral lines
b. These were named Hyperfine lines and they accompanied the other more visible lines.
c. Bohr's model could not explain why this was the case due to the lack of equipment and
development in quantum physics.
i. The reason for these lines is actually because of a hyperfine structure of atoms.
4) Stationary states
a. Although Bohr stated that electrons were in stationary, stable orbits, he could not explain why.
i. The reason that electrons do not fall all the way into the nucleus is covered later
QUANTUM MODEL OR QUANTUM MECHANICAL
MODEL = CURRENT MODEL OF THE ATOM
In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr
model of the atom one step further. Schrödinger used mathematical
equations to describe the likelihood of finding an electron in a
certain position. This atomic model is known as the quantum
mechanical model of the atom. Unlike the Bohr model, the quantum
mechanical model does not define the exact path of an electron, but
rather, predicts the odds of the location of the electron. This model
can be portrayed as a nucleus surrounded by an electron cloud.
Where the cloud is most dense, the probability of finding the
electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud. Thus, this
model introduced the concept of sub-energy levels.
FINALLY THE END, but wait, there is more!
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SIR JAMES CHADWICK IN 1932
There was still one detail that had not been totally accounted for in all of the
models. Scientists knew that electrons and protons existed, however when adding
the masses of these 2 particles together it did not equal the mass of the atom. This
led scientists to realize that there is another particle in the atom that adds mass to
the atom. Rutherford proposed an idea that there was another particle in the
nucleus, but he had no evidence to show for his idea. Other scientists proposed
there were electrons inside the nucleus with the protons, but electrons have an
extremely small mass (about 1/2000 of a proton) which would not account for the
increase in mass. James Chadwick was the scientist that discovered this particle
known as the neutron.
Chadwick took the pure element Beryllium and bombarded it with alpha overall
positively charged) particles. The result of the impact knocked out parts of the atom which then struck paraffin
wax. Those parts knocked out the protons from the paraffin wax. It was detected the protons from the paraffin
wax moved in a straight line. Scientists knew that since likes repel, a positively charged particle would have
caused this particle to move in deflected angles. The protons from the paraffin wax moved in a straight line
which supported the idea that these “neutrons” were neutral in charge.
THE END