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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 “not to be cut.”
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 with his Atomic theory as a result of his research into gases. He
Billiard Ball Model
discovered that certain gases only could 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 of Dalton’s Atomic Laws or Model. These laws focus on five basic
theorems.
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1: Pure Elements consist of particles called atoms.
2: Atoms of an element are all the same for that element. That means gold is gold and oxygen is
oxygen down to the last atom.
3: Atoms of different elements have different atomic weights, that means that Hydrogen is the lighest
element while Unubium is the heaviest.
4: Atoms of elements combine to form chemical compounds.
5: Atoms can neither be created or destroyed in chemical reaction. The atoms in each compound in the
chemical reaction are broken and recombined with the other atoms.
*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
Thomson's atomic theory proposed a model of atom which is known as plum pudding
model or Christmas pudding or chocolate chip cookie model.
Till the end of the nineteenth century the concept of atom was similar to a small solid
billiard ball.
In the year 1897 Joseph John Thomson (1856–1940) totally changed the view of an atom by
discovering electron. Thomson’s atomic theory suggested that the atom is not indivisible as it was of
smaller pieces – electrons and protons.
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An atom consists of a sphere of positive charge with negatively charged electron
embedded in it.
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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.
His experiment was using a
Cathode-Ray Tube. If you are
looking at the picture you will see
that Thomson using electricity forced
a stream of particles to be ejected
from the metal plate(Cathode) and
he observed that this stream of
particles attracted a positive plate,
which lead him to the conclusion that
the stream of particles were
negatively charged due to the rule of
“opposite attract”. He called each
negative particle the “electron”. Old
televisions are made up of
thousands of tiny small cathode ray
tubes that sit behind the class
screen.
*What experiments/labs have we done in class that prove that JJ Thomson was right in showing that atoms
have parts that have positive and negative charges? _______ (go whisper the answer to your teacher) ;)
Was JJ Thomson’s atomic model right? Read more to find out!
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ERNEST RUTHERFORD “THE GOLD FOIL EXPERIMENT
In 1911, Ernest Rutherford performed an experiment to test the plum pudding model. He fired
energetic alpha particles at a foil, and measured the deflection of the particles as they came out the
other side. From this he could deduce information 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 something about the internal structure of the mound if we know the
difference between the initial (before it hits the pile) and final (after it emerges from the pile)
trajectories 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 a’s 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 were undisturbed, 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 result lead him to believe 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 a few years earlier, discovered the 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 give off a different line spectra? Why?
(Whisper the answer to your teacher… ;)
He quickly realized that the two problems were connected and after some thought came up with the Bohr model
of the atom. Bohr's model of the atom revolutionized atomic physics.
The Bohr model consists of four principles:
1. Electrons assume 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 E1, the next closest E2 and
so on.
3. Light is emitted when an electron jumps
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 orbit
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. These energies naturally lead
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 spectra 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) The Zeeman effect
a. It was found that, when hydrogen gas was excited in a magnetic field, the produced emission
spectrum was split.
b. Bohr's model could not account for this
i. Solved by accounting for the existence of a tiny magnetic moment of each electron.
5) Stationary states
a. Although Bohr stated that electrons were in stationary states, he could not explain why.
i. The reason is covered later
QUANTUM MODEL OR QUANTUME MECHANICAL
MODEL
In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr
atom model 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.