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Lecture 15
The Atom
Ozgur Unal
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Consider the carbon.
You crush it into dust and get very small particles.
If you keep dividing into smaller and smaller particles,
eventually you will end up with the smallest carbon particle
that cannot be divided into other particles without changing
the properties of carbon.
The smallest particle of an element that retains the
properties of the element is called an atom.
How small do you think atoms are?
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Atoms are quite small, so is it possible to SEE them?
An instrument called scanning tunneling microscope (STM)
allows individual atoms to be seen.
Seeing atoms allows scientist to form shapes and patterns by
using them.
This lead to a new field
called nanotechnology.
www.princeton.edu/~kahnlab/STMImages.html
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Once the scientists were convinced that atoms exist, they
asked more questions about its structure.
Were atoms uniform throughout?
Were they made up of even smaller particles?
These questions eventually lead to the discovery of electron.
Scientists in the meantime started to make connections
between electric charge and matter.
Cathode ray tube is a device that
scientists used to study the
relationship between mass and
charge.
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English Physicist Sir William Crookes noticed a green flash
of light (called cathode ray) within a cathode ray tube.
This accidental discovery lead to the following results by
other scientists:
Cathode rays were a stream of charged particles.
The particles carried a negative charge (exact value not
known).
Since changing the metal of the electrodes or varying the
gas did not affect the cathode ray, researchers concluded that
this negative charge existed in all forms of matter.
These negatively charged particles are now known as
electrons.
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No one could determine the mass of a single cathode ray
particle…
Until English physicist JJ Thomson who began a series of
cathode ray tube experiments to determine the ratio of its
charge to its mass.
He was able to determine this ratio.
This ratio was much less than that of a hydrogen atom (the
lightest known atom).
This conclusion was shocking because it meant there were
particles smaller than the atom (Dalton had been incorrect!)
Atoms were divisible into smaller subatomic particles.
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After JJ Thomson, the next significant development about
electrons came with American physicist Robert Millikan.
He was able to determine the charge of an electron using oildrop experiment.
The charge of one electron is 1.602 x 10-19C.
This charge is now equated to a single unit of negative
charge.
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After finding the charge, Millikan was able to calculate the
mass of an electron, which is 9.1 x 10-28 kg, or 1/1840 times the
mass of one hydrogen atom.
The Plum Pudding Model:
Electrons are negatively charged and matter is neutral.
If the electrons are so small, what accounts for the rest of the
mass in a typical atom?
JJ Thomson proposed a model of atom to answer these
questions.
The model consisted of a spherically shaped atom composed
of a uniformly distributed positive charge in which the
individual electrons reside.
Check out Figure 4.9!
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Lecture 16
The Nucleus
Ozgur Unal
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Dalton’s Atomic Theory
J.J. Thomson’s Atomic Model
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Like charges repel each other, unlike charges attract each
other.
Alpha particles are positively charged.
Ernest Rutherford, in 1911, started experimenting to see if
alpha particles would be deflected as they passed through
thin gold foil.
According to plum-pudding
model of Thomson, alpha
particles should not be deflected
much.
This is because the positive
charge in an atom is uniformly
distributed within the atom.
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The set up of the experiment:
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According to the results of this experiment, most of the
alpha particles passed through the foil undeflected, some
particles were deflected at a small angle.
However, few alpha particles were deflected at large angles.
This was inconsistent with
Thomson’s atomic model.
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Rutherfod concluded that Thomson’s atomic model was
incorrect.
He calculated that an atom consisted mostly of empty space
through which electrons move.
He also concluded that atom’s positive charge and almost all
of its mass were concentrated in a tiny, dense region at the
center, which he called nucleus.
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Nucleus contains positively charged particles called protons.
It also contains neutral particles called neutrons.
The mass of a proton is nearly equal to the mass of a
neutron.
The repulsive force between the
positive nucleus and positive alpha
particles causes the deflections.
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All atoms are made up of electrons, protons and neutrons.
Atoms have dense nucleus, which consists of protons and
neutrons.
Most of atom consists of fast moving electrons traveling
through empty space surrounding the nucleus.
Most of atoms mass is
concentrated at the nucleus.
The nucleus occupies one
ten-thousandth of the total
volume of the atom.
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