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Wonders of the
Atom
Within the atom
A positive particle with the same charge as an
electron (1.6 × 10−19 𝐶), but nearly 2,000
times the mass (1.7 × 10−27 𝑘𝑔). An element
has the same number of protons as electrons.
A negative charge. Very far away from
the protons, weigh only 9.1 × 10−31 𝑘𝑔.
Hang out with the protons, same
mass (more or less) not always same
numbers of neutrons as protons.
An Atom’s two
regions
Nucleus  very small region located near the center of an atom.
1.
a)
b)
In the nucleus there is at least one positively charged particle called the proton.
Usually at least one neutral particle called the neutron.
2. Surrounding the nucleus is a region
occupied by negatively charged particles
called electrons.
Nucleons
How Big?
Car… still much
bigger than a pea!
Atoms are different sizes, but they are on the scale of the nucleus (all those protons and
neutrons) packed into a pea….Picture that pea sitting in the middle of a stadium The
electrons would be whizzing away somewhere in the stands.
Review
Name
The atom has come along way through history…
Points
Democritus • Different for different elements.
• Smallest possible “object.”
• Indivisible.
Dalton
• Spherical.
• Combine in set ratios.
Thomson
• Positive “pudding.”
• Negative “plums.”
Rutherford • Small, dense, positive nucleus.
• Electron cloud.
• Divisible.
Bohr
• Electrons orbit nucleus.
• Electrons have set energy levels.
Picture
Up &
Atom
Models of Atoms Through the Years.
This model of the atom may look familiar
to you.
This is the Bohr model.
In this model, the nucleus is orbited by
electrons, which are in different energy
levels.
http://www.colorado.edu/physics/2000/quantumzone/bohr.html
What a
BohR
Electrons Have Specific Energy Levels
• Bohr’s model aimed to explain spectral lines.
• When electrons lose energy they emit particular
frequencies of light.
• Bohr showed these particular energies as
“orbitals,” similar-looking to the solar system.
An electron orbital is a region around an atomic nucleus (not seen) in which one or a pair of electrons is
most likely to exist. For each orbital, The red area is where an electron has a positive wavefunction, and the
blue area is where the wavefunction is negative. The number and distribution of electrons in an atom's
orbitals plays a major role in determining the reactivity and chemical properties of the atom.
Let’s look at a few elements…
Hydrogen
1= Proton
1= electron
Let’s look at a few elements…
Helium
2=protons
2=electrons
2=neutrons
Let’s look at a few elements…
Lithium
3=protons
3=electrons
4=neutrons
Let’s look at a few elements…
Fluorine
9=protons
9=electrons
10=neutrons
Let’s look at a few elements…
Argon
18=protons
18=electrons
22=neutrons
Decay types
Decay
What Happens?
How’s the Nucleus?
Alpha
An alpha particle emitted from nucleus
2 less protons
2 less neutrons
Beta
(minus)
A nucleus emits an electron
1 neutron changes to a proton
1 less electron
Beta
(plus)
A nucleus emits an positron
1 proton changes to a neutron
1 less electron
Gamma
Excited nucleus releases a high-energy photon Parts remain the same, but
(gamma ray)
nucleus is less excited
Neutron A neutron ejected from nucleus
1 less neutron
𝑬=
𝟐
∆𝒎𝒄
Energy and mass are exchangable
Energy cannot be created nor destroyed, and energy, in all of its
forms, has mass. Mass also cannot be created nor destroyed,
and in all of its forms, has energy.
For example, a water molecule weighs a little less than two free
hydrogen atoms and an oxygen atom; the minuscule mass
difference is the energy that is needed to split the molecule into
three individual atoms (divided by c²), and which was given off as
heat when the molecule formed (this heat had mass).
𝑬=
𝟐
∆𝒎𝒄
Energy and mass are exchangable
Likewise, a stick of dynamite in theory weighs a little
bit more than the fragments after the explosion.
but this is true only so long
as the fragments are cooled
and the heat removed….
In this case the mass difference is the energy/heat that is released
when the dynamite explodes, and when this heat escapes, the mass
associated with it escapes, only to be deposited in the surroundings
which absorb the heat (so that total mass is conserved).
𝑬=
𝟐
∆𝒎𝒄
Whenever energy is added to a
system, the system gains mass.
• A spring's mass increases whenever it is stretched or compressed.
Its added mass is the added potential energy stored within it, which
is bound in the stretched electron bonds linking the atoms within the
spring.
𝑬=
𝟐
∆𝒎𝒄
Whenever energy is added to a
system, the system gains mass.
• Raising the temperature of an object (increasing its heat energy)
increases its mass.
For example, consider the world's primary mass standard for the kilogram, made
of platinum/iridium. If its temperature is allowed to change by 1°C, its mass will
change by 1.5 pg (1 pg = 1 × 10−12 g).
𝑬=
𝟐
∆𝒎𝒄
Whenever energy is added to a
system, the system gains mass.
• A spinning ball will weigh more than a ball that is not spinning. Its increase of mass is
exactly the equivalent of the mass of energy of rotation,
• Which is itself the sum of the kinetic energies of all the moving parts of the ball).
For example, the Earth itself is more massive due to its daily rotation, than it would be with
no rotation. This rotational energy (2.14 x 1029 J) represents 2.38 billion tonnes of added
mass.
Note that no net mass or energy is really
created or lost in any of these examples and
scenarios. Mass/energy simply moves from
one place to another.
Fission and Fusion
Atoms are the building blocks from which matter is formed. Everything around us is made up of atoms. Nuclear
energy is contained within the centre of the atom in a place known as the nucleus. Particles within the nucleus are
held together by a strong force. If a large nucleus is split apart (fission), generous amounts of energy can be
liberated. Small nuclei can also be combined (fusion) with an accompanying release of energy. Using this strong force
that holds the nucleus together to produce energy is essentially what the field of nuclear power generation is about.
The Power of
the sun
You tube video:
http://www.youtube.com/watch?v=TOErr4xnt
HE&feature=related
Reactions:
http://science.howstuffworks.com/sun2.htm
Reactions in Detail:
http://zebu.uoregon.edu/~soper/Sun/fusions
teps.html
Fate of the Sun:
http://science.howstuffworks.com/sun6.htm
Fission In The Sun
Nuclear
Power
http://www.ho
wstuffworks.c
om/nuclearpower.htm
Power Derived from Nuclei
Control rods
Cadmium alloys, generally 80% Ag, 15% In, and 5% Cd, are a common
control rod material for pressurized water reactors. It has good
mechanical strength and can be easily fabricated. It has to be encased in
stainless steel to prevent corrosion in hot water.
They absorb the neutrons
Boron is another common
neutron absorber.
Mechanical properties of
boron in its elementary form
are unfavourable, therefore
alloys or compounds have to
be used instead.
In The
Reactor
They absorb the neutrons
The graphite
core slows the
neutrons down
which increases
the likelihood of
a collision.
Critical mass
A critical mass is the smallest amount of fissile material needed for a
sustained nuclear chain reaction. The critical mass of a fissionable
material depends upon its nuclear properties: its density, its shape, its
purity, its temperature and its surroundings.
Hydrogen bomb
Energy produced by fusion of lighter elements.
Binding
Energy
Energy produced by fusion of lighter elements
and fission for heavier elements
Black Body
All matter emits electromagnetic radiation when it has a temperature above absolute zero. The radiation represents a
conversion of a body's thermal energy into electromagnetic energy, and is therefore called thermal radiation. It is a
spontaneous process of radiative distribution of entropy.
Conversely all matter absorbs electromagnetic radiation to some degree. An object that absorbs all radiation falling
on it, at all wavelengths, is called a black body.
A black body is absorbs all wavelengths of light, and is also the perfect emitter of light.
At Earth-ambient temperatures this emission is in the infrared region of the electromagnetic spectrum
and is not visible. The object appears black, since it does not reflect or emit any visible light.
As the temperature increases past a few hundred degrees Celsius, black bodies start to emit visible
wavelengths, appearing red, orange, yellow, white, and blue with increasing temperature. When an object
is visually white, it is emitting a substantial fraction as ultraviolet radiation.
Black Body Radiation
Max
Planck
Plancking to the max since 1858 (before it was mainstream)
Planck was convinced that matter was quantised.
Planck turned his attention to the problem of black-body radiation. He had been commissioned by electric
companies to create maximum light from lightbulbs with minimum energy.
The question was this: How does the intensity of the radiation emitted by a black body depend on the
frequency and temperature?
The question had been explored experimentally, but no theoretical idea agreed with experimental values.
Planck proposed that electromagnetic energy could be emitted only in quantized form, in other words,
the energy could only be a multiple of an elementary unit 𝑬 = 𝒉𝒇, where ℎ is Planck's constant.
Louis de Broglie generalized this relation by postulating that the
𝑝 ∝ ℎ𝜆
Planck constant represents the proportionality between the
momentum and the quantum wavelength of not just the photon, but
any particle. This was confirmed by experiments soon afterwards.
The Planck constant, ℎ was first
described as the proportionality
constant between 𝐸 and 𝑓.
Electron volt
Is approximately 1.602×10−19joule (symbol J).
By definition, it is equal to the amount of kinetic energy gained by a single unbound electron
when it accelerates through an electric potential difference of one volt.
– Thus it is 1 volt (1 joule per coulomb) multiplied by the electron charge (1.602176565×10−19 C).
Therefore, one electron volt is equal to 1.602176565×10−19 J
By mass-energy equivalence,
the electron volt is also a unit of mass
We use this as a much
more convenient unit
instead of dealing with
tiny numbers.
Escapist Electrons
How do electrons remove themselves
from the strong hold of the nucleus?
As you know: Electrons are “bound” to the nucleus by the weak nuclear force, this is similar to
the way the earth is bound in orbit by the sun, although the weak nuclear force is actually much
stronger than the gravitational force!
• Too small an amount and the rocket would fall back to Earth, the electron would fall back into it’s orbit.
• Just the right amount and the electron will escape the nucleus but with no extra kinetic energy, i.e.
𝑣 = 0 𝑚𝑠 −1 .
• Any amount of energy the electron has extra to the particular energy needed to break free of that
metal will be transformed into kinetic energy.
Electrons must do work to escape the
nucleus, just as a rocket must do work
to escape the gravity of the Earth.
The Work Function 𝝓
Work function is the energy (or work) required to withdraw
an electron completely from a metal surface.
This is a measure of how tightly a particular metal holds its
electrons.
The more energy needed to remove an electron, the higher the
work function.
𝒉𝒇 = 𝝓 + 𝑬𝑲
Functions of different
elements
Compare Silver and Gold on the periodic
table to Calcium and Sodium
emission spectra
These are the specific
frequencies of light that
different elements emit.
Scientists were puzzled
for many years, they
decided to focus on trying
to explain the “simplest”
atom: Hydrogen.
Fun Fact:
Sodium is used in
many street lamps,
you can see the
emission spectra
shows yellows, hence
the tell-tale yellow of
the street lamp.
Hydrogen spectrum
Absorption spectra
show all the
frequencies the
element absorbs.
Emission spectra
show all the
frequencies the
element emits.
Hydrogen spectrum
Because 𝐸 = ℎ𝑓
these lines show
not only different
frequencies, but
different energies.
http://www.ucolick.org/~bolte/AY4_00/week2/atomic_spectra.html
Hydrogen spectrum
http://www.ucolick.org/~bolte/AY4_00/week2/atomic_spectra.html
Hydrogen spectrum
Schrödinger'sTheCat
Cat that Defies Logic
http://www.tcd.ie/Physics/Schools/
what/atoms/quantum/cat.html
Bohr
Towards the end of his career Bohr took a more interpretative role and
struggled more and more with the philosophical issues of quantum
mechanics First, he came up with the idea of complementarity.
• He noted that the wave and particle views of an object exclude each other totally but
conceded that both are needed in order to fully understand the properties of the object.
He suggested that the interpretation to use depends on what
apparatus are used to view the object.
• Electrons look like particles if probed with photons,
• but like waves if diffracted through a crystal lattice.
Bohr dragged the ideas of matrix mechanics, the Heisenberg
uncertainty principle.
Experimental results
For a given metal, with a particular work function (𝝓) and incident radiation,
with frequency (𝒇):
• The rate at which photoelectrons are ejected is directly proportional to the intensity of the
incident light.
• There exists a certain minimum frequency of incident radiation below which no photoelectrons
can be emitted. This frequency is called the threshold frequency.
• Increase in intensity of incident beam increases the the photoelectric current, though stopping
voltage remains the same. It does not change the kinetic energy of the photoelectrons.
• Increase in frequency of incident beam increases the maximum kinetic energy with which the
photoelectrons are emitted. Thus the stopping voltage increases.
(In practice the number of electrons does change because the probability that each photon results in an emitted
electron is a function of photon energy).
• The time lag between the incidence of radiation and the emission of a photoelectron is very
small, less than 10−9 second.
Explanation
The photons of a light beam have a characteristic energy determined by the frequency of the light.
•
•
In the photoemission process, if an electron within some material absorbs the energy of one photon and thus has
more energy than the work function, it is ejected.
If the photon energy is too low, the electron is unable to escape the material.
Increasing the intensity of the light beam increases the number of photons in the light beam, and thus
increases the number of electrons excited, but does not increase the energy that each electron
possesses.
The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the
energy or frequency of the individual photons. It is an interaction between the incident photon and the
outermost electron.
•
Electrons can absorb energy from photons when irradiated, but they usually follow an "all or nothing" principle.
All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or
else the energy is re-emitted. If the photon energy is absorbed, some of the energy liberates the electron from
the atom, and the rest contributes to the electron's kinetic energy as a free particle.
The threshold frequency is typically visible light for alkali metals, near
ultraviolet for other metals, and extreme ultraviolet for non-metals.
Photoelectric effect
http://www.nobelprize.org/educational/physics/quantised_world/
The intensity of the light had no effect on
the energy of the ejected electrons.
Moreover, experiments showed that
there was a threshold frequency below,
which not a single photoelectron was
ejected. Below this frequency, the
brightness of the incident light made no
difference at all! Classical physics had
failed again – it could not explain either
of these observations
PhotoVoltaic effect
In the photoelectric effect, electrons are ejected from a material's
surface upon exposure to radiation of sufficient energy.
The photovoltaic effect is the creation of a voltage (or a corresponding
electric current) in a material upon exposure to light.
Though the photovoltaic effect is directly related to the photoelectric
effect, the two processes are different.
Hydrogen spectrum
In fact, these precise spectral lines, have precise
frequencies and therefore precise wavelengths.
A man by the name of Balmer presented a formula, but
could not explain it. Twenty years later, Einstein and
Planck explained it for him using quantum mechanics.
1
1
1
=𝑅 2− 2
𝜆
2
𝑛
This was later generalised to the Reidberg formula:
1
1
1
=𝑅 2− 2
𝜆
𝑆
𝐿
𝑅 is the Reidberg
constant.
𝑅 = 1.1 × 107
𝑆 = 1 for UV, 𝑆 = 2 for visible, 𝑆 = 3 for Infrared
𝐿 is the orbital
(Starting value for 𝐿 is 𝐿 = 𝑆 + 1)
Different Series
Ultraviolet
Visible
Light
Infrared
Bohr’s Model
Bohr came to the conclusion that a
circular orbit would be unstable:
The electron would simply spiral
into the nucleus (like water down a
drain).
He proposed orbitals as standing
waves. Each 𝑛𝑡ℎ orbital having 𝑛
wavelengths.
Failings
While the Bohr model was a major step toward understanding the
quantum theory of the atom, it is not in fact a correct description of
the nature of electron orbits. Some of the shortcomings of the model
are:
1. It fails to provide any understanding of why certain spectral
lines are brighter than others. There is no mechanism for
the calculation of transition probabilities.
2. The Bohr model treats the electron as if it were a miniature
planet, with definite radius and momentum. This is in direct
violation of the uncertainty principle which dictates that
position and momentum cannot be simultaneously
determined.
The Bohr model gives us a basic conceptual model of electrons orbits and
energies. The precise details of spectra and charge distribution must be
left to quantum mechanical calculations, as with the Schrodinger equation.