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UNIT FIVE
The Atom and
Its Nucleus
Chapter 18
The Structure of the Atom
Lecture PowerPoint
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Have you ever seen an atom?
Why do we
think
atoms
exist?
Do you
believe
in
atoms?
Although atoms can’t be seen
directly, atomic phenomena are
evident in our everyday world...
Televisions
Diagnostic
X rays
Chemical
changes
in our
bodies
The Existence of Atoms:
Evidence from Chemistry
 Why believe in the existence of something we
have never seen?




Observations can provide convincing evidence.
Much early evidence of atoms came from
chemistry, the study of the differences in
substances and how they can be combined to
form other substances.
Early Greek philosophers believed in four
elements: fire, earth, water, and air.
Since certain substances were retrievable, early
scientists were also tempted to believe that certain
elements were made up of tiny indivisible atoms.
The Birth of Modern
Chemistry
 Lavoisier discovered that
the total mass of chemical
reactants and products is
conserved in chemical
reactions.
 Combustion combines
oxygen from air with
carbon in coal or wood to
form carbon dioxide and
water vapor.

The role of these gases is
easily overlooked.
 Dalton established that the reactants combine in
the same proportions by mass.


8 grams of oxygen always combine with 1 gram of
hydrogen to form water.
Dalton’s law of definite proportions
 Each element’s atoms have
the same mass.
 In chemical reactions, atoms
of one element combine with
atoms of another element in
definite proportions to form
molecules.
 By studying several reactions
involving the same elements,
the relative atomic masses
can be determined.
If water is formed by a ratio of 8 grams of
oxygen to 1 gram of hydrogen, and carbon
dioxide is formed by a ratio of 8 grams of
carbon to 3 grams of oxygen, what are the
relative masses of oxygen, carbon, and
hydrogen atoms?
a)
b)
c)
d)
8:3:2
16:6:2
16:12:1
32:12:2
c) If two oxygen atoms combine with one carbon
atom in a ratio 8:3 (or 32:12) and one oxygen
atom combines with two hydrogen atoms in a
ratio 8:1 (or 16:2) then the relative masses are
Two oxygen atoms : one carbon atom : four
hydrogen atoms = 32:12:4
One oxygen atom : one carbon atom : one
hydrogen atom = 16:12:1
Fuel Cells and the
Hydrogen Economy
 Almost all energy comes from fossil fuels: oil,
natural gas, and coal



These produce significant amounts of pollution.
They also produce carbon dioxide, a greenhouse
gas that contributes to global warming.
They are also nonrenewable.
 One alternative to fossil fuels is the hydrogen
fuel cell.


Converts hydrogen and oxygen into water,
generating electricity.
Similar to a battery, except the hydrogen and oxygen
are stored externally and so can be replenished.
Fuel Cells and the
Hydrogen Economy
 The proton exchange membrane (PEM) fuel
cell is most promising for use in cars and light
trucks.





Hydrogen gas is fed to the anode.
H2 molecules react chemically with the platinum catalyst and
splits into two protons and two electrons.
The PEM allows the protons through to the cathode.
The electrons must travel through a circuit through an electric
motor to get to the cathode.
The protons and electrons arriving at the cathode react with
oxygen molecules to form oxygen.
2H2 + O2 => 2H2O
Fuel Cells and the
Hydrogen Economy
 Uses oxygen from the air
 Pollution-free, producing only electricity and
water
 NASA has been using fuel cells for years
 Practical challenges:



Developing reasonably priced, long-lasting fuel cells
Developing effective hydrogen delivery and storage
systems
Developing environmentally sound methods for
producing the hydrogen
 By listing the elements in order of increasing
atomic mass, Mendeleev organized the elements
into a table with elements of similar properties
aligned into columns. This is called the periodic
table.
Cathode rays, Electrons,
and X-rays
 By the end of the nineteenth century, chemists
were using the concept of atoms to explain their
properties.
 Physicists were less convinced.

The discovery of cathode rays was the beginning of
atomic physics.
Two electrodes are sealed in a glass tube.
As the tube is evacuated, a glow discharge
appears in the gas between the electrodes.
With further evacuation, the discharge
disappears, and a glow appears on the end
of the tube opposite the cathode.
 An invisible radiation seemed to emanate from
the cathode to produce the glow on the
opposite wall of the tube.

The invisible radiation was called cathode rays.
 If the north pole of a magnet is brought down
toward the top of a cathode-ray tube, the spot
of light is deflected to the left across the face of
the tube.

This indicates the cathode rays are negatively
charged particles.
Two electrodes are sealed in a glass
tube.
As the tube is evacuated, a glow
discharge appears in the gas between
the electrodes.
With further evacuation, the discharge
disappears, and a glow appears on the
end of the tube opposite the cathode.
 J. J. Thomson used both electric fields and
magnetic fields to deflect the beam.
 The combined effect allowed him to estimate the velocity of
the particles.
 With the deflection produced by the magnetic field alone,
this allowed him to estimate the mass of the particles.
 We now call these particles electrons.
An electron beam in a cathode-ray tube passes
between two parallel plates that have a voltage
difference of 300 V across them and are separated
by a distance of 2 cm. In what direction will the
electron beam deflect?
a)
b)
c)
d)
upward
downward
out of the page
into the page
a) The electron beam is deflected upward because the force
on a negative charge is opposite to the direction of the
electric field.
An electron beam in a cathode-ray tube passes
between two parallel plates that have a voltage
difference of 300 V across them and are separated
by a distance of 2 cm. What is the value of the
electric field between the plates?
a)
b)
c)
d)
6 N/C
150 N/C
600 N/C
15,000 N/C
d) From V = Ed:
E = V / d = (300 V) / (0.02 m) =15,000 N/C
An electron beam in a cathode-ray tube passes
between two parallel plates that have a voltage
difference of 300 V across them and are separated
by a distance of 2 cm. What is the magnitude of the
force exerted on the electron?
a)
b)
c)
d)
2.4 x 10-15 N
24,000 N
1.5 x 1015 N
15,000 N
a) From F = qE, q = 1.6 x 10-19 C:
F = (1.6 x 10-19 C)(15,000 N/C) = 2.4 x 10-15 N
An electron beam in a cathode-ray tube passes
between two parallel plates that have a voltage
difference of 300 V across them and are separated
by a distance of 2 cm. What is the acceleration of
the electron?
a)
b)
c)
d)
2.64 x 10-15 m/s2
2,640 m/s2
26,400 m/s2
2.64 x 1015 m/s2
d) From F = ma, m = 9.1 x 10-31 kg:
a = F / m = (2.4 x 10-15 N) / (9.1 x 10-31 kg) = 2.64 x 1015 m/s2
An electron beam in a cathode-ray tube passes
between two parallel plates that have a voltage
difference of 300 V across them and are separated
by a distance of 2 cm. What path will the electron
follow?
straight line, up and
to the right
straight line, down
and to the right
26,400 m/s2
parabolic curve, up
and to the right
parabolic curve,
down and to the
right
a)
b)
c)
d)
c)
It will move in a parabolic path toward the positive charged
plate, similar to a body moving in a constant gravitational field.
 You probably use cathode rays almost
every day.
 The heart of most television sets is the
cathode ray tube, or CRT.
Do you
know how
a TV
works?
 The electrodes that produce and focus the electron beam are called
the electron gun.



An electric current passes through the filament to heat the cathode to emit
electrons.
Electrons are accelerated from the cathode to the anode by the high
voltage.
Electrons passing through the hole in the anode make up the electron
beam.
 After leaving the electron gun,
the beam of electrons travel
across the tube, producing a
bright spot of light when it
strikes the glass face of the
tube.
 Magnets deflect the beam so
that it strikes different points on
the face of the tube at different
times.
 The beam scans across the
entire face of the tube in a
fraction of a second, to form the
picture.
 Thomson’s discovery provided the first known subatomic
particle.



The mass of an electron is 9.1 x 10-31 kg.
The charge of an electron is 1.6 x 10-19 C.
The electron was the first possible candidate for a building block of
atoms.
 The study of cathode rays led Roentgen to discover yet
another type of radiation.
 He noticed that a fluorescent material would glow when
placed near his covered cathode-ray tube.
 Cathode rays could not travel through air, but this new
radiation did.
 Because they were unknown, Roentgen called this new
radiation X-rays.
 X-rays are produced by collisions of the cathode rays
(electrons) with the walls of the tube or with the anode.
 The strongest X-ray beams are produced by placing the
metal anode at a 45o angle to the beam, such as in an Xray tube used in a diagnostic X-ray machine.
The discovery of X-rays led to the discovery of natural
radioactivity...
Radioactivity and the
Discovery of the
Nucleus
 When Becquerel placed a piece of phosphorescent
material on a covered photographic plate, the
developed plate showed a silhouette of the sample.
 Radiation apparently was passing from these
materials to expose the film.
 The penetrating radiation did not seem to be
connected with the phosphorescence.
 Becquerel named this new radiation natural
radioactivity because it seemed to be
produced continuously by compounds
containing uranium or thorium.




Where did these rays come from?
How could rays continue to be emitted when no
energy was being added to the samples?
Was this radiation somehow a property of the
atoms themselves?
Is more than one type of radiation involved?
 Rutherford noticed when the beam of radiation from a
uranium sample passes through a magnetic field, it splits
into three components.


Alpha deviates slightly to the left,
indicating positively charged
particles.
Beta is bent in the opposite
direction, indicating negatively
charged particles.



Beta is also bent much more,
indicating less massive particles
than those in the alpha beam.
Further study indicated that
these beta rays were electrons.
The gamma rays were undeviated
by the magnetic field.

These are electromagnetic waves
similar to X-rays but with shorter
wavelengths.
 The alpha particles were established to be helium atoms
stripped of their electrons.
 Rutherford realized that they would make effective probes
for studying the structure of the atom.





A beam of alpha particles was scattered from a thin gold foil.
The size of the gold atoms would affect the angle of the scattered
alpha particles.
Most of them went straight through the foil, as expected.
This was consistent with the plum-pudding model of the atom.
BUT, a few particles scattered back at much larger angles.
 The backward scattering was as if someone had fired
bullets into a piece of tissue paper and the bullets bounced
back.
 If most of the alpha particles go through, but a few are
scattered through large angles, there must be very dense
but small centers somewhere within the atoms.



The nucleus of the atom had been discovered!
The nucleus is the very dense center of the atom, containing most of
its mass and all of its positive charge.
The electrons are responsible for most of the atom’s size, but very
little of its mass.
Atomic Spectra and the
Bohr Model of the Atom
 The atom has a positively charged nucleus
that contains most of the mass, and
negatively charged electrons are arranged
somehow around this nucleus.



Could the atom be arranged like a tiny solar
system, with the electrons in orbit around the
nucleus?
Why didn’t the electron spiral into the nucleus
as it lost energy due to radiating
electromagnetic waves?
Why are the electromagnetic waves emitted
only with particular wavelengths?
 If a substance is heated and the emitted light is observed
through a prism, each substance produces characteristic
colors or wavelengths, called the atomic spectrum of that
substance.
 The spectrum of hydrogen is simple, with just four
wavelengths in the visible portion.
 Balmer discovered that the wavelengths of these four lines
could be computed from a simple formula:
where n and m are both integers
 1
1
1 
and R is the Rydberg constant:
 R 2  2 
n

m 
R = 1.097 x 107 m-1
Quantization of light energy
 To complete the picture of the atom, there was yet another
piece of the puzzle that needed to be discovered...
 Both Planck and Einstein contributed to the idea of light
quanta.




A blackbody radiator is represented by a hole in a metal or ceramic box.
The hole appears black at room temperatures.
When the box is heated, the hole emits a continuous spectrum of
electromagnetic radiation.
The average wavelength depends on the temperature.
Quantization of light energy
 Planck derived a formula that predicted the distribution of
wavelengths emitted, depending on the temperature.

His formula required that light could only be absorbed or emitted in
discrete chunks or quanta, whose energy depended on the
frequency or wavelength.
E  hf
where h = 6.626 x 10-34 J s
is called Planck’s constant.
 This idea was indeed radical.
 Einstein showed that the quantization of light energy
explains a number of other phenomena.

 The idea of light quanta (photons) having energies E = hf
prepared the way for a new model of the atom.
Bohr’s model of the atom
 Bohr combined all these ideas:
 the discovery of the nucleus
 knowledge of the electron
 the regularities in the hydrogen spectrum
 the new quantum ideas of Planck and Einstein
 He pictured the electron as orbiting
the nucleus in certain quasi-stable
orbits.
 Light is emitted when the electron
jumps from one orbit to another.
Bohr’s model of the atom
 The energy between the two orbits
determines the energy of the
emitted light quantum.
Bohr’s model of the atom
 The hydrogen spectrum
can be explained by
representing the
energies for the
different electron orbits
in an energy-level
diagram.
 The blue Balmer line is
produced by the
indicated jump.
What is the wavelength of the photon emitted
in the transition from n = 4 to n = 2?
∆E = E4 - E2
= - 0.85 eV - (-3.4 eV)
= 2.55 eV
E = hf where h = 6.626 x 10-34 J s
= 4.14 x 10-15 eV s
f=E/h
= (2.55 eV) / (4.14 x 10-15 eV s)
= 6.16 x 1014 Hz
= c / f
= (3 x 108) / (6.16 x 1014 Hz)
= 487 nm
Particle Waves and
Quantum Mechanics
 Bohr’s model of the atom generated intense
activity in physics.


Unanswered questions included why only certain
orbits were stable.
Quantum mechanics provided the answer.
 If light waves sometimes behave like
particles (as shown by Planck and Einstein),
could particles sometimes behave like
waves?
 De Broglie suggested that certain things traditionally
thought of as particles, such as the electron, might
sometimes behave like waves.
 The frequency and wavelength of the wave depend on the
particle’s energy and momentum:
E
f 
h
h

p
 If the electron in Bohr’s model of the atom were pictured as
a standing wave wrapped
around a circular orbit,
de Broglie showed that its
wavelength could only
take on certain values.
 These values yield the
quasi-stable orbits
predicted by Bohr.
 h 
L  n 
2 
 Schrödinger developed a theory of the atom that used
three-dimensional standing waves to describe the orbits of
the electron about the nucleus.

 Quantum mechanics was born.
Heisenberg Uncertainty Principle
 In quantum mechanics, the orbits are not simple curves as
in the Bohr model.
 Instead, they are three-dimensional probability distributions
centered on the nucleus.
 These describe the probability of finding electrons at certain
distances and orientations about the nucleus.
The waves cannot tell us exactly where the
particle is located.
The more precisely we know the particle’s
momentum, the less we know about the
particle’s location.
This limitation was
introduced by Heisenberg:
h
px 
2
 Quantum mechanics successfully predicts the structure and
spectra of atoms with many electrons.




Quantum numbers describe the various possible stable orbits.
No two electrons in an atom can have the same set of quantum
numbers.
Once an orbit is filled, other electrons
must take on higher values for at
least one of the quantum numbers.
The regularities of the periodic table
can be explained by the way the
electrons fill the available orbits.
 The theory also predicts the ways
different elements combine to
form compounds.
 Quantum mechanics has become
the fundamental theory of chemistry
as well as atomic, nuclear, and
condensed-matter physics.