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UNIT 7: ATOMIC MODELS AND PHOTOELECTRIC EFFECT
1) Basing on the figure above,
a) How is the structure/arrangement of balls shown in the figure related
to an atom? You can use chemistry knowledge from O’level.
b) Relate the arrangement of electrons in an atom to how the balls in the
figure above are arranged.
c) Explain how movement of particles in an atom leads to release or
absorption of energy
2) It is important to realise that a lot of what we know about the structure
of atoms has been developed over a long period of time. This is often how
scientific knowledge develops, with one person building on the ideas of
someone else.In attempt to explain an atom, different scientists suggest
different models. An atomic model represents what the structure of an
atom could look like, based on what we know about how atoms behave.
It is not necessarily a true picture of the exact structure of an atom.
a) Why did these scientists use the word Model not exact structure of an
atom?
b) Can you explain some of the scientific models that tried to explain thestructure of an Atom?
7.1. Bohr model of the atom and energy levels
ACTIVITY 7.1
In year 1, you discussed about Rutherford model and this was a great
step in understanding atomic structure of an atom but it still had some
limitations that are listed below.
- Why doesn’t the electron fall into the nucleus since it revolves
around the nucleus
- Rutherford’s model could not explain the observed line spectra
of elements. As electrons spiraled towards the nucleus with
increasing speed, they should emit all frequencies of radiation not
just one. Thus, the observed spectrum of the element should be a
continuous spectrum not a line spectrum.
- The model did not explain the distribution of electrons outside
the nucleus.
a) Basing on Rutherford’s limitations above, suggest some of corrections
that would be made
b) Talk about the energy possessed by these electrons as they are in
energy levels.
c) Does an electron remain with same energy if it
i) Jumpsii) Drops from one energy level to another?
7.1.1. Bohr’s atomic model
In 1900 Max Planck (1858–1947)investigated the relationship between the
intensity and frequency of the radiation emitted by very hot objects. Planck
showed that the radiation from a hot body was emitted only in discrete
quantities or “packets” called quanta. The energy, E, of each quantum wasshown to be proportional to the frequency, f, of the radiation emitted:
Bohr’s thinking on a new atomic model was also guided by the work that had
been done on the spectrum of hydrogen.
In 1913 he develops a theory of the atom in which he assumes that: “Electrons
are arranged in definite shells, or quantum levels, at a considerable distance fromthe nucleus”.
7.1.2. Orbital radii, orbital speed and Energy level
Starting with these four postulates and using a mixture of Classical and Quantum
Physics, Bohr derived equations for:
The radii of the various stationary states and the velocity of an electron in
a particular stationary state;
Let’s assume the electron’s orbit is a circular obit with radius r which is
approximately the size of the hydrogen atom. Since the electron in Fig.7.2ismoving in a circle, there must be a force directed toward the center of the circle.
When light coming from a discharge tube containing hydrogen gas is passed
through a prism, a series of lines is observed in the visible part of the spectrum:
this is termed Balmer series. Some are found in the IR (Infra-Red) and UV
(ultraviolet) regions. Those lines detected in the UV are known as Lyman series
and those detected in the IR were discovered by Paschen, Brackett, and Pfund.
From 1884 to 1886, Johann Balmer, a Swiss school teacher, suggested a
mathematical formula to fit the known wavelengths of the hydrogen emission
spectrum:
When light coming from a discharge tube containing hydrogen gas is passed
through a prism, a series of lines is observed in the visible part of the spectrum:
this is termed Balmer series. Some are found in the IR (Infra-Red) and UV
(ultraviolet) regions. Those lines detected in the UV are known as Lyman series
and those detected in the IR were discovered by Paschen, Brackett, and Pfund.
From 1884 to 1886, Johann Balmer, a Swiss school teacher, suggested a
mathematical formula to fit the known wavelengths of the hydrogen emissionspectrum:
where
- m is an integer with a different value for each line (m = 3, 4, 5, 6)
- b is a constant with a value of 364.56 nm.
This formula produces wavelength values for the hydrogen emission spectral
lines in excellent agreement with measured values. This series of lines has
become known as the Balmer series.
Balmer predicted that there should be other series of hydrogen spectral lines
and that their wavelengths could be found by substituting values higher thanthe 2 shown on the right hand side of the denominator in his formula.
The great success of Bohr’smodel is that:
- It gives an explanation for why atoms emit line spectra, and accurately
predicts the wavelengths of emitted light for hydrogen.
- It explains absorption spectra: photons of just the right wavelength can
knock an electron from one energy level to a higher one. To conserve
energy, only photons that have just the right energy will be absorbed.
This explains why a continuous spectrum of light entering a gas will
emerge with dark (absorption) lines at frequencies that correspond to
emission lines.
- It ensures the stability of atoms. It establishes stability by decree: the
ground state is the lowest state for an electron and there is no lower
energy level to which it can go and emit more energy.
- It accurately predicts the ionization energy of 13.6 eV for hydrogen.
However, the Bohr model was not so successful for other atoms
Limitations of the Bohr model:
As with any scientific model, however, there were limitations. The problems
with the Bohr model can be summarized as follows:
- Bohr used a mixture of classical and quantum physics, mainly the former.
He assumed that some laws of classical physics worked while others did
not.
- The model could not explain the relative intensities of spectral lines.
Some lines were more intense than others.
- It could not explain the hyperfine structure of spectral lines. Some spectral
lines actually consist of a series of very fine, closely spaced lines.
- It could not satisfactorily be extended to atoms with more than one electron
in their valence shell because it does not account for the electrostatic
force that one electron exerts on another.
- It could not explain the “Zeeman splitting” of spectral lines under the
influence of a magnetic field.The Zeeman Effect is the splitting of atomic
energy levels and the associated spectral lines when the atoms are placed
in a magnetic field.- It could not explain the Stark effect (splitting up in electric field).
The picture above is a section of solar panels that were installed in
Rwamagana district to supplement power in the region.
a) From your experience what do you know about solar panels.
b) Using the knowledge of black bodies, explain how a solar panel
operates.
c) Explain why the same project may not work well in areas like
Musanze and Gicumbi.
d) Basing on your answers provided in the above questions, does lightcarry energy?
7.2.1. Photoelectric Effect
Photoelectric effect is the emission of electrons from the surface of metal
when illuminated with electromagnetic radiation of sufficient frequency. Amaterial that exhibits photoelectric effect is said to be Photosensitive.
4. Stopping voltage
The stopping potential does not depend on intensity, but does depend only on
frequency.
The only effect of increasing the intensity is to increase the number of electrons
per second and hence the photocurrent i. If the intensity of light is held constant
but the frequency is increased, the stopping potential also increases.
In other words, Greater intensity at a particular frequency means a greater
number of photons per second absorbed, and thus a greater number of electrons
emitted per second and a greater photocurrent. The greater the light frequencyis, the higher the energy of the ejected photoelectrons is.
7.2.4. Applications of photoelectric effect
The photoelectric effect has a number of applications. Digital cameras, studying
nuclear processes, chemically analyzing materials based on their emitted
electrons, image intensifiers andnight-vision scopes use it to convert light
energy into an electric signal that is reconstructed into an image.
On the moon, sunlight striking the surface causes surface dust to eject electrons,
leaving the dust particles with a positive charge. The mutual electric repulsion
of these charged dust particles causes them to rise above the moon’s surface, a
phenomenon that was observed from lunar orbit by the Apollo astronauts.
It led physicists to think about the nature of light and the structure of atoms in
an entirely new way
(a) Photo emissive cells
These are used in reproduction of sound in a film sound track and also in
controlling lift doors. Photo emissive cells are also used in security alarms. The
symbol for a photo emissive cell is shown below. Light falling on the cathode
ejects electrons which are attracted to the anode and a current flow.
(b) Photovoltaic cells
In photovoltaic cells, the ejected electron travels through the emitting material
to enter a solid electrode in contact with the photo emitter (instead of travelling
through a vacuum to the anode) leading to the direct conversion of radiant
energy to electrical energy. The more intense the light falling on the photocell,
the greater the conductivity of the photocell and the greater the current
measured by the ammeter (A).
Photovoltaic cells are used in calculators and light exposure metres in cameras.They can also drive small machines.
(c) Photoconductive cells.
Examples of photoconductive cells are photodiodes, photo resistors (lightdependent resistors, LDR) and phototransistors. These work on the principle that light reduces the resistance of some semiconductor materials such ascalcium sulphide.
7.3.1. Production of Cathode rays
Thermoelectric emission is the process by which electrons are emitted from a
metal surface when it has been electrically heated. The least energy an electron
requires to break away from the surface is called the work function and this
value varies from one metal to another. Substance with low work functions
emits electrons at lower temperatures compared with metals with higher work
function.
Cathode rays are stream of electrons that are moving at high speed. Cathode
rays are produced in a discharge tube which is a long (about 30 cm or more)
hard glass tube with two electrodes attached at its two ends. The electrodes
are made of any metal which is a good conductor such as copper, aluminium or
platinum, and are connected externally to a high voltage source. The discharge
tube has a facility to connect it to a vacuum pump. Gaseous discharge takes
place between the two electrodes and hence the name of the tube.
If the temperature of the metal is raised, the thermal velocities of the electrons
will be increased. The chance of electrons escaping from the attraction of the
positive ions, fixed in the lattice, will then also be raised. Thus by heating a
metal such as tungsten to a high temperature, electrons can be boiled off. This
called thermionic emission.
The electrode connected to the negative terminal is known as the cathode and
the terminal connected to the positive terminal is known as the anode. As the
discharge tube is evacuated, various changes take place.
- The air inside the discharge tube is a non-conductor of electricity and
therefore initially tube looks intact.
- As the air pressure inside reduces, the gas starts ionizing. Since a potential
difference is maintained inside the tube, when one gas atom is ionized, the
electrons escaping from it ionize other gas atoms. This creates a stream
of positive ions and negative electrons. These start moving towards thecathode and the anode respectively and generate a current.
- When the pressure is not very low, the gas movement looks like bluish
streaks. As the pressure reduces further, the gas inside looks pink.
- When the discharge tube is evacuated to a high degree, the inside will
start looking black, as there is no gas inside to conduct a current. This
dark space is called Faraday’s dark space. A small glow can be observed
at the cathode and the anode. This is due to residual gases.
- As the vacuum is reduced further, there will be a greenish glow behind the
anode. The rays or particles come from the cathode towards the anode.
Some of them overshoot the anode and reach the inner surface of the
tube. This causes the glow. These rays are called cathode rays. Since the
cathode rays come towards the anode, they must be negatively charged.
It has been proved that the cathode rays are nothing but electrons. As the
discharge tube is evacuated, the electrons at the cathode get attracted to the
anode due to the high potential difference. Cathode rays are not seen when the
potential difference is low or if he gas pressure is high.
7.3.2. Properties of cathode rays.
Cathode rays are moving electrons and have the following properties:
- They travel in straight lines and They carry negative charge.
- They are deflected by electric and magnetic fields.
- Cathode rays cause fluorescence on striking certain materials.
- They have energy and momentum.
- Cathode rays are capable of ionizing gas atoms if the potential difference
is large and the gas pressure is not high.
- Depending on their energy, cathode rays can penetrate thin sheets of
paper or metal foils.
- When cathode rays are stopped suddenly, they produce X-rays.
- They affect photographic plates.
7.3.3. Applications of cathode rays
a) Cathode ray oscilloscope
A Cathode Ray Oscilloscope (CRO) also called Oscillograph is an instrument
generally used in a laboratory to display, measure and analyze various
waveforms of electrical circuits. A cathode ray oscilloscope is a very fast X-Yplotters that can display an input signal versus time or other signal.
Cathode ray oscilloscopes use luminous spots which are produced by striking
the beam of electrons and this luminous spot moves in response variation in
the input quantity.
Nowadays, with the help of transducers it is possible to convert various physical
quantities like current, pressure, acceleration etc to voltage thus it enable us
to have a visual representations of these various quantities on cathode ray
oscilloscope.
The main part of cathode ray oscilloscope is cathode ray tube (CRT) which is
also known as the heart of cathode ray oscilloscope.
The CRT is a vacuum tube in which a beam of electrons is accelerated and
deflected under the influence of electric or magnetic fields. The electron beam
is produced by an assembly called an electron gun located in the neck of the
tube. These electrons, if left undisturbed, travel in a straight-line path until they
strike the front of the CRT, the “screen,” which is coated with a material that
emits visible light when bombarded with electrons. Electrons leaving the hot
cathode C are accelerated to the anode A. The beam of electrons produced is
called Cathode rays. In addition to accelerating electrons, the electron gun isalso used to focus the beam of electrons, and the plates deflect the beam.
This tube is commonly used to obtain a visual display of electronic information
in oscilloscopes, radar systems, television receivers, and computer monitors.
Functions of a Cathode Ray Oscilloscope
A cathode ray oscillograph is essentially an electrostatic instrument which
consists of a high evacuated glass tube. The features of a CRO (Cathode ray
oscilloscope) can be split into 3 main sections: The electron gun, the deflection
system and the fluorescent screen.
- Electron Gun: The role of this section is to produce electrons at a high,
fixed, velocity and focus them on the screen. This is done through a process
known as thermionic emission. A filament in the cathode is heated to the
point where its electrons become loose.
An anode with a high voltage applied to it accelerates the electrons towards
the screen due to electrostatic attraction. On the way, the electrons pass
through a series of control grids which control the brightness of the
image produced. The more negative the grid, the darker the image and
vice versa.
- Deflection system: The role of the deflection system is to control the image
produced by controlling the position that the electrons hit the screen.
It consists of Two perpendicular sets of Electric/Magnetic fields. This
allows control over both horizontal and vertical axes. By controlling the
Voltage applied to the fields, it is possible to vary the deflection through
Electrostatic force/Motor effect.
- Fluorescent screen: The role of this part is to display where the electrons
are hitting the CRT. It is a screen coated with a material that emits light
when struck by electrons. Zinc sulfide or Phosphorus are two commonly
used materials. The CRO is a perfect voltmeter as its input resistance is
very high. It is usually placed in parallel with a component.
The voltage is measured on the vertical axis, which is controlled by the
Y-plates.It can also be used as an ammeter by placing it across a resistor
of known resistance.The CRO is used to analyze waveforms. It can be used
to determine the peak voltage of an a.c. waveform and the period, which
in turn allows one to work out its frequency.
b) Televisions
A CRT TV works by having the electron beam “scan” the screen at a rate faster
than our eyes can perceive. This means that it shoots across the screen like a
machine gun, and the images we see are actually made from many fluorescent
dots.
The fluorescence caused by the beam striking the screen lasts a bit longer so
that the next scan can be made without the previous image disappearing. It
scans twice each time, first filling in the odd “holes” then the even ones. Each
scan is about 1/50 of a second.
Colour CRT TVs has electron guns rather than a single one, a shadow mask,
and a modified fluorescent screen. The 3 electron guns are needed as there are
three primary colours (Red, Green and Blue) that can be adjusted in different
amounts to create any colour.
The colours are formed as a result of the shadow mask, which is a layer with
holes in it that controls the angle of the incoming electron beams. This is
because the fluorescent screen is separated into multi-coloured phosphors
that are placed adjacent to each other at small intervals. Thus it isn’t actually a
single coloured pixel, but rather 3 very small pixels that join together to form a
larger dot.
The vertical sensitivity defines the voltage associated with each vertical
division of the display or the amplitude of the displayed signal. Virtually all
oscilloscope screens are cut into a crosshatch pattern of lines separated by 1
cm in the vertical and horizontal directions.
This section carries a Volts-per-Division (Volts/Div) selector knob, an AC/DC/
Ground selector switch and the vertical (primary) input for the instrument.
Additionally, this section is typically equipped with the vertical beam position
knob.
7.3.4. Fluorescence and Phosphorescence
When an atom is excited from one energy state to a higher one by the absorption
of a photon, it may return to the lower level in a series of two (or more)
transitions if there is at least one energy level in between. The photons emitted
will consequently have lower energy and frequency than the absorbed photon.
When the absorbed photon is in the UV and the emitted photons are in the
visible region of the spectrum, this phenomenon is called fluorescence.
The wavelength for which fluorescence will occur depends on the energy levels
of the particular atoms. Because the frequencies are different for different
substances, and because many substances fluoresce readily, fluorescence is a
powerful tool for identification of compounds. It is also used for determining
how much of a substance is present and for following substances along a natural
metabolic pathway in biological organisms.
For detection of a given compound, the stimulating light must be monochromatic,
and solvents or other materials present must not fluoresce in the same regionof the spectrum.
Sometimes the observation of fluorescent light being emitted is sufficient to
detect a compound. In other cases, spectrometers are used to measure the
wavelengths and intensities of the emitted light.
Fluorescent light bulbs work in a two-step process. The applied voltage
accelerates electrons that strike atoms of the gas in the tube and cause them
to be excited. When the excited atoms jump down to their normal levels, they
emit UV photons which strike a fluorescent coating on the inside of the tube.
The light we see is a result of this material fluorescing in response to the UV
light striking it.
Materials such as those used for luminous watch dials, and other glow-in thedark products, are said to be phosphorescent. When an atom is raised to a
normal excited state, it drops back down within about .
In phosphorescent substances, atoms can be excited by photon absorption to
energy levels called metastable, which are states that last much longer because
to jump down is a “forbidden” transition. Metastable states can last even a few
seconds or longer.
In a collection of such atoms, many of the atoms will descend to the lower state
fairly soon, but many will remain in the excited state for over an hour. Hencelight will be emitted even after long periods.
- Clean a zinc plate with fine emery paper or steel wool.
- Attach the plate to the top disc on a gold leaf electroscope, so there
is good electrical contact.
- Charge the zinc plate and inner assembly of the electroscope
negatively, e.g. by rubbing the zinc plate with a polythene rod
which has been rubbed with wool or fur. [Charging by induction
using a perspex rod is more reliable, but might be considered too
confusing!]
- The leaf should now be raised, because the leaf and the back plate
are both charged negatively and repel each other. The leaf should
temporarily rise further if the charged polythene rod is brought
near the zinc plate.
- Place an ultraviolet lamp near the zinc plate. Switch it on. The leaf
should be seen to fall.
- Safety note: Don’t look at the ultraviolet lamp (when it’s turned
on!)] Clearly the plate (and inner assembly of electroscope) islosing charge.
- Repeat the procedure, but charging the zinc plate and inner
assembly of the electroscope positively, e.g. by rubbing the plate
with a charged perspex rod.
- Observe what happen
This time the ultraviolet does not affect the leaf. Charge is not lost. The
simplest explanation is the correct one. The ultraviolet causes electrons
to be emitted from the zinc plate. If the plate is charged positively, the
electrons are attracted back again. If the plate is charged negatively theemitted electrons are repelled and lost from the plate for ever.
- A metal plate P made in caesium and a smaller electrode C
(collecting electrode) are placed inside an evacuated glass tube,
called a photocell. The two electrodes are connected to an
ammeter and a source of emf, as shown Fig.7.14. Note the polarity
of the power supply.
- Any electrons emitted from the caesium surface will be collected
by the ‘collecting electrode’.
- If the photocell is covered the current is zero; if light falls on thecaesium electrode there is current.
