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Discovery of Electrons
In the 1800s electricity was new, exciting and the subject of a lot of study. The
English chemist Humphrey Davy (1778-1829) had built the world's largest battery
(over 250 metallic plates) and pushed the very high currents this battery could
generate through all kinds of solutions, compounds and substances in the hope that
the high energies involved would pull apart the chemical constituents. Using this
technique, he discovered many things, including new elements, and aroused the
interest of his pupil - Michael Faraday (1792 - 1867).
Faraday coined many of the terms still used today, including electrolysis,
electrolyte, electrodes, anode, anions, cathode and cations. He also got the idea to
pass an electrical current (discharge) through a complete vacuum, just to see what
happened - if anything.
Unfortunately his methods of producing an appropriate vacuum were not good
enough and he never really succeeded, but a German glass blower - Heinrich
Geissler - certainly did. His apparatus consisted of a glass tube in which an anode
(the positive pole, or plate) was at one end, and the cathode (the negative pole, or
plate) was at the other end. His superior vacuum pump removed all the air from the
tube, and he connected the anode and the cathode to the appropriate ends of a
powerful battery.
At high enough voltages electricity certainly seemed to be able to leap across the
vacuum between the oppositely charged plates, but that was not all. On the wall
opposite to the negative cathode, the glass glowed a strange, greenish color.
Discovery of Cathode Rays
By 1876 Eugen Goldstein was certain that the glow was being caused by a new kind
of radiation that started at the negative plate and radiated across the vacuum until it
hit the glass. Since Faraday had called the negative pole or plate the cathode,
Goldstein called his radiation, cathode rays.
What could they be?
German researchers were convinced that the cathode rays traveled in waves, like
light, whereas the British researchers thought that the rays were composed of tiny,
tiny charged particles. This difference in concept caused a lot of rivalry at the time
and, although none of them were ever to know it, they were both right - sort of!
William Crookes (an English physicist), among several others, including Julius
Plucker, showed that bringing a magnet next to the sides of the tube caused the
cathode rays to bend in a way that strongly suggested that they were made up of
electrically charged particles - not waves. This observation allowed Arthur Schuster
(in 1890) to calculate the charge-to-mass ratio of these "particles" from the amount
of bending he observed in a cathode ray tube when it was magnetized.
Joseph John Thomson
All these interesting discoveries attracted the attention of the English physicist
Joseph John Thomson ("J.J." - to his friends). By 1894 Thomson was able to
announce that these rays traveled much more slowly than rays of light (he found a
value of 1.9 x 107 cm/sec. Light travels at 3.0 x 1010 cm/sec.). So it was more likely
that they were particles than waves.
Thomson also carried out a series of experiments using a better designed cathode
ray tube that incorporated two small plates, between which the rays had to travel.
By connecting these plates to the opposite poles of a battery, an electric field was
generated at right angles to the path of the rays. This had a dramatic effect. Just as
had happened with a magnetic field, so with the electric field, the rays were bent!
By April 30th, 1897 J.J. Thomson was ready with his big announcement; the
cathode rays consisted of negatively charged particles (which he called
"corpuscles") that were only less than 1/1000th of the mass of a hydrogen atom.
This meant that they could not be simply charged atoms (ions), or any then known
particle. This was something very new.
The electron
Thomson was awarded the Nobel Prize in 1906 for his "discovery" of the first subatomic particle; the electron. This discovery strongly implied that Dalton was
wrong and that the atom was not the smallest particle of matter. It looked as if the
atom could be broken down into even smaller pieces, and to Thomson these smaller
pieces were his negatively charged electrons.
Very rarely in science is a new discovery the result of a single person's work or
ideas, and the idea that cathode rays were interesting, possibly particles, traveled in
straight lines, carried energy and had other intriguing properties were the conclusion
of many other researchers, including the "positivist" Walter Kaufmann, who
actually had many of his results before those of Thomson. However, Thomson gets
the historical credit for "discovering" the electron because he had the courage to
insist that his corpuscle was one of the building blocks of matter (and atoms).
No one else had this courage.
In 1904 he was bold enough to try and build the first model of what an atom might
look like. He had one major problem; all electrons were negatively charged, without
some way of neutralizing these charges there was no way of keeping thousands of
electrons together, in one place, so they could become part of an atom.
Thomson proposed this solution:
... suppose that the atom consists of a number of corpuscles moving about in a
sphere of uniform positive electrification...
This is often called Thomson's "plum pudding model", or more likely these days the
"chocolate chip cookie" model in which the dough of the cookie is a cloud of
"uniform positive electrification" and the chocolate chips are the small, negatively
charged electrons.
As a model of atomic structure it had a very short life, but it got people thinking,
and one of those who thought more than others was Ernest Rutherford, whose work
took the search to the next level.
2. Cathode rays
Almost all discoveries related to particles such as electrons, ions, etc.
finds its origins in the experiments conducted by scientists with
discharge tubes. J.J. Thomson was the first to show, with the help of
experiments with discharge tubes that electrons are same in all
substances and that atom is made up of particles which are not
indivisible as suggested by Daltonís atomic theory.
Discharge tube is a hard glass tube which is long (about 30 cm or more)
and has two electrodes attached at its two ends. The electrodes are
made of any metal which is a good conductor, such as copper,
aluminum, platinum, etc. The discharge tube has a facility to connect it to
a vacuum pump. The electrodes can be connected externally to a high
voltage source. The discharge tube is named so because of the gaseous
discharge that takes place between the two electrodes. This we shall
see in the ensuing discussions.
Let the electrodes have a potential difference applied to them (about 10
kV). 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, measure the pressure
inside in terms of millimeters of Hg. You will notice various changes in it.
The air inside the discharge tube is a non-conductor of electricity. So
initially the 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 electron escaping from it, ionizes
other gas atoms. There a stream of positive ions and negative electrons
gets created. These start moving towards cathode and anode
respectively. This generates 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 any current. This dark space is called
Faradayís 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 reason for this can be inferred from the direction. The
rays or particles are coming 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 are coming towards anode, they must be
negatively charged. It has been proved that cathode rays are nothing but
electrons.
The cause for the production of cathode rays can be easily explained. 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 the gas pressure is
high.
Properties of cathode rays
1. Take a modified discharge tube where direction of cathode and anode
can be changed. Keep a fluorescent screen opposite to that of the
cathode. You will notice that the cathode rays are always emitted
perpendicular to the surface of the cathode.
2. Keep a strip of paper designed in the shape of a cross in the path of
the cathode rays. You will see a shadow of the design on the fluorescent
screen. This indicates that the cathode rays travel in a straight line.
3. Make a special arrangement to keep a lightweight paddle wheel in the
path of the cathode rays. Let the paddle wheel be placed in such a
manner that only its upper portion is exposed to the cathode rays. You
will notice that the wheel will start rolling in the direction of the cathode
rays, away from the cathode itself. This shows that the cathode rays
exert mechanical force on the object in their path. This also proves that
cathode rays are particulate in nature. They are nothing but negatively
charged electrons.
4. Notice first the position of the spot on the fluorescent screen where
the cathode rays are striking. Now keep a horseshoe magnet near the
discharge tube. You will notice that the spot of the cathode rays is
getting deflected from its initial position. Change the direction of the
horseshoe magnet. You will see the spot on the fluorescent screen is
getting deflected in the opposite direction. This clearly shows that the
cathode rays are affected by the presence of a magnetic field.
5. An application of an electric field will show that the cathode rays get
deflected towards the positive plates. This is because the negatively
charged electrons or the cathode rays are attracted towards the positive
plate and is repelled by the negative plates.
6. As seen before, cathode rays are capable of ionization of gas atoms if
the potential difference is large and the gas pressure is not high.
7. Cathode rays also produce fluorescence in some materials. As they
are energetic electrons, when they strike a certain substance, the
substance starts to glow.
8. Depending on the energy of the cathode rays, they can penetrate thin
sheets of paper or metal foils.
9. When cathode rays are stopped they produce X-rays (see the next
section). X-rays are very small wavelength rays which find many
practical applications.
10. Cathode rays also affect photographic plates when they strike them.
Uses of cathode rays
1. A cathode ray tube (CRT) is widely used in research laboratories to
convert any signal (electrical, sound, etc) into visual signals. These are
called CRT or oscilloscopes.
2. CRT is the basic component in all television and computer screens.
The signals are sent to the vertical and horizontal deflecting plates,
which produce a pattern on the fluorescent screen.
High energy cathode rays when stopped suddenly produce X-rays. The
X-rays have many medical and research applications.
Eugen Goldstein
The Raisin Pudding Model of the Atom (Eugen Goldstein)
In 1886 Eugen Goldstein noted that cathode-ray tubes with a perforated cathode
emit a glow from the end of the tube near the cathode. Goldstein concluded that
in addition to the electrons, or cathode rays, that travel from the negatively
charged cathode toward the positively charged anode, there is another ray that
travels in the opposite direction, from the anode toward the cathode. Because
these rays pass through the holes, or channels, in the cathode, Goldstein called
them canal rays.
When the cathode of a cathode-ray tube was perforated, Goldstein observed
rays he called "canal rays," which passed through the holes, or channels, in
the cathode to strike the glass walls of the tube at the end near the cathode.
Since these canal rays travel in the opposite direction from the cathode rays,
they must carry the opposite charge.
(*) Site address for the above content:
http://chemed.chem.purdue.edu/genchem/history/goldstein.html
Rutherford’s Experiment:
Chadwick’s Experiment:
Neutrons from beryllium
The alpha-particles from the radioactive source hit the beryllium nuclei
and transformed them into carbon nuclei, leaving one free neutron. When
this neutron hit the hydrogen nuclei in the wax it could knock a proton
free, in the same way that a white snooker ball can transfer all its energy
to a red snooker ball.
Rutherford gave the best description of a neutron as a highly penetrating
neutral particle with a mass similar to the proton. We now know it is not a
combination of an electron and a proton. Quantum mechanics restricts an
electron from getting that close to the proton, and measurements of
nuclear 'spin' provide experimental proof that the nucleus does not
contain electrons.
(*) Site address for the above content:
http://www-outreach.phy.cam.ac.uk/camphy/neutron/neutron5_1.htm
X – Rays:
In the latter half of the year 1895, a German scientist called Roentgen
was working in his laboratory at the Physical Institute of the University of
W¸rzburg, Germany, experimenting with a type of discharge tube called
Crookeís tube. The tube displayed a fluorescent glow when a high
voltage current was passed though it. When he shielded the tube with
heavy black cardboard, he found that a greenish fluorescent light could
be seen on a fluorescent screen kept some 9 feet away. Roentgen
concluded that a new type of ray was emitted from the tube that could
pass through the black covering. The rays could pass through most
substances, including the soft tissues of the body, but left the bones and
most metals visible. One of his earliest photographic plates from his
experiments was that of a film of his wife, Bertha's hand with a ring.
Roentgen named the invisible radiations as X-rays (or unknown rays).
Bertha's hand with a ring
Detailed experiments showed that when cathode rays or electrons are
stopped, X-rays are emitted. The kinetic energy of the electrons is
transferred to the anode material of the discharge tube. The atoms of the
anode emit X-rays. These days the origin of X-rays is well known and
tabulated. When the cathode rays strike the atoms in the anode, the
electrons of these atoms jump to higher excited orbits. When the
electrons jump back to their original orbits, they emit X-rays. These are
called characteristic X-rays. Each element emits specific X-rays only. Xrays are electromagnetic radiations and have energies of the order of a
few eV to a few 10s of keV. Their energies are much less than radiations.
Production of X-rays:
X-rays are produced by modified discharge tubes that are called
Coolidge tubes. Cathode is in the form of a filament, which emits
electrons on heating. Anode is made up of solid copper, molybdenum or
any other material whose X-ray energies are useful for further
applications. Anode is also called the target. A high potential difference
between the cathode and anode is maintained by an external high
voltage source. A separate battery that supplies high current heats the
cathode filament. The filament is generally made out of tungsten and is
coiled to provide high resistance to the passing current.
The electrons emitted from the filament, experience the potential
difference and are accelerated towards the anode. Thus the electrons
gain energy. When such electrons hit on the anode surface, they are
stopped. They transfer their kinetic energy to the electrons of the anode
material. These then give rise to the X-rays.
The intensity of the X-rays depends on the number of electrons striking
the anode or the intensity of the electron beam.
High energy X-rays are called hard X-rays and they have very high
penetrating power. Low energy X-rays are called soft X-rays and they
have low penetration powers.
Properties of X-rays
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X-rays are invisible electromagnetic radiations. Their wavelength is less
than that of the visible light photons and is larger than the -ray photons.
They are generally of the order of a few 10s of nano-meters.
X-rays posses all properties of visible light photons, such as reflection,
diffraction, rectilinear propagation, etc. Refraction of X-rays is not yet seen.
Being electromagnetic waves, they travel at the speed of light 3 x 108 m/s.
Since they are not charged particles, they are not affected by either electric
or magnetic fields.
X-rays can ionize materials through which they pass such as gases. X-rays
like -rays produce fluorescence and affect photographic plates.
X-rays are highly penetrating. They can easily pass through thin sheets of
paper, metal foils, tissues etc.
X-rays are stopped by high density materials such as lead, bones, etc.
Uses of X-rays
1. Medical uses :
X-rays are widely used by radiologists to see if the bones are broken or
not. They can be used to see if a foreign object such as a bullet has
lodged in a body, if there are any other defects such as kidney or gall
bladder stone is present or not. X-rays are also used to test the density
of the bones.
2. Industrial uses :
Hard X-rays are routinely used to see if there are any faults deeper in
any structure such as pipes, machine parts, airplane bodies, etc. X-rays
are used to see the characteristic qualities of gemstones, the
genuineness and purity of precious metals such as gold, silver, etc. Xrays are also used to detect the percentage of certain elements in an ore
before the ore is taken for commercial mining.
3. Scientific uses :
X-rays undergo diffraction. This is useful in studying the nature of the
crystal and the arrangements of atoms within the crystal. X-rays are also
extensively used to study the way chemical reactions occur.
Characteristic X-rays give information about the structure of atoms.
Differences and similarities between cathode rays and X-rays
Both cathode rays and X-rays were discovered when discharges from a
discharge tube were studied in details. But the rays are completely
different from each other.
Differences :
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Cathode rays are charged particles. They are negatively charged
electrons. X-rays on the other hand are electromagnetic
radiations. X-rays have no charges.
Cathode rays emanate from the cathode itself. X-rays are emitted
when high-energy electrons are stopped.
Cathode rays have low penetrating powers. X-rays have high
penetrating powers, although
-rays are much more penetrating
than the X-rays.
Cathode rays travel at the speed given by the potential difference
between the anode and the cathode. X-rays always travel at the
speed of light.
Cathode rays are deflected by electric and magnetic fields. X-rays
are unaffected by both the electric as well as the magnetic fields.
Similarities :
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Both rays are capable of ionizing materials though which they
pass, especially gases.
Both rays cause fluorescence when they strike any fluorescence
material such as zinc sulphide.
Both rays affect photographic plates.
The Rutherford Model
In 1909 Ernest Rutherford conducted what is now a famous experiment where
he bombarded gold foil with alpha particles (Helium nuclei). A source which
undergoes alpha decay is placed in a lead box with a small hole in it. Any of
the alpha particles which hit the inside of the box are simply stopped by the
box. Only those which pass through the opening are allowed to escape, and
they follow a straight line to the gold foil. The animation below shows the
experiment in action.
Observations
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Most of the alpha particles pass straight through the gold foil.
Some of the alpha particles get deflected by very small amounts.
A very few get deflected greatly.
Even fewer get bounced of the foil and back to the left.
Conclusions
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The atom is 99.99% empty space.
The nucleus contains a positive charge and most of the mass of the atom.
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The nucleus is approximately 100,000 times smaller than the atom.
The deflection of the alpha particle through large angles and even bouncing off
the gold foil is best described by the animation below. Keep in mind that the
gold nuclei have a charge of +79 and the alpha particle has a charge of +2.
(remember your electrostatics) These two positive charges repel each other.
The closer they get, the greater the force. The greater the force, the greater the
amount of deflection.
(*) Site address for the above content [Animation of deflection of alpha particle]
http://www.regentsprep.org/Regents/physics/phys05/catomodel/ruther.htm
Rutherford’s Nuclear Model of Atomic structure:
With his model, Bohr explained how electrons could jump from one orbit to
another only by emitting or absorbing energy in fixed quanta. For example, if an
electron jumps one orbit closer to the nucleus, it must emit energy equal to the
difference of the energies of the two orbits. Conversely, when the electron
jumps to a larger orbit, it must absorb a quantum of light equal in energy to the
difference in orbits.
Absorption and Emission of Radiation by an Atom (Transition of electron)
(*) Site address:
http://online.cctt.org/physicslab/content/applets/JavaPhysMath/java/atomphot
on/index.html
ATOMIC SPECTRA AND PHOTONS
We can describe the allowed orbits of electrons by the energies associated
with them. This diagram indicates the particular energy levels associated
with a Hydrogen atom and some of the different transitions which its
single electron can make. The lowest energy state is the known as the
ground state. The next highest energy state is known as the first excited
state and so on. The energy difference between states rapidly gets
smaller and converges to a particular value, if the electron obtains more
energy than this value it is no longer considered part of the atom and
becomes ionized. In diagram A an atom is raised to its first excited state
by absorbing a photon of just the right energy. In diagram B an excited
atom decays back to the ground state by emitting a photon of the same
fixed energy. However, there are many other possible transitions and in
diagram C the atom emits a lower energy photon in moving from the third
excited state to the first excited state. The atom can interact via a whole
family of fixed energy transitions but above certain energy the electron is
ripped from the atom and becomes ionized, as in diagram D. Because it is
no longer constrained by fixed orbits the electron can have any arbitrary
energy and so at energies beyond the point at which it becomes ionized
the atom can interact with a continuous range of energies both in
absorption or emission when a free electron is captured by a previously
ionized atom.
Discrete Spectrum
Close examination of the spectra from the Sun and other stars reveals that the rainbow of
colors has many dark lines in it, called absorption lines. They are produced by the cooler
thin gas in the upper layers of the stars absorbing certain colors of light produced by the
hotter dense lower layers. You can also see them in the reflected light spectrum from planets.
Some of the colors in the sunlight reflecting off the planets are absorbed by the molecules on
the planet's surface or in its atmosphere. The spectra of hot, thin (low density) gas clouds are
a series of bright lines called emission lines. In both of these types of spectra you see spectral
features at certain, discrete wavelengths (or colors) and no where else.
The type of spectrum you see depends on the temperature of the thin gas. If the thin gas is
cooler than the thermal source in the background, you see absorption lines. Since the spectra
of stars show absorption lines, it tells you that the density and temperature of the upper layers
of a star is lower than the deeper layers. In a few cases you can see emission lines on top of
the thermal spectrum. This is produced by thin gas that is hotter than the thermal source in
the background. Unlike the case for absorption lines, though, the production of emission lines
does NOT require a thermal source be in the background. The spectrum of a hydrogenemission nebula (``nebula'' = gas or dust cloud) is just a series of emission lines without any
thermal spectrum because there are no stars visible behind the hot nebula. Some objects
produce spectra that is a combination of a thermal spectrum, emission lines, and absorption
lines simultaneously!
What is very useful about discrete spectra is that the pattern of lines you see depends on the
chemical composition of the thin gas. Each element or molecule produces a distinct pattern
of lines---each element or molecule has a ``fingerprint'' you can use to identify it. This allows
you to remotely determine what stars, planets, nebulae, etc. are made of!
The composition canNOT be found from just one line because one element may have one
spectral line at the same wavelength as another element's spectral line. However, an element's
pattern of lines is unique. Using a single line to identify a gas would be like identifying the
name of someone using just one letter of their name---many people will have that same letter
in their name, but the pattern of letters (which letters and how they are arranged) is unique to
that one person. Of course, stars, planets, nebulae, etc. are made of more than one type of
material, so you see the discrete spectra of many elements and molecules superimposed on
each other---all of the spectral lines add together. An experienced astronomer can disentangle
all the different patterns and sort out the elements and molecules (but it does take time!).
ORBITALS
(*) Orbital definitions
 An orbital is a region in space where the probability of finding the
electron in question is high (90%)
 An orbital is a region in space where the electron wave density is high
(90%)
 Schrodinger's wave equation can be solved for the h atom and other
one electron ions
 It can't be solved exactly for 2+ electron systems
 It can be solved approximately for 2+ electron systems
 Gives results consistant with observations so we will use it!
 To describe the orbitals need three quantum numbers and their
selection rules
N = prinicipal quantum number
N = 1, 2, 3, 4, 5.... (integer values)
Same n as in bohr's equation
N relates to relative energies as well as the size of the orbital
Orbital with n = 3 is higher in energy than n = 2
Orbital with n = 3 is larger than n = 2
L = angular momentum quantum number
L = 0, 1, .... (n - 2), (n - 1) (integer values)
L describes the shape of the orbital
Diffferent values of l have different names
L name
0
s
1
p
2
d
3
f
4
g
5
h
Letter designations came out of early spectroscopy
All orbitals with the same n are said to be in the same "shell"
3s, 3p, 3d
All orbitals with the same n and l are said to be in the same "subshell"
The three 2p orbitals
Ml = magnetic quantum number
Ml = -l, (-l +1), ... 0 ...(l - 1), +l (integer values)
Ml describes orientation in space
X lies on the x axis
Px, py, pz
Xy is between the x and y axes, etc.
Dxy, dxz, dyx, dx2 - y2, dz2
These three quantum numbers describe specific orbitals
n l ml ORBITAL NAME
1 0 0 1s
2 0 0 2s
2 1 -1 2px
2 1 0 2py
2 1 +1 2pz
3 0 0 3s
3 1 -1 3px
3 1 0 3py
3 1 +1 3pz
3 2 -2 3dxy
3 2 -1 3dxz
3 2 0 3dyz
3 2 +1 3dz2
3 2 +2 3dx2 - y2
4 0 0 4s
What are the shapes of the orbitals?
1s
1s
2px
2py
2pz
ALL THREE 2p ORBITALS
3dz2
3dxz
3dxy
The images above come from the sites below and the individuals
involved deserve a lot of credit. By viewing these sites you can also see
how different individuals portray orbitals in different formats. Sometimes
a particular site might be very slow. By linking the above images here,
we are minimizing traffic at their sites.
animated orbitals through g by f. James holler, university of kentucky
http://images.google.co.in/imgres?imgurl=http://www.uwplatt.edu/~sundi
n/pdb/S.JPG&imgrefurl=http://www.uwplatt.edu/~sundin/114/l114_20.htm
&h=200&w=300&sz=13&tbnid=qzr2pKKViJQJ:&tbnh=74&tbnw=111&hl=e
n&start=2&prev=/images%3Fq%3D%2522Shapes%2Bof%2Bthe%2Borbit
als%2522%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DN
Rotatable shapes by john j. Nash, university of purdue, requires chime plugin
quicktime movies of orbitals from washington university atomic and molecular
orbitals using chime plugin from w. R. Salzman of the university of arizona
(*) the orbitron: atomic and molecular orbitals from mark winter of the
university of sheffield
The fourth quantum number is associated with specific electrons
Ms = spin quantum number
Ms = -1/2, +1/2
Electrons behave like magnets
Caused by spinning charge
Have two orientations in a magnetic field
Each unique set of four quantum numbers describes a unique electron in a
given atom
(*) Site address for various orbital shapes:
http://images.google.co.in/imgres?imgurl=http://www.uwplatt.edu/~sundin/pdb/S.JPG&imgr
efurl=http://www.uwplatt.edu/~sundin/114/l114_20.htm&h=200&w=300&sz=13&tbnid=qzr
pKKViJQJ:&tbnh=74&tbnw=111&hl=en&start=2&prev=/images%3Fq%3D%2522Shapes%
2Bof%2Bthe%2Borbitals%2522%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DN