Nats 101 S04 #16
Reading: T&H 200-215
The nature of the atom, quantum model
The Bohr atom
Ground and excited states
Photons, absorption and emission
Spectroscopy, fingerprinting elements
Lasers, how they work
Pauli exclusion principle
The periodic table, why it works
The Bohr atom
We left off last lecture with the conclusion that the Rutherford model
of the atom was not correct.
The first person to present a better model was Bohr (1913), 2 years later.
Bohr was studying the spectra of the hydrogen atom. Rather than giving
off light with a continuous wavelength, H only gives off light that has
certain fixed wavelengths.
Although Rutherford’s model was wrong, it still provided insight as to
why electromagnet radiation is given off. It is due to accelerating electrons,
whirling about the nucleus. Bohr’s contribution is that the radius of the
orbit of the electrons must be constrained to certain values, and not just
any value. At such a place the electron could orbit the nucleus and
not give off any radiation.
Therefore, we have a set of allowed radii. The electron is not allowed
to exist at any other distance. While at one of these special radii, the
energy of the electron is constant.
We can make an analogy, the energy levels, or radii, are similar to steps
on a staircase. You can stand on any given step but you cannot stand somewhere
in between the steps. Personally, this analogy does not do it for me. Because,
although you cannot stand anywhere other than on a step, your foot exists
while it is moving from one step to another. This is not the case with
the electron. It does not exist in between steps.
However, the analogy of energy with steps is a good one. Each time you
change steps on a staircase, the gravitational potential energy also changes.
Each time an electron changes orbits, its energy also changes.
An electron in the lowest energy level, closest to the nucleus, is said
to be in the ground state, all others are called excited
Photons: Particles of light
What these states and orbits imply is that if an electron is in an excited
state, then there is a lower energy state available for it to occupy. When
it drops down to this lower energy state, its energy drops exactly by the
difference in the energies of the two orbits. This difference in energy
is given off as radiation.
This was Bohr’s great insight!
The radiation that is given off is called a photon.
Every time an electron changes its orbit, from a higher energy one to a
lower energy one, it gives off a photon of light whose energy is the difference
in the energy between the two states.
This is the big success of the Bohr atom. Whenever a group of atoms are
being excited, say by heat or whatever, then electrons are changing energy
levels and light is being given off. The light from the camp fire or from
the element on your stove, is light that is being emitted
by excited electrons.
Likewise, if light with the right energy strikes an atom then its electrons
will be excited and rise to a higher energy state, and the light will be
The electron cannot exist anywhere between the two radii. Such a process,
that of moving from one energy level to another is called a quantum
leap, or quantum jump.
Furthermore, the leap from the excited state can follow many different
paths, producing all sorts of different wavelengths of light. See figure
8-9 of your text. Thus, the jump shown in the figure above (between (b) and (c))
can produce 3 different wavelengths of light.
Another way to raise the energy of an atom is to heat it. This causes the electrons to jump into excited states.
When they drop back down to the ground state the atom gives off light. This is why a heated object glows.
An Intuitive leap
The Bohr model is quite strange. It took another 20 years for people to
start to understand it. This field is called quantum mechanics. However,
people believed the model right away, because it worked.
Like Newton’s realization that the gravity field of the earth extended
all the way to the moon, this is one of the great intuitive achievements
of all time.
Adding energy to a large collection of atoms will excite a large number
of electrons. Some of the excited electrons will jump to higher energy
states. Later, they will jump back down to the ground state. When they
jump back down, they give off electromagnetic radiation. If the radiation
is in the visible range, then the atoms will glow.
Some common examples are mercury or sodium lights. Mercury gives off a
Sodium gives off a yellow color. The atmosphere of the moon is sodium.
This was determined by analysis of its spectra by scientists here at the
The atoms of different elements give off different spectra. This is the
basis of many ways to identify the presence of different elements. Each
atom has a characteristic spectrum, which can be considered
like a fingerprint. The spectra allow us to identify the
Not only do atoms give off energy, but they absorb it too. So if white
light is passing through some material, then certain wavelengths will be
absorbed, and will be missing in the spectra. These are called absorption
Astronomers use emission lines to identify the composition of stars. They
use absorption lines to identify the composition of interstellar dust and
atmospheres of the planets.
The branch of science that identifies things this way is called spectroscopy.
Another example: The analysis of the exhaust from our cars.
Another: my Raman probe.
The word laser is an acronym for “light amplification by stimulated
emission of radiation”
Most lasers are made of some material such as a crystal of ruby or a gas
that is enclosed in a glass tube, like argon.
Suppose an atom was in an excited state. I.e. an electron was in a higher
energy orbit. If a photon of light, that has the same energy as the difference
between the excited state and the ground state, passes by the atom then
it might cause the excited electron to drop to its ground state and emit
another photon of the same wavelength. This is the stimulated emissions
part of the definition.
Furthermore, the second photon is exactly in phase with the first photon.
When many photons are in phase, we say that the light is coherent.
Now then, suppose that the ruby crystal or the gas is constantly being
excited. Then if a first photon is created, and this would happen spontaneously,
then it will create a second one that is coherent. As they travel along
the tube or crystal, they both create more photons, all of which are coherent.
Therefore, by constructive interference the light quickly becomes bright.
The crystal or the gas is constantly being excited so after the photon
is emitted, the atoms quickly get re-excited.
The excited atoms are enclosed with mirrors at both ends. This way the
photons hit the mirror and are bounced back to pass through the material
repeatedly. Only the photons that are travelling precisely along the axis
of the tube will remain, as the others will bounce right out of the sides of
the tube. Thus a beam of very bright, coherent light is created that is
aligned precisely through a cascading effect. The photons will bounce back and
forth millions of times.
The mirrors are not perfect. They let out about 5% of the light. The light
that escapes forms the laser beam.
Periodic chemical properties
The most important aspect of the periodic
table is the similarity of elements within a given column.
For instance, everything in column 1 combines with everything in column
7 in a 1:1 ratio. E.g. NaCl. All can be dissolved by water.
Everything in column 2 combines with atoms in column 6 in a 1:1 ratio,
e.g. MgO. They are all colorless and have very high melting temperatures.
Everything in column 8, the noble gases, are colorless gases that do not
react with many things at all. E.g. He is used to fill balloons. The only
other lighter-than-air gas is H, which is very explosive because of its
reactivity. Argon, or Ne are used in light bulbs because other gases, like
N or O would react with the filament.
Why the periodic table works
When Mendeleev invented the periodic table it was accepted but only because
it worked. No one knew why.
Bohr’s model of the atom explains this.
When two atoms, say C and O meet close enough to undergo a reaction, say
burning charcoal and O, then it is their outermost electrons that meet
first. These electrons determine chemical properties.
One more fact about the electronic structure of atoms, the Pauli
exclusion principle. No two electrons can occupy the same state.
We can imagine that there is a region around the atom in which an electron
can exist at a constant energy level. It fills this whole space and there
is no space left over for other electrons. However, we also model that
electrons spin in opposite ways, like the revolution of the planet around
its own axis, providing night and day. These are considered different states.
Only 1 of each can be at the same energy level.
Therefore, H has 1 electron. He has two, with both spinning in opposite
directions. If a third electron were to come along, then it must go to
a new energy level. That is why there are only two atoms in the first row.
Lithium has 3 electrons, two that fill the innermost orbit and 1 in the
next higher energy orbit. Since it only has 1 electron in its outermost
orbit, it is similar to H, and it goes in the same column as H.
The region of the second orbit is further away from the nucleus than the
first orbit, so it has more space, and can take 8 electrons. Therefore,
atoms with 3-10 electrons occupy the second row of the periodic table.
Ne is under He, not because it has 2 electrons, but because its orbit is
The next orbit can fit 18 electrons, so there are 18 different atoms in
this row. Etc.