Bohr model of the atom

Bohr model of the atom
Lasing action is a process that occurs in matter.
Since matter is composed of atoms, we need to understand about
the structure of the atom, and its energy states.
We shall start with the semi-classical model, as suggested in
1913 by Niels Bohr, and called: The Bohr model of the atom.
According to this model, every atom is composed of a very
massive nucleus with a positive electric charge (Ze), around
it electrons are moving in specific paths.
Z = Number of protons in the nucleus,
e = Elementary charge of the electrons:
e = 1.6*10-19 [Coulomb]
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The figure illustrates a simple, but adequate, picture of the atom,
the Bohr model
Every "allowed orbit" of the electron around the nucleus, is connected to a specific
energy level.
The energy level is higher as the distance of the "orbit" from the nucleus increases.
Since for each atom there are only certain "allowed orbits", only certain discrete
energy levels exist, and are named: E1, E2, E3, etc.
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Energy States (Levels)
Every atom or molecule in nature has a specific structure for its
energy levels.
The lowest energy level is called the ground state, which is the
naturally preferred energy state. As long as no energy is added to the
atom, the electron will remain in the ground state.
When the atom receives energy (electrical energy, optical energy, or
any form of energy), this energy is transferred to the electron, and raises
it to a higher energy level.
The atom is then considered to be in an excited state.
The electron can stay only at the specific energy states (levels) which are
unique for each specific atom. The electron can not be in between
these "allowed energy states", but it can "jump" from one energy level
to another, while receiving or emitting specific amounts of energy.
These specific amounts of energy are equal to the difference
between energy levels within the atom.
Each amount of energy is called a "Quantum" of energy (The
name "Quantum Theory" comes from these discrete amounts of
energy).
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Energy transfer to and from the atom
Energy transfer to and from the atom can be performed in
two different ways:
Collisions with other atoms, and the transfer of kinetic energy
as a result of the collision. This kinetic energy is transferred
into internal energy of the atom.
Absorption and emission of electromagnetic radiation.
Since we are now interested in the lasing process, we shall
concentrate on the second mechanism of energy transfer to
and from the atom.
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Photons and the energy diagrams
Electromagnetic radiation has, in addition to its wave nature, some
aspects of "particle like behavior".
In certain cases, the electromagnetic radiation behaves as an ensemble of
discrete units of energy that have momentum. These discrete units
(quanta) of electromagnetic radiation are called "Photons".
The relation between the amount of energy (E) carried by the photon,
and its frequency (?), is determined by the formula (first given by
Einstein):
E = h?
The proportionality constant in this formula is Planck s constant (h):
h = 6.626*10-34 [Joule-sec]
This formula shows that the frequency of the radiation (?), uniquely
determines the energy of each photon in this radiation.
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E = h ?
This formula can be expressed in different form, by using the
relation between the frequency (?) and the wavelength: c = ?*?
to get:
E = h * c/ ?
This formula shows that the energy of each photon is
inversely proportional to its wavelength. This means that
each photon of shorter wavelength (such as violet light) carries
more energy than a photon of longer wavelength (such as red
light).
Since h and c are universal constants, so either wavelength
or frequency is enough to fully describe the photon.
Dr. Hazem Falah Sakeek www.physicsacademy.org & www.hazemsakeek.com
28/10/1431
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