Download We will have a brief glimpse of atomic structure. The notion of atomic

This lecture by Dr. Shubhajit Roy Chowdhury is an introduction to atomic
models, namely Rutherford's and Bohr's. It explores the different types of
bonds that exist in solids. The concept of valence and conduction bands is
introduced.
We will have a brief glimpse of atomic structure. The notion of atomic
structure dates back to the days of Rutherford, when he proposed his
famous atomic structure, based on the notion of our solar system. So,
Rutherford basically performed an alpha ray scattering experiment,
wherein he understood that most of the space in an atom is empty. And
the electrons which are negatively charged revolve around the nucleus in
orbits and the positively charged are basically concentrated at the centre
of the atom in a space called nucleus.
While performing his alpha ray scattering experiment, he found that
most of the alpha particles pass through the gold foil and a very small
fraction of the alpha particles was deflected by small angles. However,
one in 20000 alpha particles touched the gold foil and then bounced
back with a velocity that is greater than the velocity with which it was
targeting the gold foil. So, immediately he came to the conclusion that
there must be some centre of positive charges at some point in the gold
foil towards which the alpha particles were approaching. And because
they were strongly repelled by those positive charges, they came back
with a velocity greater than the velocity with which they were
approaching. Immediately he proposed his atomic structure based on the
notion of a solar system where just like the sun is located at the centre of
the solar system, here you have the positive charges located at the centre
of the atom called nucleus. And then electrons equal in number to the
number of positive charges in the nucleus, revolve around the nucleus in
circular well-defined orbits.
However, this concept works well as long as the particles are neutralas it
is well known from the classical theory of electrodynamics that if a
charged particle revolves around the electric field of another charged
particle, then the revolving charged particle gradually emits radiations,
loses energy and falls into the other charged particle. However, in case
of an atomic structure, we know that it is fairly stable and therefore these
kinds of collapses do not take place. Therefore, there must be some flaw
in Rutherford's atomic model. And this was actually corrected by Niels
Bohr, who was Rutherford's student. He propounded that the energy of
an electron in an orbit remains unchanged as long as the electron
remains in that orbit, and these orbits are called stationary orbits or
stationary states.
The key features of Bohr's modelare:
1. Energy of electrons is emitted when electrons move from a higher
energy level to a lower energy level or in other words from a
higher stationary state to a lower stationary state.
2.Energy thus released in the form of radiation appears in the form of
packets of energy or quanta of energy.
If energy in the outer orbit of an electron is E2 and in the inner orbit is
E1 then the change in energy is E2-E1 which is emitted as a quantum of
energy. The angular momentum of an electron is quantized whose
angular momentum is an integral multiple of h/2 , where h is the
Planck's Constant.
So the notion of electrons moving within in an atom cannot be explained
by Classical Mechanics, because there are lots of aspects of electronic
behaviour which can only be explained with the help of Quantum
Mechanics which is a theoretical science that deals with the study of the
motion of microscopic objects that have both observable wave and
particle like properties. Previously Davisson and Germershowed an
excellent experiment where they could show the diffraction of electrons.
But it is very well known that diffraction is a property that can only be
exhibited by waves and not by particles.Thus since waves and particles
both show diffraction, it was Louis de Broglie who gave the famous
hypothesis of wave-particle duality of matter.
Naturally, the next question came up as to how to develop the notion of
matter waves. The answer was actually given by Erwin Schrödinger, in
his famous Schrödinger's wave equation, which was basically built on
the notion of Heisenberg's uncertainty principle. So for a subatomic
particle moving with a very high velocity, it is almost impossible to
predict position and momentum at the same time. Therefore, in order to
reach a balance between the Heisenberg's uncertainty principle and the
Bohr's atomic model, it was proposed that instead of moving in orbits
there are well-defined spaces called the orbitals, where the electronic
motion is restricted. And electrons can exist at any point within an
orbital. So, instead of just having a principal quantum number, which
basically describes the size and energy of the orbital, they came up with
the notion of azimuthal quantum number. Then apart from this
azimuthal quantum number, people found that under a strong magnetic
field the spectra of the atoms showed some closely spaced lines which
means that different orbitals have different magnetic moments. This led
to the notion of a magnetic quantum number. Under the influence of a
weak electrical field they again showed further splitting of the lines that
were split with the help of strong magnetic fields. This led to the notion
of spin quantum number, with each electron having spin-up or spindown states.
The study of solid state structure reveals that there are three different
types of primary bonds:
1.The ionic bond in which there is an electrostatic force of attraction
between the positively charged ions and negatively charged ions.
2.The covalent bond, which is formed by the sharing of electrons
between two atoms.
3.The metallic bond, where the valence electron is shared throughout
An energy gap exists between the valence band and the conduction
band. The valence band is the highest level of energy in which the
electrons are present at absolute zero temperature, For example, Lithium
has an electronic configuration of 1s2,2s. The highest energy state filled
at absolute zero temperature is 2s. These energy states form the valence
band. The unfilled energy orbital with no electrons, i.e. 2p orbital, in this
case, corresponds to the conduction band.