This document provides an overview of the Bohr atomic model and its limitations. It discusses:
- Bohr proposed his atomic model in 1913 to explain stable electron orbits, requiring electrons to move in fixed circular paths (orbits) at discrete energy levels.
- The model's main postulates include electrons revolving in fixed orbits with discrete energies and angular momenta that are whole number multiples of a fundamental constant.
- However, the Bohr model fails to explain phenomena like the Zeeman effect and contradicted the uncertainty principle. It also could not explain spectra of multi-electron atoms or molecular bonding.
- De Broglie introduced the concept of matter waves, relating the wavelength of a particle to
2. V th SEM- OPEN COURSE
MODULE -1 : ATOMIC STRUCTURE
Dr. Renjusha S
Asst.Professor
Dept. of Chemistry
N.S.S. College, Pandalam
3. BOHR ATOMIC MODEL
In 1913 Bohr proposed his quantized shell model of the atom to
explain how electrons can have stable orbits around the nucleus.
The motion of the electrons in the Rutherford model was unstable
because, according to classical mechanics and electromagnetic
theory, any charged particle moving on a curved path emits
electromagnetic radiation; thus, the electrons would lose energy
and spiral into the nucleus.
To remedy the stability problem, Bohr modified the Rutherford
model by requiring that the electrons move in orbits of fixed size
and energy.
4. MAIN POSTULATES OF BOHR ATOM MODEL
• Electrons revolve around the nucleus in a fixed circular path termed
“orbits” or “shells” or “energy level.”
• Every circular orbit will have a certain amount of fixed energy. The
electrons will not radiate energy as long as they continue to revolve
around the nucleus in the fixed orbital shells. The orbits are termed
as “stationary orbit.”
• Only those orbits are permitted in which the angular momentum of
the electrone is a whole number multiple of h/2π
• The different energy levels are denoted by integers such as n=1 or
n=2 or n=3 and so on. The different energy levels or orbits are
represented in two ways such as 1, 2, 3, 4… or K, L, M, N…..
shells. The lowest energy level of the electron is called the ground
state
• The change in energy occurs when the electrons jump from one
energy level to other. In an atom, the electrons move from lower to
higher energy level by acquiring the required energy. However,
when an electron loses energy it moves from higher to lower energy
level.
5. LIMITATIONS OF BOHR’S MODEL OF ATOM
• Failed to explain Zeeman Effect (how atomic spectra are
affected by magnetic fields).
• It contradicts Heisenberg Uncertainty Principle.
• Unable to explain how to determine the spectra of larger
atoms containing more than one electron
• Does not provide any clue to explain the shape of molecules
arising out of the directional bonding between atoms
6. DUAL NATURE OF THE ELECTRON
de Broglie stated that if light waves can behave like a stream of
particles (photons), then particles such as electrons could
possess wave properties.
Thus, the waves can behave like particles and particles can
exhibit wavelike properties. So, de Broglie deduced that the
particle and wave properties are related by the expression,
λ = h/mv
Where h is Planks constant, λ is the wave length associated with
a moving particle.
7. CONCEPT OF ORBITALS
The region of space around the nucleus where
the probability of finding the electron is
maximum. Each orbital has a definite amount
of energy. So each orbital is said to be
quantised.
