MODEL OF ATOM
TOPIC: -
MODEL OF THE ATOM
INSTRUCTIONAL MATERIALS: - Chart
showing model of the atom.
REFERENCES BOOK: -
1. FARINDE
O. E e tal, ESSENTIAL PHYSICS FOR SSS, Tonad Publishing Limited.
2.
M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR SENIOR SECONDARY
SCHOOLS, Africana first publishers.
3. INTERNET
PREVIOUS KNOWLEDGE: - Students have been familiar with gadgets
uses electromagnetic principles.
OBJECTIVES: - At the end of
the lesson students should be able to: -
I.
State and discuss what chemical evidence there is for the
existence of atom.
II.
State and discuss what experimental evidence for believing that
matter is electrical in nature
III.
State limitations of Thompson’s model and Rutherford’s model.
CONTENT: -
MODEL OF THE ATOM
All elements are consists of very small invisible
particles, called atoms. Atoms of same element are exactly same and atoms of
different element are different.
There are three particles which make up of an atom.
These are: Electrons, Protons and Neutrons.
THE VARIOUS MODEL OF THE ATOM
1.
Thomson’s Atomic Model
Every atom is uniformly positive charged sphere of radius of the
order of 10-10 m, in which entire mass is uniformly
distributed and negative charged electrons are embedded randomly. It is like a
plums in a pudding. The atom as a whole is neutral.
Limitations of Thomson’s Atomic
Model
1. It could not explain the origin of spectral series of hydrogen
and other atoms.
2. It could not explain large angle scattering of α – particles.
2.
Rutherford’s Atomic Model
On the basis of this experiment, Rutherford made following
observations
(i) The entire positive charge and almost entire mass of the
atom is concentrated at its centre in a very tiny region of the order of 10-15 m, called nucleus.
(ii) The negatively charged electrons revolve around the nucleus
in different orbits.
(iii) The total positive charge 011 nucleus is equal to the
total negative charge on electron. Therefore atom as a overall is neutral.
(iv) The centripetal force required by electron for revolution
is provided by the electrostatic force of attraction between the electrons and
the nucleus.
Limitations of Rutherford Atomic
Model
(i) About the Stability of Atom According to
Maxwell’s electromagnetic wave theory electron should emit energy in the form
of electromagnetic wave during its orbital motion. Therefore, radius of orbit
of electron will decrease gradually and ultimately it will fall in the nucleus.
(ii) About the Line Spectrum Rutherford atomic
model cannot explain atomic line spectrum.
3
Bohr’s Atomic Model
In
a process of overcoming the defects of Rutherford nuclear model of the atom
Neil Bohr proposed new atomic model called Bohr atomic model. He explained that
the electrons revolve round the nucleus containing circular orbits or shell
located at some distances from the nucleus.
Neil Bohr’s postulations
are as follows:
1. That the angular momentum of the electrons is quantized. This
means that Rutherford's model which says that electron can be found anywhere is
modified to imply that it can only exist in a permissible orbit.
2. As long as any electron stays in a permissible orbit, it does
not radiate energy, despite the demand of classical electrodynamics.
3. When an electron jumps from the
higher energy state to the lower energy state, the difference of energies of
two states is emitted as the radiation of definite frequency which is called
spectral line.
Failure
of Bohr’s Model:
a)
This model is applicable only to hydrogen-like atoms and fails in case of
higher atoms.
b) It
could not explain the fine structure of the spectral lines in the spectrum of
hydrogen atom.
ENERGY
QUANTIZATION
Quantization
is the concept that a physical quantity can have only certain discrete values.
Electrical charge, energy, light, angular momentum and matter are all quantized
on the microscopic level.
Energy quantization is a process by which a
packet of radiation energy known as photon or quanta is emitted when heat is
applied or when a temperature of a substance is high.
Excited
energy: This is the maximum amount of energy needed to raise an atom from the
ground state to higher energy state i.e. E2-Eo.
Hydrogen Spectrum Series
Each element emits a spectrum of radiation, which is
characteristic of the element itself. The spectrum consists of a set of
isolated parallel lines and is called the line spectrum.
(i) Lyman Series When electron jumps from n =
2, 3,4, …orbit to n = 1 orbit, then a line of Lyman series is obtained.
This series lies in ultra violet region.
(ii) Balmer Series When electron jumps from n =
3, 4, 5,… orbit to n = 2 orbit, then a line of Balmer series is obtained.
This series lies in visual region.
(iii) Paschen Series When electron jumps from n
= 4, 5, 6,… orbit to n = 3 orbit, then a line of Paschen series is obtained.
This series lies in infrared region
(iv) Brackett Series When electron jumps from n
= 5,6, 7…. orbit to n = 4 orbit, then a line of Brackett series is obtained.
This series lies in infrared region.
(v) Pfund Series When electron jumps from n =
6,7,8, … orbit to n = 5 orbit, then a line of Pfund series is obtained.
This series lies in infrared region.
HYDROGEN ENERGY LEVEL
Eo – Ground state or lowest energy level. E1, E2, E3 are called discrete energy level. Energy level is expressed in term of number (n) where n = 1,2,3 etc are called quantum numbers.
The ground state is – 13.6 eV (- 21.76x10-19J)
other excited levels can be calculated using
The table below shows the first five energy level of a hydrogen atom.
|
Energy Level |
Energy (eV) |
|
1 |
-13.6 |
|
2 |
-3.4 |
|
3 |
-1.51 |
|
4 |
-0.85 |
|
5 |
-0.54 |
Let’s say the electron wants to jump from the first energy level n= 1 to the second energy level n=2. The second energy level has higher energy than the first, so to move from n=1 to n=2, the electron needs to gain (E2-E1) -3.4- (-13.6) = 10.2eV of energy to make it up to the second energy level.
The electron can gain the energy it needed by absorbing light. If the electron jumps from second energy level down to the first energy level, it must give off some energy by emitting light. The atom absorbs or emits light in discrete packed called PHOTONS, and each photon has a definite energy.
To find the wavelength a light absorbed or emitted, this
equation is used: E=
A photon with an energy of 10.2eV has a
wavelength of 1.21x10‑7m, in the ultraviolet part of the spectrum.
Example
An electron jumps from one energy level
to another in an atom radiating 4.5x10-19J. If the plank’s constant
is 6.6x10-34Js. What is the wavelength of the radiation n? [ take
velocity of light= 3x108m/s]
Solution
E=
4.5x10-19J; h= 6.6x10-34Js;
c= 3x108m/s ; wavelength=?
Wavelength =
PRESENTATION
Step I: The teacher revises the previous topic.
Step II: The teacher introduces the new topic.
Step III: The teacher explains Thompson’s and
Rutherford’s atomic model.
Step IV: The students chorus the limitation of
Rutherford atomic model.
Step V: The teacher explain Bohr’s atomic model
Step VI: The teacher leads the students to solve problem on a.c
circuit.
EVALUATION
The teacher evaluates the students by asking these questions:
I.
State and discuss what chemical evidence there is for the
existence of atom.
II.
State and discuss what experimental evidence for believing that
matter is electrical in nature
III.
State limitations of Thompson’s model and Rutherford’s model.
ASSIGNMENT
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