ch 1-atomic structure

8/13/2019 Ch 1-Atomic Structure

1/20

Atomic Structure

Dr. R. D. Senthilkumar,

Assistant Head, Dept. of Math & Appl. Science


2/20

Dr. R.D. Senthilkumar, Asst. Head, Dept. of Math & Appl. Sciences, MEC

Intended Learning Outcomes

After the completion of this class, you will be

able to:

Describe the atom and its sub particles

Recall the properties of atom and its sub-particles

Explain the structure of atom (Bohr atomic model)

Calculate Bohr radius of orbit

Determine the velocity and frequency of revolving

electron


3/20


What is fundamental ?

A eternal question

People have long asked that what is the world made up of?


4/20


Answer is

Atom


5/20


What is an atom?

A fundamental particle of matter

Very smaller in size

Consists of electron, nucleus (proton andneutron)

Has both negative and positive charges

Atoms are stable Atoms can stick together to form molecules


6/20


Structure of an atom

Electron

Proton Neutron

Electron Proton Neutron

Discovered by JJ Thomson Rutherford James Chadwick

Charge -1.6 x 10-19 C +1.6 x 10-19C No Charge

Mass 9.1110-31Kg 1.673 10-27 Kg 1.675 10-27 Kg


7/20Dr. R.D. Senthilkumar, Asst. Head, Dept. of Math & Appl. Sciences, MEC

Atomic models

To explain the structure of atom, several

theories have been proposed called as atomic

models and they are

Thomsons model

Rutherfords nuclear model

Bohrs atom model Sommerfelds relativistic model

Vector atom model



Thomson atom model

According to this model, Atom is a heavy sphere about 10-10m in

diameter

Electrons and positive particles are uniformlydistributed inside the spherical atom

Repulsive force between two adjacentelectrons is balanced by the equal andopposite attractive forces acting betweenelectron and positive particle



Thomson atom model contd

Electron

Positive

particles



Rutherfords nuclear atomic model

Rutherford, in 1911, proposed a new type ofmodel of the atom

According to this model,

Positive charge of the atom is concentrated in avery small volume (< 10-13cm in dia) at its centre.This central core is called nucleus

Nucleus is surrounded by Electrons are revolving

around the nucleus in a spiral orbits. Hence, the entire atom becomes electrically

neutral



Rutherfords model

Spiral orbit



Drawbacks of Rutherfords nuclear

atomic model

The stability of the atom as a whole

Distribution of electrons around the nucleus


13/20



Bohrs atom model contd

Electron

Proton Neutron

Circular orbit



Bohrs theory of hydrogen atom

Bohr explained the hydrogen atom based onthe following assumptions:

Electron in an atom moves in a circular orbitaround the nucleus under the influence ofCoulomb force of attraction between the

electron and nucleus (entire atom is stable asCoulomb force of attraction is balanced byNewtonian centrifugal force as shown in fig.)




contd

2

0

2

4 r

e

r

mv

2

=

Coulomb force

Centrifugalforce

+Z e

- ev

r




contd

Only particular orbits are allowed in an atom

Electron moving in such allowed orbit does notemit or absorb the radiation (total energy of

the electron revolving in any one of the so manyallowed orbits remains constant)

Radiation is emitted when an electron jumpsfrom higher energy(E2) orbit to lower energy (E1)

orbit. Frequency (h) of emitted radiation isrelated by the equation

E2E1= h


18/20



Formulae

Bohrs radius of orbit rn= n2x ao

where, n - no. of orbit & ao= 0.529x10-10metre

Velocity of revolving electron v = nh/2mr

Where, h is Plancks constant = 6.626x10-34Joule-sec

m is mass of electron = 9.11x10-31kg

r is radius of nthorbit

Frequency of revolving electron f = v/2r

Where v is the velocity of electron.


20/20D R D S thilk A t H d D t f M th & A l S i MEC

ch 1-atomic structure

Documents