THOMSON MODEL OF ATOM

J. J. Thomson proposed that atom possesses a spherical shape in which positive charge is uniformly distributed and. The electrons are embedded in it in such a manner as to give most stable electronic configuration.

RUTHERFORD’S MODEL OF ATOM

Rutherford ‘s gold foil experiment.

Observations and conclusions

1. most of the space in an atom is empty as most of the ά-particles passed through the foil undeflected

2. A small fraction of ά-particles were deflected, the deflection must be due to repulsion showing positive charge is not spread all over.

3. A very few ά-particles were bounced back i.e. were deflected by 180 degree. This must be the point of positive charge which Rutherford named nucleus radius of nucleus is 10-15m.

Rutherford’s Nuclear Model of Atom

The positive charge in an atom is concentrated in very small portion of atom called nucleus.

nucleus is surrounded by electrons that move around it in vey high speed in circular paths called orbits.

Electrons and nucleus are held together by strong electrostatic force.

Drawbacks of Rutherford’s model

Rutherford’s model cannot explain stability of atom. Because when charged particles move in an orbit they must emit radiation and thus the orbits will spiral up into nucleus which does not happen.

Rutherford’s model does not say anything about the electronic configuration of elements.

Atomic number and Mass number The number of protons in an atom gives

atomic number(Z). Hydrogen has one proton hence atomic no 1.

Mass number of an atom is determined by the number of nucleons present in an atom(A).

Isotopes and Isobars

o Isobars are atoms with same mass no. but different atomic no. Such as 14

6C and 14

7N, Both have same mass 14.

o Isotopes are atoms of same element with same atomic no and different mass no. such as 1

1H(protium), 21H(deuterium)

, 31H(tritium).

WAVE NATURE OF ELECTROMAGNETIC RADIATION

James Maxwell suggested that when charged particles move under acceleration alternating electric and magnetic fields are produced and are transmitted in form of waves called Electromagnetic Radiations.

Planck’s quantum theory

Black body radiations- the ideal body which emits and absorbs energy of all frequencies, is called a black body. Radiations emitted by such a body are called black body radiations.

Photoelectric effect- when certain metals like cesium, potassium were exposed to a beam of light they eject electrons. This phenomena is called photoelectric effect.

Atomic spectra

When white light is passed through a prism it splits into a series of color bands. There are two types of atomic spectra-

i. Emission spectra- it is observed when the radiations emitted from a substance that have absorbed energy are analyzed with a spectroscope.

ii Absorption spectra- when a continuous electromagnetic radiation is allowed to passed through a gas or solution of some salts and transmitted light.

Orbitals and Quantum numbers

To describe an electron in an atom we need three set of numbers, and an additional no. which specifies the spin of an electron.

1. Principal quantum no.- it determines the main energy shell in which the electron is present. It is denoted by ‘n’.

n 1

2

3

4

j…….

shell K L

M N

o…….

ii Azimuthal quantum no.- this quantum no. determines the angular momentum of the electron. It is denoted by ‘l’ and gives subshell in principal energy shell.

l = 0,1,2,3,4………(n-1).

ex:- if n=4 ,then l= 0,1,2,3

l`

0 1 2 3 4……

shell s (sharp)

p (principal)

d (diffuse)

f(fundamental)

g……

iii magnetic quantum no.- this quantum no. describes behavior of electron in a magnetic field it is denoted by ‘lm’. It can have value –l to l. there are 2l+1 values of m for each value of l.

ex:- if l=2 then ml = -2,-1,0,1,2.

Value of l 0 1 2 3 4 5

Subshell notations

s p d f g h

Number of orbitals

1 3 5 7 9 11

iv spin quantum no.- this quantum no. describes the spin orientation of the electron. Electron can have two spin clockwise or anticlockwise so it can have only two values +1 or -1 .

2 2

Aufbau’s principal

In ground state of an atom electron enters the orbital of lowest energy subshell first and subsequent electrons are filled in order of increasing energy.

The order in which shells are filled is given as:-

1s >2s >2p >3s >3p >4s >3d >4p >5s >4d >5p >6s >4f >5d >6p >7s >5f >6d >7p.

Pauli Exclusion Principal

“Only two electrons may have the same orbital and these electrons must have opposite spin.”

No two electrons In an atom can have same set of quantum no.

The maximum no of electrons in a shell is 2n2

Hund’s rule of maximum multiplicity Pairing of electrons in Orbitals belonging to

same subshell will not take place until each orbital belonging to that subshell has got one electron each.

H ˥ ˩

Electronic configuration of atoms The distribution of electrons in an atom is called

its electronic configuration.

SOME IMPORTANT formulae

Speed of light in vacuum is 3.00 ×108 m/s. and is given by symbol ‘c’. The frequency(ν), wavelength(λ) and (c) are related as

c = νλ Wave no. is another important term in

spectroscopy and is given by reciprocal of wavelength.