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Chemistry short notes, Lecture notes of Chemistry

The content is about the atomic structure

Typology: Lecture notes

2023/2024

Uploaded on 02/06/2024

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bg1
x
z
λ
A
Electric field
Magnetic field
Direction of
propogation
(a) Wavelength (λ)
(b) wave no. (υ)
(c) Frequency (υ)
(d) Time Period (T)
(e) Velocity (c)
(f) Amplitude (A)
CHARACTERISITICS
OF WAVE
. Electromagnetic spectrum is a collection
space electromagnetic waves arranged
according to frequency and wavelength.
ELECTRO-MAGNETIC
SPECTRUM
Increasing Frequency (v)
Cosmic
rays
UV-
rays
x-
rays
Gamma
rays
Infra-
red
visible
light
Micro
waves
Radio
waves
Increasing wavelength(λ)
c
υ
=
λ
EMISSION
SPECTRA
Spectrum of
radiation emitted
by a substance
that has absorbed
energy
Spectrum of the
electromagnetic radiation
emitted or absorbed by an
electron during transition
from one energy level to
another
It is like
photographic
negative of an
emission spectra
ABSORPTION
SPECTRA
ATOMIC SPECTRA p- ORBITALS
(DUMBELL SHAPE)
y
z z z
yy
2px3dyzpz
x x x
d-ORBITALS ( Double Dumbell shape )
yz
z
3dxy 3dyz 3dxz
xx
y
3dx2 - y2
x
y
3dz2
x
s-SHAPE OF
ORBITALS
RULES FOR
ARRANGING ELECTRONS
(a) Aufbau principle: Electrons
occupy lowest energy level
First and then move to the next
energy level.
(b) Pauli Exclusion principle: No two
e- can have same set of all 4
quantuM numbers.
(c) Hund's rule: If two or more
orbitals of equal energy
available, then electron will
occupy them singly before filled
in pairs.
ENERGY LEVEL
. Wavelength: Distance between successive crest and trough
. Frequency: Number of waves passed through
a point in 1 sec.
J.J. THOMSON
Discovered electron(s)
Proposed plum pudding Model
ELECTRON
POSITIVE SPACE
ERNEST
RUTHERFORD
Discovered proton (P+)
a-partical experiment
Proposed Rutherford's
model of an atom (1911)
Atom consist of two parts
nucleus and extra nucleus
part
NUCLEUS
ORBIT
+
ELECTRO-MAGNETIC WAVE THEORY
Neutron
Discovered by James
Chadwick. Charge
on Neutron is 0.
Mass of a p+
1.6 × 10 kg
TOWARDS QUANTUM MECHANICAL MODEL
DUAL NATURE OF MATTER
Every microscopic particle in the motion
has dual nature (wave and particle
nature) and produce matter waves.
wavelength of matter waves
(De=Broglie's wavelength)
λ ==
h
mv
h
p
HEISENBERG'S
UNCERTAINITY PRINCIPLE
It is impossible to measure
simultaneously the exact momentum
and exact position of a microscopic
moving particle.
X . P h
7π
QUANTUM NUMBER
QUANTUM MECHANICS
. Fundamental equation was developed by Schrodinger know as
Schrodinger wave equation.
. The electrons in an atom have quantized values of energy.
. By evaluating ψ2 at different points around the nucleus in aN
atom, we can predict the probability of finding the electron.
d2ψ(E - U)ψ = 0
+
dx2
d2ψ
dy2+d2ψ
dz2+8π2m
h2
BOHR'S MODEL OF AN ATOM
STRUCTURE
OF ATOM
. Electron in H atom can move around the nucleus in a circular
path of fixed radius
. Each orbit has a definite energy and is known as energy
level or stationary level.
. When an electron jumps from a lower energy level to to
higher one, energy is absorbed and vice versa.
. Angular momentum of electron
Radius (r) = n2
Z
0.529 × A
°Energy (E) = Z2
n2
-13.6 × ev
Velocity (v) = Z
n
2.18 × 106 × m/sec
LIMITATION
. Applicable to only one e- system eg: H, He+
. It could not explain Zeeman effect and stark effect.
Beam of
light
Ejection of
electron
PHOTOELECTRIC
EFFECT
E = hυ0 + K.E.
BLACK BODY
RADIATION
A perfect obsorber or
emitter of light.
i.e Absorber or emits all
type of frequency/
radiation
PLANK'S THEORY
Space or region, wher finding the
probability of e- is zero
Radial node = (n - l -1)
Angular node = l
Total node = n - 1
TYPES:-
NODES
1s orbital
y
x
Z
1. Principle quantum No. (n) = 1, 2, 3, 4... shell = K, L, M,
2. Azimuthal Quantum No. (l) = for given value of n, l can have
values from 0 to n - 1)
3. Magnetic quantum no. (m) = for subshells with 'l' value,
m can have values from -l to +l and Total value of m= 2l + 1
4. Spin quantum number = s = + ,
1
2
1
2
-
N
Mass of a Neutron is
1.6 × 10 kg
27
W0 = Hυ0
Hυ = W0 + K.E.
E = hυ
h = 6.623 × 10–34 Js
Where h = Plank’s Constant
. Wavelength of visible light is from 400nm
to 750nm.
Light travells in the form of small energy packets
known as "Photons".
SERIES
Lyman n1 = 1
n1 = 2
n1 = 3
n1 = 4
n1 = 5
n2 = 2, 3...
n2 = 3, 4...
n2 = 4, 5...
n2 = 5, 6...
n2 = 6, 7...
Balmer
Paschen
Bracket
Pfund
charge of an e-
–1.6022 × 10–19 c
Mass of an e
9.1 × 10 kg
31
27
charge of a proton (p+)
+1.6 × 10-19 c
Sub atomic particles
. Wavelength of radiation emitted when an
e- jumps from n2 to n1.
RH = Rydberg’s constant
1
λ=RH Z21
n2
1
1
n2
2
-
= 109677 cm-1
n h
2πn = 1, 2, 3
= mevr =
Hydrogen spectrum
Proton
Electron
pf3
pf4
pf5
pf8
pf9
pfa
pfd
pfe
pff
pf12
pf13
pf14
pf15
pf16
pf17
pf18
pf19
pf1a
pf1b
pf1c
pf1d
pf1e
pf1f
pf20
pf21
pf22
pf23
pf24
pf25
pf26
pf27
pf28
pf29
pf2a
pf2b
pf2c
pf2d
pf2e
pf2f
pf30
pf31
pf32
pf33
pf34
pf35
pf36
pf37
pf38
pf39
pf3a
pf3b
pf3c
pf3d
pf3e
pf3f
pf40
pf41
pf42
pf43
pf44
pf45
pf46
pf47
pf48
pf49
pf4a
pf4b
pf4c
pf4d
pf4e
pf4f
pf50
pf51

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x

z

λ

A

Electric field

Magnetic field

Direction of

propogation

(a) Wavelength (λ)

(b) wave no. (υ

(c) Frequency (υ)

(d) Time Period (T)

(e) Velocity (c)

(f) Amplitude (A)

CHARACTERISITICS

OF WAVE

. Electromagnetic spectrum is a collection

space electromagnetic waves arranged

according to frequency and wavelength.

ELECTRO-MAGNETIC

SPECTRUM

Increasing Frequency (v)

Cosmic

rays

UV-

rays

x-

rays

Gamma

rays

Infra-

red

visible

light

Micro

waves

Radio

waves

Increasing wavelength(λ)

c

υ

λ =

EMISSION

SPECTRA

Spectrum of

radiation emitted

by a substance

that has absorbed

energy

Spectrum of the

electromagnetic radiation

emitted or absorbed by an

electron during transition

from one energy level to

another

It is like

photographic

negative of an

emission spectra

ABSORPTION

SPECTRA

ATOMIC SPECTRA p- ORBITALS

(DUMBELL SHAPE)

y

z z z

y y

2p

x

3d

y

zp

z

x x x

d-ORBITALS ( Double Dumbell shape )

y

z

z

3d

xy

3d

yz

3d

xz

x

x

y

3d

x

2

y

2

x

y

3d

z

2

x

s-SHAPE OF

ORBITALS

RULES FOR

ARRANGING ELECTRONS

(a) Aufbau principle: Electrons

occupy lowest energy level

First and then move to the next

energy level.

(b) Pauli Exclusion principle: No two

e- can have same set of all 4

quantuM numbers.

(c) Hund's rule: If two or more

orbitals of equal energy

available, then electron will

occupy them singly before filled

in pairs.

ENERGY LEVEL

. Wavelength: Distance between successive crest and trough

. Frequency: Number of waves passed through

a point in 1 sec.

J.J. THOMSON

• Discovered electron(s

• Proposed plum pudding Model

ELECTRON

POSITIVE SPACE

ERNEST

RUTHERFORD

  • Discovered proton (P

)

  • a-partical experiment
  • Proposed Rutherford's

model of an atom (1911)

  • Atom consist of two parts

nucleus and extra nucleus

part

NUCLEUS

ORBIT

ELECTRO-MAGNETIC WAVE THEORY

Neutron

Discovered by James

Chadwick. Charge

on Neutron is 0.

Mass of a p

1.6 × 10 kg

TOWARDS QUANTUM MECHANICAL MODEL

DUAL NATURE OF MATTER

  • Every microscopic particle in the motion

has dual nature (wave and particle

nature) and produce matter waves.

  • wavelength of matter waves

(De=Broglie's wavelength)

h

mv

h

p

HEISENBERG'S

UNCERTAINITY PRINCIPLE

It is impossible to measure

simultaneously the exact momentum

and exact position of a microscopic

moving particle.

∆X. ∆P ≥

h

7 π

QUANTUM NUMBER

QUANTUM MECHANICS

. Fundamental equation was developed by Schrodinger know as

Schrodinger wave equation.

. The electrons in an atom have quantized values of energy. . By evaluating ψ

2

at different points around the nucleus in aN

atom, we can predict the probability of finding the electron.

d

2 ψ

  • (E - U)ψ = 0

dx

2

d

2 ψ

dy

2

d

2 ψ

dz

2

8 π

2 m

h

2

BOHR'S MODEL OF AN ATOM

STRUCTURE

OF ATOM

. Electron in H atom can move around the nucleus in a circular

path of fixed radius

. Each orbit has a definite energy and is known as energy

level or stationary level.

. When an electron jumps from a lower energy level to to

higher one, energy is absorbed and vice versa.

. Angular momentum of electron

Radius (r) =

n

2

Z

0.529 × A

Energy (E) =

Z

2

n

2

-13.6 ×

ev

Velocity (v) =

Z

n

2.18 × 10

6

× m/sec

LIMITATION

. Applicable to only one e -

system eg: H, He

. It could not explain Zeeman effect and stark effect.

Beam of

light

Ejection of

electron

PHOTOELECTRIC

EFFECT

E = hυ

0

  • K.E.

BLACK BODY

RADIATION

A perfect obsorber or

emitter of light.

i.e Absorber or emits all

type of frequency/

radiation

PLANK'S THEORY

Space or region, wher finding the

probability of e

is zero

  • Radial node = (n - l -1)
  • Angular node = l
  • Total node = n - 1

TYPES:-

NODES

1s orbital

y

x

Z

1. Principle quantum No. (n) = 1, 2, 3, 4... shell = K, L, M,

2. Azimuthal Quantum No. (l) = for given value of n, l can have

values from 0 to n - 1)

3. Magnetic quantum no. (m) = for subshells with 'l' value,

m can have values from -l to +l and Total value of m= 2l + 1

4. Spin quantum number = s = + ,

N

Mass of a Neutron is

1.6 × 10 kg

27

W

0

= Hυ

0

Hυ = W

0

+ K.E.

E = hυ

h = 6.623 × 10

Js

Where h = Plank’s Constant

. Wavelength of visible light is from 400nm

to 750nm.

Light travells in the form of small energy packets

known as "Photons".

SERIES

Lyman n^ 1

= 1

n

1

= 2

n

1

= 3

n

1

= 4

n 1

= 5

n

2

= 2, 3...

n

2

= 3, 4...

n

2

= 4, 5...

n

2

= 5, 6...

n

2

= 6, 7...

Balmer

Paschen

Bracket

Pfund

charge of an e

–1.6022 × 10

c

Mass of an e

  • 9.1 × 10 kg

31

27

charge of a proton (p

+1.6 × 10

c

Sub atomic particles

. Wavelength of radiation emitted when an

e

  • jumps from n 2

to n 1

.

RH = Rydberg’s constant

1

= R^ H

Z

2 1

n

2

1

1

n

2

2

= 109677 cm

n

h

= m n = 1, 2, 3

e

vr =

Hydrogen spectrum

Proton

Electron

John Dalton coined the term atom. The atom is the fundamental particle of

matter and considered to be indivisible and indestructible.

In fact, the atom as the whole is electrically neutral as number of protons in

it is equal to number of electrons.

How small is an atom?

Atoms are very small – they are about 0.00000001 cm wide. Think about the

thickness of a crisp. The number of atoms you would need to stack up to make

the thickness of a crisp, is approximately the same number of crisps you would

need to stack up to make the height of Mount Everest!

That’s roughly 7 million crisps!

Electron , proton , neutron are the main fundamental particles of an atom.

Dalton’s Theory

Limitations of Dalton’s Theory

  • It fails to explain why atoms of different kinds should differ in mass and valency etc.
  • The discovery of isotopes and isobars showed that atoms of same elements may have different atomic masses

(isotopes) and atoms of different kinds may have same atomic masses (isobars).

  • The discovery of various sub-aomic particles like X-rays, electrons, protons etc. during late 19th century lead

to the idea that the atom was no longer an indivisible and smallest particle of the matter.

STRUCTURE OF ATOM

iii. They carry negative charge , the negatively charged material particles constituting the cathode rays are

called electrons.

Cathode rays get deflected when they placed in an electric field & magnetic field. Direction of deflection

shows that they are negatively charged.

iv. They produce heating effect.

v. They cause ionization of the gas through which they pass.

vi. They produce X-rays when they strike against the surface of hard metals like tungsten, molybdenum etc.

vii. They produce green fluorescence on the glass walls of the discharge tube exp: ZnS.

viii. They affect the photographic plates.

ix. They possess penetrating effect (i.e., they can easily pass-through thin foils of metals).

x. The nature of the cathode rays does not depend upon the nature of the gas , taken in the discharge tube

and the nature of cathode material.

xi. For each cathode rays, the ratio of charge (e) to mass (m) is constant

Discovery of proton – study of Anode rays:

Goldstein discovered the presence of positive rays. He performed discharge tube experiment in which he took

perforated cathode and a gas at low pressure was kept inside a discharge tube.

On applying high voltage between electrodes, new rays were coming from the side of anode and passing through

the hole in the cathode gives fluorescence on the opposite glass wall coated with zinc sulphide.

These rays were called anode rays or canal rays or positive rays.

Origin of anode or positive rays :

In the discharge tube the atoms of gas lose negatively charged electrons. These atoms, thus, acquire a positive

charge. The positively charged particle produced from hydrogen gas was called the proton.

H → H

+ (proton) + e

-

Fundamental particles:

1) Electron : Electron is a universal constituent discovered by the J.J. Thomson.

Charge : It was determined by Mullikan by oil drop experiment as - 1.602x

  • 19 coulombs

or 4.803x

  • 10 e.s.u.

Mass :9.11x

  • 28 g (nearly equal to 1/

th of mass of hydrogen atom).

Specific charge :e/m ratio is called specific charge & is equal to 1.76x

8 coulombs/gm.

Mass of one mole of electrons : It is 0.55 mg.

Charge on one mole of electron is 96500 coulombs or 1 faraday.

Density : 2.17x

17 g/cc.

2. Proton: ( +1p

0 or 1 H

1 )

 It was discovered by Goldstein.

Charge :It carries positive charge i.e.1.602 x 10

  • 19 coulombs or 4.803x - 10 esu.

Mass :1.672x

  • 24 g or 1.672x - 27 kg.It is 1837 times heavier than an electron.

Specific charge (e/m): 9.58x

4 coulomb/gm.

3. Neutron ( 0 n

1

  • It was discovered by Chadwick by bombarding Be atom with high speed -particles.

𝟗

𝟐

𝟒 → 𝟔

𝟏𝟐

𝟎

𝟏

  • Charge : Charge les s or neutral particle.

  • Mass :1.675x

  • 24 g or 1.675x - 27 kg.
  • Density :1.5x

14 g/cm

3 and is heavier than proton by 0.18%.

  • Specific charge : It is zero.

  • Among all the elementary particles neutron is the heaviest and least stable.

Properties of Electron, Proton and Neutron

Properties Electron Proton Neutron

Discovery J.J.Thomson Goldstein Chadwick

Charge - 1.6022x

  • 19 C 1.6022x - 19 C Zero

Mass 9.109x

  • 31 kg 1.672x - 27 kg 1.675x - 27 kg

Spin ½ ½ ½

Charge - 1 +1 0

Location Outside the nucleus In the nucleus In the nucleus

2) Rutherford’s Atomic Model:

Rutherford, performed  - ray scattering experiment in which he bombarded thin foils of metals like gold, silver,

platinum or copper with a beam of fast-moving radioactive particles originated from a lead block. The presence

of 𝛼 particles at any point around the thin foil of gold after striking it was detected with the help of a circular

zinc sulphide screen. The point at which a𝛼 particle strikes this screen; a flash of light is given out.

Observations and Conclusions

i. Most of the -particles passed through the gold foil without any deflection from their original path.

Because atom has largely empty space as most of the -particles passed through the foil undeflected.

ii. A few of alpha particles are deflected fairly at large angles while some are deflected through small

angles.

Bcz there is heavy positive charge at the center of the atom which causes repulsions.The entire mass of

the atom is concentrated in the nucleus.

iii. A very few -particles are deflected back along their path.

According to Rutherford,

  1. Atom is spherical & mostly hollow with lot of empty space in it.
  2. It has a small +ly charged part at its center known as nucleus.
  3. The nucleus is surrounded by electrons. Electrons revolve round the nucleus

with very high speeds in circular paths called orbits.

  1. The number of extra nuclear electrons is equal to the number of units of positive

charge in the nucleus. Therefore, the atom is electrically neutral. Electrons and the nucleus are held

together by electrostatic forces of attraction.

  1. Rutherford’s model has resemblances with solar system. Hence, it’s also known as planetary model of the

atom.

  1. There is an empty space around the nucleus called extra nuclear part. In this part electrons are present.

As the nucleus of the atom is responsible for the mass of the atom, the extra nuclear part is responsible

for its volume.

Drawbacks:

1. According to the electromagnetic theory of Maxwell ,

when a charged particle moves under the influence of

attractive force it loses energy continuously in the

form of electromagnetic radiation. Therefore, an

electron in an orbit will emit radiation.

As a result of this, the electron should lose energy at

every turn and move closer and closer to the nucleus following a spiral path. Ultimate result is that it will

fall into the nucleus thereby making the atom unstable.

i.e., Rutherford’s model cannot explain the stability of the atom.

2. If the electrons lose energy continuously, the spectrum is expected to

be continuous but the actual observed spectrum consists of well-defined

lines of definite frequencies. Here the loss of energy by the electrons

is not continuous in an atom.

Isotopes, Isobars and Isotones:

Isotopes: The atoms of the same element which have the same atomic number but different mass numbers

are called isotopes.

Exp-

12 13 14

6 6 6

C , C , C ,

16 17 18

8 8 8

O , O , O ,

35 37

17 17

Cl , Cl

Isotopes of an element differ in the number of neutrons present in the nucleus. But they have the same number

of protons and electrons.

Because of same number of electrons, they show same chemical properties. They, have different number

of neutrons , so they will have different masses and hence different physical properties.

Isobars: The atoms of different elements which have the same mass number but different atomic numbers

are called isobars.

Exp:

40

18

Ar ,

40

19

K ,

40

20

Ca

They have same number of nucleons. But they are differed chemically because the chemical characteristics

depend upon the number of electrons which is determined by the atomic number.

1 proton

0 neutrons

1 electron

hydrogen

1 proton

1 neutrons

1 electron

deuterium

1 proton

2 neutrons

1 electron

tritium

Isotones: Isotones are the atoms of different elements which have the same number of neutrons.

Eg:

14

6

C ,

15

7

N ,

16

8

O (n = 8)

30 31 32

14 15 16

Si , P , S (n = 16)

Isotones show different physical and chemical properties.

CLASS EXERCISE

  1. The number of neutrons present in 19

K

is:

a) 39 b) 19 c) 20 d) None of these

  1. The nucleus of the atom (Z > 1) consists of:

a) Proton and neutron b) Proton and electron

c) Neutron and electron d) Proton, neutron and electrons

  1. The number of electrons in a neutral atom of an element is equal to it’s:

a) Atomic weight b) Atomic number c) Equivalent weight d) Electron affinity

  1. The specific charge of the canal rays:

a) Is not constant but changes with gas filled in discharge tube

b) Remains constant irrespective of the nature of gas in discharge tube

c) Is maximum when gas present in discharge tube is hydrogen

d) Is 9.58 x 10

coulombs/g

  1. Proton is:

a) Nucleus of deuterium b) Ionized hydrogen molecule

c) Ionized hydrogen atom d) An α-particle

  1. According to the Rutherford which statement is correct?

a) Electron revolves in fixed circular path around the nucleus

Nature of Light ( Electromagnetic Radiation ):

Electromagnetic radiation does not need any medium for propagation e.g visible, ultra violet, infrared, x-rays, -

rays, radio waves, radiant energy etc.

Two theories were proposed to explain the nature and the propagation of light

i. Corpuscular theory: This theory was proposed by Newton. According to this theory light is

propagated in the form of invisible small particles. i.e. light has particle nature.

The particle nature of light explained some of the experimental facts such as reflection and refraction of

light but it failed to explain the phenomenon of interference and diffraction. Therefore, was discarded

and ignored.

ii. Wave theory of light (electromagnetic wave theory) : was explained by James Clark Maxwell

in 1864 to explain & understand the nature of electromagnetic radiation.

Features of this theory are:

a) The light is a form of electromagnetic radiations.

b) Light radiations consist of electric & magnetic fields oscillating perpendicular to

each other.

c) Vertical component of wave, ‘E’ indicates the change in the

strength of the electric field and the horizontal

component of the wave ‘H’ indicates the change in the

strength of the magnetic field.

d) These radiations do not require any medium for propagation.

e) The radiations possess wave character and travel with the velocity of light

i.e. 3x

8 m/sec because of the above characteristics, the radiation is called electromagnetic radiations or waves.

Electromagnetic radiation is explained by following characteristics:

1. Wave length:

The distance between two successive crests, troughs or between any two consecutive identical points in the

same phase of a wave is called wave length. It is denoted by the letter (lambda).

The wave length is measured in terms of meters (m) , centimeters (cm) , angstrom units (A

) nanometers (nm) ,

picometers (pm) and also in millimicrons (m).

The S.I. unit of wavelength is meter , m

1A

0

  • 10

m or 10

  • 8

cm

1nm = 10

  • 9

m or 10

  • 7

cm = 10A

0

1pm = 10

  • 12

m or 10

  • 10

cm = 10

− 2 A

0

2.Frequency:

The number of waves that pass-through a given point in one second is known as frequency of radiation. It

is denoted by the ‘v (nue).

SI unit of frequency is per second (s

) or Hertz (Hz). A cycle is said to be completed when a wave consisting of

a crest and a trough passes through a point.

3.Velocity:

Distance travelled by the wave in one second is called velocity or speed of the wave (C).

SI unit is meters per second (ms

C of electromagnetic radiation in vaccum is a constant commonly called the speed of light and is denoted by ‘ c ’.It

is equal to 3 × 10

ms

4.Wave number:

Number of waves that can be present at any time in unit length is called wave number.

It is denoted by (nue bar).

It is the reciprocal of wave length.

Wave number =  =

It is expressed in per centimeter (cm

) or per meter (m

The SI unit of wave number is m

Wave length, wave number 𝝂 ̅ , frequency 𝝂 and velocity c are related as follows

.

Limitations of Electromagnetic Wave Theory:

This theory was successful in explaining the properties of light such as interference, diffraction etc.

But it could not explain the following:

(i) The phenomenon of black body radiation.

(ii) The photoelectric effect.

(iii) The change heat capacity of solids as a function of T.

(iv) The line spectra of atoms with special reference to hydrogen.

These phenomena could be explained only if electromagnetic waves are supposed to have particle nature.

Black body radiation:

When a radiant energy falls on the surface of a body, a part of it is absorbed, a part of it is reflected and the

remaining energy is transmitted.

An ideal body is expected to absorb completely the radiant energy falling on it is known as a black body. A black

body is not only a perfect absorber but also a perfect emitter of radiant energy.

A hollow sphere coated inside with a platinum black, which has a small hole in its wall can act as a near black body.

The radiation emitted by a black body kept at high

temperature is called black body radiation. A black

body radiation is the visible glow that the solid object

gives off when heated.

A graph is obtained by plotting the intensity of

radiation against wave length gives the following

details.

  1. The nature of radiation depends upon the T of

the black body.

  1. If the energy emitted is continuous the curve should be as shown by the dotted lines.
  1. At a given temperature the intensity of radiation increases with the wave length, reaches maximum and then

decreases.

  1. The intensity of radiation is greatest at the medium wave lengths and least at highest and lowest wave

lengths.

  1. As the temperature increases the peak of maximum intensity shifts towards the shorter wave lengths.

Planck’s quantum theory:

In order to explain black body radiation, Max Planck proposed quantum theory of radiation.

Postulates

  1. Emission of radiation from a body is due to vibrations of the charged particles in the body.
  2. Energy is emitted or absorbed by a body discontinuously in the form of small packets of energy called quanta.
  3. Energy of each quantum of light is directly proportional to the frequency of the radiation.

E   or E = h

Where ‘h’ is known as Planck’s constant.

The value of ‘h’, 6.6256 × 10

Jsec

  • or 6.6256 × 10

ergs sec

  1. In case of light, the quantum of energy is called a photon.

Total amount of energy emitted or absorbed by a body is some whole number multiple of quantum,

E = nh , where n is an integer such as 1,2,.....

This means that a body can emit or absorb energy equal to hv, 2hv, 3hv..... Or any other integral multiple of

h. This is called quantization of energy.

  1. The emitted radiant energy is propagated in the form of waves.

Photoelectric Effect:

When radiations with certain minimum frequency (ν 0

) strike the surface of a metal, the electrons are ejected

from the surface of the metal. It is called photoelectric effect , electrons emitted are called photoelectron.