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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
)
- 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
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
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
31
27
charge of a proton (p
+1.6 × 10
c
Sub atomic particles
. Wavelength of radiation emitted when an
e
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
or 4.803x
Mass :9.11x
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
14 g/cm
3 and is heavier than proton by 0.18%.
Properties of Electron, Proton and Neutron
Properties Electron Proton Neutron
Discovery J.J.Thomson Goldstein Chadwick
Charge - 1.6022x
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,
- Atom is spherical & mostly hollow with lot of empty space in it.
- It has a small +ly charged part at its center known as nucleus.
- The nucleus is surrounded by electrons. Electrons revolve round the nucleus
with very high speeds in circular paths called orbits.
- 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.
- Rutherford’s model has resemblances with solar system. Hence, it’s also known as planetary model of the
atom.
- 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
- The number of neutrons present in 19
K
is:
a) 39 b) 19 c) 20 d) None of these
- 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
- 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
- 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
- Proton is:
a) Nucleus of deuterium b) Ionized hydrogen molecule
c) Ionized hydrogen atom d) An α-particle
- 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
m or 10
cm
1nm = 10
m or 10
cm = 10A
0
1pm = 10
m or 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.
- The nature of radiation depends upon the T of
the black body.
- If the energy emitted is continuous the curve should be as shown by the dotted lines.
- At a given temperature the intensity of radiation increases with the wave length, reaches maximum and then
decreases.
- The intensity of radiation is greatest at the medium wave lengths and least at highest and lowest wave
lengths.
- 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
- Emission of radiation from a body is due to vibrations of the charged particles in the body.
- Energy is emitted or absorbed by a body discontinuously in the form of small packets of energy called quanta.
- 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
ergs sec
- 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.
- 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.
