Smallest particle of an element which shows all properties of element is called atom.
Some characteristics of "atoms" are as follows:
Atom takes part in chemical reactions independently.
Atom can be divided into a number of sub-atomic particles.
Fundamental particles of atom are electron, proton and neutron.
CHARACTERISTICS OF ELECTRON
1. Charge: It is a negatively charged particle.
2. Magnitude of charge: Charge of electron is 1.6022 x 10-19 coulomb.
3. Mass of electron: Mass of electron is 0.000548597 a.m.u. or 9.11 x 10-31 kg.
4. Symbol of electron: Electron is represented by "e".
5. Location in the atom: Electrons revolve around the nucleus of atom in different circular orbits.
CHARACTERISTICS OF PROTON
Charge: Proton is a positively charged particle.
Magnitude of charge: Charge of proton is 1.6022 x 10-19 coulomb.
Mass of proton: Mass of proton is 1.0072766 a.m.u. Or 1.6726 x 10-27 kg.
Comparative mass: Proton is 1837 times heavier than an electron.
Position in atom: Protons are present in the nucleus of atom.
CHARACTERISTICS OF NEUTRON
1. charge: It is a neutral particle because it has no charge.
2. Mass of neutron: . Mass of neutron is 1.0086654 a.m.u. Or 1.6749 x 10-27 kg.
3. Comparative mass: Neutron is 1842 times heavier than an electron.
4. Location in the atom: Neutrons are present in the nucleus of an atom.
ATOMIC NUMBER
Total number of protons present in the nucleus of an atom
is called "Atomic number" or "Charge number"
Since the total number of protons and the total number of electrons in an atom are equal therefore atomic number may also be defined as:
"Total number of electron in an atom is called Atomic number"
SYMBOL: It is denoted by "z".
MASS NUMBER
Total number of protons and neutrons present in the nucleus of an atom is called "Mass number".
SYMBOL: It is denoted by "A".
A = p + n
SYMBOL OF NUCLEUS
A nucleus is represented by the symbol :
zXA
Where X= symbol of element (Cl, O, H , Na etc.)
A= mass number.
Z= atomic number.
1H1, 11Na23, 17Cl35, 7N14, 13Al27
ION
If an electron or more than one electrons are added to or removed from an atom then it becomes a negative or a positive charged particles which is known as "ION".
CATION
If an electron or more than one electrons are removed from an atom,
it becomes a positive ion which is also known as "cation".
for example: Na+1, K+1, Ca+2, Mg+2, Al+3.
ANION
If an electron or more than one electrons are added to an atom
it becomes a negative ion which is also known as "anion".
for example: Cl-1, Br-1, O-2, N-3, F-1
IONIZATION POTENTIAL
"Amount of energy required to remove an electron from
an isolated gaseous atom is called ionization potential."
ATOM + ENERGY è CATION + ELECTRON
A + Energy è A+ + e-
UNIT:
kJ / mole
FACTORS AFFECTING IONIZATION POTENTIAL:
(i) ATOMIC SIZE (ii) NUCLEAR CHARGE (iii) SHIELDING EFFECT OF INNER ELECTRONS
1. Big atoms have small I.P. values
2. Small atoms have high values of I.P.
3. Greater the nuclear charge , higher is the I.P. of atom.
I.P. IN A GROUP
I.P. decreases in a group from top to bottom due to increase in atomic size from top to bottom in a group.
I.P. IN A PERIOD
I.P. increases in a period from left to right.
This increase in I.P. is due to increase in nuclear charge.
Main postulates of Dalton atomic theory are as follows:
Matter is composed of very tiny or microscopic particles called "Atom".
Atom is an indivisible particle.
Atom can neither be created nor it is destroyed.
Atoms of an element are identical in size, shape, mass and in other properties.
Atoms of different elements are different in their properties.
Atoms combine with each other in small whole numbers.
All chemical reactions are due to combination or separation of atoms.
DEFECTS IN DALTON’S THEORY:
Postulate number 2, 3, 4 and 6 are not correct as described below:
DEFECT NO: 1
Atom can be divided into a number of sub-atomic particles such as electron, proton and neutron etc.
DEFECT NO: 2
Atoms of an element may be different in their masses.
For example:
1H1, 1H2, 1H3
17Cl35, 17Cl37
DEFECT NO: 3
All compounds do not have small number of atoms.
For example:
Decane C10H22.
Sugar C12H22O11.
DEFECT NO: 4
Atom can be destroyed by fission process in
Atom bomb.
Nuclear reactor.
On the basis of above defects, Dalton's atomic theory has failed now.
RUTHERFORD’S EXPERIMENT
APPARATUS :
Alpha particles.
Gold foil. (0.0004 cm thick)
Zinc sulphide screen.
Electron Gun.
EXPERMENT
In his experiment, Rutherford bombarded alpha particles on very thin gold foils and record their observations.
OBSERVATIONS
1. He observed that most of the alpha particles were pass through the foil undeflected.
2. Very few particles were deflected when passed through the foil.
3. One particle out of 8000 particles was rebound.
MAIN POINTS OF RUTHERFORD’S THEORY
Major portion of the atom is empty.
The whole mass of the atom is concentrated in the center of atom called nucleus.
The positively charged particles are present in the nucleus of atom.
The charge on the nucleus of an atom is equal to (+z.e).
The electrons revolve around the nucleus in different circular orbits.
Size of nucleus is very small as compare to the size of atom.
DEFECT OF
RUTHERFORD’S THEORY
According to electromagnetic theory, being a charge particle, a revolving electron must emit energy. Due to emission of energy, the orbit of electron will decrease. Consequently electron will fall into the nucleus. But this is against the actual situation and this shows that atom is unstable.
If the electrons emit energy continuously, they should form continuous spectrum .But actually line spectrum is obtained
ELECTRONEGATIVITY
"Relative tendency or relative power of an atom to attract shared
pair of electrons towards itself is called ELECTRONEGATIVITY."
E.N depends upon the size of atom .
Small atoms have large values of E.N.
Big atoms have small values of E.N.
E.N decreases in a group.
E.N increases in a period.
Most Electronegatively element is "Flourine". E.N = 4
ATOMIC RADIUS
Half of the distance between the nuclei of two adjacent similar atoms is called "Atomic Radius".
Unit of atomic radius is Angstrom (A)
1 A. = 10-10 m
BOHR’S ATOMIC THEORY
Main postulates of Bohr’s atomic theory are:
1. Electrons revolve around the nucleus in circular path, which are known as "ORBITS" or "ENERGY LEVEL".
2. Energy of an electron in one of its allowed orbits is fixed.As long as an electron remains in one of its allowed orbit, it can not absorb or radiate energy.
3. If an electron jumps from lower energy level to a higher energy level, it absorbs a definite amount of energy.
...
4. If an electron jumps form higher energy level to a lower energy level, it radiates a definite amount of energy.
..
5. Energy released or absorbed by an electron is equal to the difference of energy of two energy levels.
Let an electron jumps from a higher energy level E2 to a lower energy level E1.The energy is emitted in the form of light .Amount of energy released is given by:
E = E2 - E1
E2 - E1= h
Where
h = Planck's consrant ( 6.6256 x 10-34 j.s)
= Frequency of radiant light
6. Spectrum of light emitted from an electron is a "LINE SPECTRUM".
7. Angular momentum of an electron is given by:
m v r = nh /2
Where n =1, 2, 3, ………..
m = mass of electron
V = velocity of electron
r = radius of orbit
DISCHARGE TUBE EXPERIMENT AND PROPERTIES OF CATHODE RAYS
DISCHARGE TUBE
Discharge tube is also called "CROOCK TUBE". It is made of a glass tube which consists of two metallic plates. One plate is connected to positive terminal of high voltage power supply and the other to negative terminal. The plate connected to the positive terminal is called "ANODE" the other connected to negative terminal is called "CATHODE". The tube is filled with any gas.
EXPERIMENT
In discharge tube experiment, at low pressure and at very high voltage, an electric current is passed. Due to passage of electric current, a stream of rays is passed in the tube originating from cathode. These rays are called "CATHODE RAYS".
PROPERTIES OF CATHODE RAYS
These rays originate from cathode.
Cathode rays travel in straight line.
Cathode rays carry negative charge.
Cathode rays are deflected by electric field.
Cathode rays are deflected by magnetic field.
These rays consist of material particles.
The ratio e / m of these particles was 1.76 x 108 col / gm.
Cathode rays consist of "electron".
The rays, upon striking glass or certain other materials, cause them to glow.
The mass of each negative particle was found equal to1/1837 of the lightest hydrogen atom.
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