CHAPTER 6- ATOMIC STRUCTURE
Q1) Describe the postulates of Dalton atomic theory? What are the draw
backs in this theory?
Ans:Daltonatomic theory:-The fundamental postulates of Dalton atomic
theory are stated below:
1) Matter is composed of extremely tiny invisible particles called atom.
2) Atom can neither be created or destroyed.
3) Atom of a particular element are identical in size shape mass and
all other properties & differ from atom of other element in these
properties.
4) Chemical combination takes place between small whole number of atom.
According to the latest research on the structure of atom &matter
neutral particle following defeat are observed in Dalton atomic model.
Drawbacks of dalton atomic theory:-
1) Atom consist of charged particle called proton and electron &
neutral particle called neutron.
2) The atom of the same element may differ in their atomic mass and
such atomof an element are called isotopes.
3) The ratio between atom of various element in many molecules of the
organic compound is not simple.
4) Matter can be converted into energy by Einstein equation E=mc2
Q2) How was electron discovered? describe in detail the experiment
performed for the discovery of electron?
Ans:Discovery of electron:-Electron is thesmallest particle of atom.
the most convising evidence for the existence of electron came from
the experiment performed by crooker, faraday, and JJ Thomson. A high
voltage electric current was pass through gases at a very low
pressure(1mm of hg) in a dis charge tube. When high voltage is applied
across the two metallic electrode sealed in a discharge tube the rays
begin ti originate from cathode which are called cathode ray. These
rays travel in straight line towards the anode. If an electric field
is applied , these rays bend towards the positive pole which show
their negative nature.
Properties of cathode rays:-
1) Cathode rays travel in straight line away from the cathode.
2) Cathode rays are negatively charged asthey bend towards the
positive pole of the magnetic field.
3) The rays upon striking glass or certain other material cause these
material to glow.
4) The charge to mass ratio (e/m) of those particle was found equal to
that of electron.
e/m = 1.76 * 10 power8 coulomb per gram
5) The mass of each negative particle wasfound equal to 1/1837 of the
lightest hydrogen atom.
On the basic of those properties it was concluded that cathode rays
are negatively charged particle called electron.
Q3) Discuss bohrs atomic model&describe its significance in atomic structure?
Ans:Bohrs atomic model:-Neil bohr proposed a new model for the
structure of the atom in 1913.The salient features of this model are
that.
1) Electron revolve around the nucleus in fixed circular paths which
he called orbit or energy level.
2) As long as an electron revolve in a particular energy level it does
not emit orabsorb energy.
3) When an electron absorb energy it moves to a higher energy level
further away from the nucleus. When it lose energy it return to lower
energy level closer to the nucleus & the energy is emitted as light.
4) The electron loses a definite quantity of energy called quantum of
energy when it jump down from an orbit of higher energy level to a
lower energy level. The energy is emitted in the form of radiation.
The frequency of the energyemitted is directly proportional to the
difference in energy between the two level.
5) The angular momentum (mvr)of an electron in any orbit is integral
multiple of h/2x
mvr = nh/2x
Q4) Name the fundamental particle of an atom write the characteristic
of each particle?
Ans:fundamental particles of atom:-fundamental particle of an atom are:
1)Electron:-
1: It is a negatively charged particle.
2: Its mass is equal to 9.11*10 power-31 kg
3: The charge on electron is unit negatively or 1.6022*10power -19 coulomb
4: Electron are present around the nucleus of an atom.
2)Proton:
1: It is a positively charged particle.
2: Its mass is equal to 1.6726*10 power-27 kg
3: The charge on proton is equal to that of electron.
4: Proton are present in the nucleus of anatom.
3)Neutron:
1: Neutron has no charge.
2: Its mass is equal to 1.67492*10 power-27 kg
3: It is 1842 times beaver than an electron.
4: Neutron are also present in the nucleus of an atom.
Q5) Describe the Rutherford Experiment and Discovery of Nucleus
?What are the draw backsRutherford Model?
Rutherford Experiment and Discovery of Nucleus
Lord Rutherford (1911) and his coworkers performed an experiment. They
bombarded a very thin, gold fail with Alpha particles from a
radioactive source. They observed that most of the particles passed
straight through the foil undeflected. But a few particles were
deflected at different angles. One out of 4000 Alpha particles was
deflected at an angle greater than 150.
(Diagram)
Conclusion
Following conclusions were drawn from the Rutherford's Alpha Particles
scattering experiment.
1. The fact that majority of the particles went through the foil
undeflected showsthat most of the space occupied by an atom is empty.
2. The deflection of a few particles over awide angle of 150 degrees
shows that these particles strike with heavy body having positive
charge.
3. The heavy positively charged central part of the atom is called nucleus.
4. Nearly all of the mass of atom is concentrated in the nucleus.
5. The size of the nucleus is very small as compared with the size of atom.
Defects of Rutherford Model
Rutherford model of an atom resembles our solar system. It has
following defects:
1. According to classical electromagnetic theory, electron being
charged body will emit energy continuously. Thus the orbit of the
revolving electron becomes smallerand smaller until it would fall into
the nucleus and atomic structure would collapse.
2. If revolving electron emits energy continuously then there should
be a continuous spectrum but a line spectrum is obtained.
(Diagram)
Q6)Define the following:
Atomic Number
The number of protons present in the nucleus of an atom is called
atomic number or proton number. It is denoted by z. The proton in the
nucleus of an atom is equal to number of electrons revolving around
its nucleus.
Mass Number
The total number of the protons and neutrons present in the nucleus of
an atom is called mass number. The protons and neutrons together are
called nucleon. Hence it is also known as nucleon number. It is
denoted by A. the number of neutrons present in the nucleus of an atom
is represented by N.
Mass Number = No of Protons + No of neutrons
A = Z + N
Isotopes
The atoms of same elements which have same atomic number but different
mass number are called Isotopes. The number of protons present in the
nucleus of an atom remains the same but number of neutrons may differ.
Isotopes of Different Elements
Isotopes of Hydrogen
Hydrogen has three isotopes:
1. Ordinary Hydrogen or Protium, H.
2. Heavy Hydrogen or Deuterium, D.
3. Radioactive Hydrogen or Tritium, T.
Protium
Ordinary naturally occurring hydrogen contains the largest percentage
of protium. It is denoted by symbol H. It has one proton in its
nucleus and one electron revolve around the nucleus.
*.Number of Protons = 1
*.Number of Electrons = 1
*.Number of Neutrons = 0
*.Atomic Number = 1
*.Mass Number = 1
Deuterium
Deuterium is called heavy hydrogen. The percentage of deuterium in
naturally occurring hydrogen is about 0.0015%. It has one proton and
one neutron in its nucleus. It has one electron revolving around its
nucleus. It is denoted by symbol D.
*.Number of Proton = 1
*.Number of Electron = 1
*.Number of Neutrons = 1
*.Atomic Number = 1
*.Mass Number = 2
Tritium
Radioactive hydrogen is called tritium. It is denoted by symbol T. The
number of tritium isotope is one in ten millions. It has one proton
and 2 neutrons in its nucleus. It has one electron revolving around
its nucleus.
*.Number of Proton = 1
*.Number of Electron = 1
*.Number of Neutron = 2
*.Atomic Number = 1
*.Mass Number = 3
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