CHAPTER 6- ATOMIC STRUCTURE

by • 14/07/2011 • 9th class ChemistryComments (3)1133

Q1) Describe the postulates of Dalton atomic theory? What are the draw backs in this theory?
Ans:Dalton atomic 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 atom of 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 the smallest 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 as they 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 was found 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 or absorb 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 energy emitted 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 an atom.
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 backs Rutherford 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 shows that most of the space occupied by an atom is empty.

2. The deflection of a few particles over a wide 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 smaller and 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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3 Responses to CHAPTER 6- ATOMIC STRUCTURE

  1. chalako masi says:

    @idrees….cathode rayx r electronx…n electronx 4 evry gas r same..thatx y cathod rayx r indepndnt of da natur ov gas……

  2. kami says:

    nyC woRdinG…………………………> sir

  3. idrees says:

    catode rays are independent of the nature of gas used in the discharge tube,explain

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