8th Science, 5.3, Inside the Atom, Notes

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What's app Group 2🎷

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🏄 The maximum number of electron a shell can contain is obtained by the formula ‘2n²’.

 

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Symbol    n      2n²

   ^{K              1        2}

   ^{L              2        8}

   ^{M            3        18}

   ^{N            4         32}

   ^{O            5         50}

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🌱 We have seen that 2, 8, 18, 32....electrons can be accommodated in the shells K, L, M, N .... respectively.

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 🏆 ^{Energy of an electron in the first shell (K shell) is the lowest.}

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🎯   Valency

1. 1. The combining capacity of an atom is known as its valency.
   2. valency means the number of chemical bonds formed by an atom

   The number of electrons an atom can gain, lose, or share when forming chemical bonds with other atoms.

   • The number of electrons given or taken to obtain a double or octal state is called valence.

     The outermost shell is called valence shell^..

     The electrons in the outermost shell are called valence electrons.

     Valency of an element is same as the number of its valence electrons if this number is four or less than four.

     When an element has four or more valence electrons, the number of electron by which the octet is short of completion is the valency of that element.

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^🏈 Why are the atomic numbers and atomic mass numbers always in whole numbers ?

Ans: 

 Atomic number represents the count of protons in an atom's nucleus and is a whole number

The mass number is a whole number because it is the sum of protons and neutrons.

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^🏇 Sulphur contains 16 proton and 16 neutrons. What would be its atomic number and mass number?

Ans:

The number of protons present in an atom indicates its atomic number. 

Sulphur contains 16 proton 

 Atomic number of Sulphur:- 16

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The sum of protons and neutrons is called atomic mass. 

∴   Atomic mass = p + n

∴  = 16  + 16

∴  = 32

. The atomic number is a fundamental property of an element and its chemical identity^.

 

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Write down the number of electron present in the outermost shell.

         

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Write the electronic configuration number of valence electron and valency of following elements.

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🏁 Isotopes : 

1. Some elements in nature have atoms with same atomic number but different mass number. 

2. Such atom of the same element having different mass number are called isotopes.^.  

3. 1. For example, carbon has three isotopes, namely, C - 12, C - 13, C - 14. 
   2. Hydrogen isotopes: Protium (1H), Deuterium (2H), and Tritium (3H). 
   3. The isotopes have same proton number but different neutron number

🏀 Uses of isotopes : 

Isotopes of some elements are radioactive. They are used in various fields such as industry, agriculture, medicine, research field.

1. Uranium - 235 is used for nuclear fission and production of electricity.

2. Some radioactive isotopes like Cobalt - 60 are used in the medical treatment of fatal diseases like cancer^.

3. Iodine -131 is used in the treatment of goiter, disease of thyroid gland.

4. The radioactive isotopes are used for detection of cracks (leakage) in the underground pipes. eg. Sodium -24^.

5. Cobalt-60, Radioactive isotopes are used for food preservation from microbes.

6. The radioactive isotope C-14 is used for determining the age of archeological objects.

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🎾 Nuclear Reactor : 

Nuclear reactor is a machine that generates electricity on large scale by using atomic energy. 

In a nuclear reactor, the nuclear energy in atom is released by bringing about nuclear reactions on the nuclear fuel. 

 On bombardment with slow speed of neutrons, the nucleus of the isotope Uranium - 235 undergoes nuclear fission to form nuclei of two different elements Krypton - 92 and Barium - 141 and 2 to 3 neutrons. On decreasing the speed these neutrons bring about fission of more U-235 nuclei. In this way a chain reaction of nuclear fission takes place.  

A large amount of nuclear energy is released during a chain reaction of fission. 

To control the chain reaction in the nuclear reactor it is necessary to decrease the speed and number of neutrons. For this purpose the following provision is made in a nuclear reactor. 

1. Moderator : Graphite or heavy water is used as moderator for reducing the speed of neutrons.

2. Controller : To reduce the number of neutron by absorbing them rods of boron, cadmium, beryllium etc. are used as controller.

The heat produced in the fission process is taken out by using water as coolent. Water is transformed into steam. By means of this steam, turbines are driven and electricity is generated.

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