Bohr’s Model of The Hydrogen Atom

Bohr’s Model of Hydrogen Atom :

Rutherford’s atomic model was not in accordance with the laws of Physics. In 1912 Neil Bohr presented a new atomic model based on concepts. The main postulates of the Bohr’s hydrogen atom model, based on the quantum theory, are as under :

1. In the hydrogen atom electron moves in circular orbits of definite radius and energy. These orbits are represented by 1, 2, 3, 4 ………. or K, L, M, N, O.
2. The angular velocity of electrons in these orbits (mvr) is equal to or h/2 Pai or its multiple. Here h is Planck’s constant, m = mass of electron v = velocity of electron and r = radius of the orbit.
3. There is no change in energy of the electron revolving in a particular orbit but energy is emitted and absorbed respectively when electron moves from higher orbital to lower orbital or lower orbital to higher orbital.

Atomic radius is the distance between the nucleus and the outermost orbital (shell) of an atom dissociated from a compound. But neither can an atom be dissociated nor can the distance of the shell from outermost orbital, be measured in a simple way. Therefore the atomic radii can be explained in the following manner :

1 Covalent radius :

• Covalent radius is half the distance of single covalent bond formed between similar atoms. For example half of the distance between the nuclei of two chlorine atoms is 99Aº which is considered to be its atomci radius (1Aº = 10^-8 cm).

2 Metallic radius :

• Metallic radius is half of the total distance between the nuclei of the two adjacent atoms in a metallic cluster. This is its atomic radius.

Important Points :- 

1. The basic particles of atom are electron, proton and neutron.
2. The negatively charged particles in the atom are electrons.
3. The numeric value of the charge on electron and proton is the same but their sign is opposite.
4. James Chadwick discovered neutrons. 
5. There are 6.022×10²³ particles in one mole. This is known as the Avogadro number.
6. The NTP volume of 1 mole of a gas is 22.4 litres.
7. The formula to determine the maximum number of electrons in a shell is 2n² .
8. When the atomic number is the same but mass number is different they are known as Isotopes.
9. Isobars are elements having different atomic number and the same mass number.
10. There are three isotopes of hydrogen, Protium, Deutirium and Tritium.

Atomic Structure Important Questions-Answers

1. The Plum Pudding Model of atom was given by:
(a) Neil Bohr
(b) Thomson
(c) Ernest Rutherford
(d) Goldstein

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2. The discoverer of neutron was :
(a) C.V. Raman
(b) Rutherford
(c) J.J. Thomson
(d) James Chadwick

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3. The size of atom is :
(a) 10^-6 cm
(b) 10^-15 cm
(c) 10^-2 cm
(d) 10^-8 cm

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4. The number of neutrons in the Deutirium Isotope of hydrogen is /are :
(a) one
(b) Two
(c) Three
(d) Not even one

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