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Tuesday, 7 February 2017

Structure of Atom V


1. Quantum numbers. They are used to get complete information about electron, i.e., location, energy, spin, etc. These quantum numbers also help to designate the electron present in an orbital.

2. Principal quantum number. It specifies the location and energy of an electron. It is measure of the effective volume of the electron cloud. It is denoted by _n‘.. Its possible values are 1, 2, 3,4 …..

3. Angular momentum quantum number. It is also called _azimuthal quantum number‘. It determines the shape of the orbital. It is denoted by _l‘. The permitted values of  _l‘ are 0, 1, 2, etc., upto n–1. For a given value of n, l = 0 to n 1. e.g., if value of n is 4, l can have values 0, 1, 2, 3. It determines angular momentum.
4. Magnetic quantum number. It is denoted by _m‘ and its its value depends on value of _l‘ since magnetism is due to angular momentum. It determines the magnetic orientation of an orbital, i.e., the direction of orbital relative to magnetic field in which it is placed. Its permitted values are - l to + l including zero, e.g., when l = 1, then m = -1, 0, +1. It has total number of values equal to 2l + 1.

5. Spin quantum number. It indicates, the direction in which electron revolves. Spin is magnetic property and is also quantized. It has two permitted values + ½ or – ½. The spin angular momentum of an electron is constant and cannot be changed.

6. (n+l) rule: The relative order of energies of various sub-shells in a multi-electron atom can be predicted with the help of (n+l) rule (also called Bohr-Bury rule)According to this rule a sub-shell with lower values of (n+l) has lower energy.In case two sub-shell has equal value of (n+l), the sub-shell with lower value of n has lower energy

7. Pauli’s Exclusion Principle. No two electrons in an atom can have all the four quantum numbers same. It can also be stated as
An orbital can have maximum two electrons and they must be of opposite spin quantum numbers.

8. Aufbau principle. Electrons are filled in the various orbitals in the increasing order of their energies, i.e., orbital having lowest energy will be filled first and the orbital having highest energy will be filled last.Increasing energy of atomic orbitals for multi-electron atoms
         1s < 2s < 2p < 3s <3p < 4s <3d < 4p <5s < 4d <5p < 6s < 4f < 5d < 6p < 7s

9. Hund’s rule of maximum multiplicity. No electron pairing takes place in p, d and f orbitals until each orbital in the given sub-shell contains one electron, e.g., N (7) has electronic configuration 1s2 2s2  2 p1x 2 p1y  2 p1z   p1z  according to Hund‘s rule and not 1s2  2s2  2 px2  2 p1y 

10. The valence electronic configurations of Cr and Cu, therefore, are 3d5 4s1 and 3d104s1 respectively and not 3d4 4s2 and 3d94s2. It has been found that there is extra stability(Stability of Completely Filled and Half Filled Subshells) associated with these electronic configurations.

11. Three orbitals of 2p subshell (2px, 2py, and 2pz orbitals)
12. Five orbitals of 3d subshell  (3dxy, 3dyz, 3dzx 3dx2-y2and 3dx2 orbitals).
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