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 1*s*^{2}2s^{2}2*p*^{1}*2*_{x}*p*^{1}*2*_{y}*p*^{1}*p1z according to Hund‘s rule and not 1*_{z}*s*^{2}2*s*^{2}2*p*_{x}^{2}2*p*^{1}_{y}
10. The valence electronic configurations of Cr and Cu, therefore, are 3d

^{5}4s^{1}and 3d^{10}4s^{1}respectively and not 3d^{4}4s^{2}and 3d^{9}4s^{2}. 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**(3d_{xy}, 3d_{yz}, 3d_{zx}3d_{x}^{2}_{-y}^{2}and 3d_{x}^{2}orbitals).**Click here to view complete Chemistry Free Study Materials and Notes for NEET Preparation**

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