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how to fill subshells?

Filling of sub shell in a atom:

The filling of electrons into the orbitals of different atoms take place according to the aulfbau principle which is based on the Pauli's exclusion principle, the Hund's rule of maximum multiplicity and the relative energies of the orbitals. 

1.Aufbau Principle : In the ground state of the atoms, the orbitals filled in order of their increasing energies.  

For example : 3s>2p

2.Pauli exclusion rule : No two electron in an atom can have the same set of four quantum number and only two electron can exist in the same orbital and these electron must have opposite spin.

3. Hund's Rule : Pairing of electron in the orbital belonging to the same subshell (p,d or f) does not take place until each orbitals belonging to that subshell has got one electron if each i.e., it is singly filled.  

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In order to examine the results ofthe wave theory of the electronfor eachelementin theperiodic table, we must recall the general rules that are necessary in order to predictelectron configurationsfor allatoms of the elements. To review, these rules are as follows:

1The Aufbauprinzip(building-up principle).The structure of an atommay be built up from that of the element preceding it in the periodic system by adding oneproton(and an appropriate number ofneutrons) to thenucleusand one extranuclear electron.

2The order of fillingorbitals. Each time an electron is added, it occupies the available subshell oflowestenergy. The appropriate shell may be determined from a diagram such as Fig. 1awhich arranges the subshells in order of increasing energy. Once a subshell becomes filled, the subshell of the next higher energy starts to fill.

3The Pauli exclusion principle. No more than two electrons can occupy a singleorbital. When two electrons occupy the same orbital, they must be of opposite spin (an electron pair).

4Hunds rule. When electrons are added to a subshell where more than one orbital of the same energy is available, their spins remain parallel and they occupy different orbitals. Electron pairing does not occur until it is required by lack of another empty orbital in the subshell.

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