Electronic Configuration:-

History:-
In 1923 Neil Bohr was the first scientist who proposed properties of element can be describe by the electronic structure of an atom. This proposal based on the Neil Bohr model of atom that state as "electron of an atom revolve in the fixed position from the nucleus" . During this era another chemist Sulfur also proposed his proposal about electronic configuration but his proposal is widely strange to the Neil Bohr's proposal.
A scientist E.C Stoner worked on the third quantum number, he was corrected Stoner electronic configuration proposal pridictedly. But both scientist Neil Bohr and Stoner could not describe atomic spectra in magnetic field.

Difinition:-
Arrangement of electron in different orbitals according to the some rules is called electronic configuration.

Rules of Electronic Configuration:-

• n + l rule:- This rule was given by vis visser. According to n + l rule electrons are filled in orbitals in increasing order of their n + l value where n is principle quantum number and l is azimuthal quantum number. Thus first electrons are filled in orbital with low n + l value then in orbital with high n + l value.
If two orbital have equal n + l value then first electron are filled in orbital with low value of n.

Order of Filling Orbitals According to n + l Rule:-
1s, 2s, 3s, 3p, 4s, 4p, 5s, 4d, 5p, 6s, 5d, 4f, 6p, 7s, 6d, 5f, 7p.

• Aufbau Principle:-
This principle was given by paulli meaning of Aufbau is to build up. Accorfing to this princple electrons are filled in orbitals equal to the atomic number in increasing order of their orbital energy.

Thus first electrons are filled in orbital with low energy then in orbital with high energy.

Order of Fillimg Orbital According to Aufbau Principle:-
1s, 2s, 3s, 3p, 4s, 4p, 5s, 4d, 5p, 6s, 5d, 4f, 6p, 7s, 6d, 5f, 7p.

• Hund's Rule:-
 According to the Hund's rule if degenerate orbitals are available then first electron are filled unpaired (single) to give maximum number of unpaired electrons, then pairing takes place.

Degenerate Orbitals:-
Orbitals with same energy are called degenerate orbitals px, py and pz and f sub shells have five and seven degenerate orbitals.

Ion:-
An atom with positive or negative charge is called an ion. When an atom gives one or more electrons then it changes to an ion.

Types of Ions:-
There are two types of ions.
• Cation
• ‎Anion

1) Cation:-
An atom with a positive charge is called a cation. When an atom gives one or more electrons then it acquires a positive charges to a cation.


2) Anion:-
 An atom with a negative charge is called an anion. When an atom gains one or more electrons then it acquires a negative charge and changes to anion.


Electronic configuration:-
In ions electrons are filled equal to the electrons in the ion.
The electronic configuration of an atom or molecule is the distribution of electron in the quantum chemistry.
The description of electronic configuration is that the electron is moving independently in a sub-shell.
We can understand this by the law of quantum mechanics that the level of energy of the system that contain one electron in which every level of energy is related to the electron configuration and the electron can easily move from one configuration to another but during this process electron will absorb or emit energy i.e quantum energy, but in the form of photon.
It is very help to understand the structure or periodic table because modern periodic table is based on the electronic configuration. Through electronic configuration chemical bonds can also easily described that hold the atoms together.

Shells and Sub-shells:-
Bohr's model of an atom firstly described the about the concept of electron configuration, by configuration of an atom world's come to know about the shells and sub-shells which are present in the atoms which are very help to study about the quantum mechanics.
Furthur shells described by the three quantum number which are:
1) Principle quantum numbers
2) ‎Azimuthal quantum numbers
3) ‎Spin quantum numbers.

Energy Ground State and Excite State:

The energy of an electron is all about it's orbital infact the orbital of electron is it's energy. The energies of all electron  aporoxinately is equal to the energy of configuration. Configuration of ground state of an atom has lowest electronoc energy and other configuration is excited state.
For example sodium atom ground state configuration is 1s2 2s2 2p6 3s1 this is according to Aufbau principle. Excited state firstly obtained when promoted 3s electron to 3p orbital and obtained 1s2 2s2  2p6 3p1 configuration this is abbreviate as 3p level. When an atom move from one configuration to another then it emit or absorb energy. For example sodium acquire 3p (excited)  level when it get electrically discharge. When valence electron excite it involve energy in form of photon or ultravoilate rays. Core electron can also be excite but it need more energy and it involve energy in forn of X-ray photon. Example of this case is when sodium 2p electron excite to 3s level and configuration will be 1s2 2s2 2p5 3s2 configuration.