Chemistry AUFBAU PRINCIPLE

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AUFBAU PRINCIPLE

Electronic Configuration :

The electron configuration of an atom is the particular distribution of electrons among available shells. It is described by a notation that lists the subshell symbols, one after another. Each symbol has a superscript on the right giving the number of electrons in that subshell. For example, a configuration of the lithium atom (atomic number `3`) with two electrons in the `1s` subshell and one electron in the `2s` subshell is written `1s^2 2s^1`. The notation for electron configuration gives the number of electrons in each subshell.

The electron configuration of an atom is the particular distribution of electrons among available shells. It is described by a notation that lists the subshell symbols, one after another. Each symbol has a superscript on the right giving the number of electrons in that subshell. For example, a configuration of the lithium atom (atomic number `3`) with two electrons in the `1s` subshell and one electron in the `2s` subshell is written `1s^2 2s^1`. The notation for electron configuration gives the number of electrons in each subshell.

Rules for Filling of Electrons in Various Orbitals : Aufbau Principle :

The atom is built up by filling electrons in various orbitals according to the following rules :

`text(Aufbau Principle)` : This principle states that the electrons are added one by one to the various orbitals in order of their increasing energy starting with the orbital of lowest energy. The increasing order of energy of various orbital is,

`2s`, `2p`, `3s`, `3p`, `4s`, `3d`, `4p`, `5s`, `4d`, `5p`, `6s`, `4f`, `5d`, `6p`, `5f`, `6d`, `7p` ........................

How to remember such a big sequence? To make it simple we are giving you the method to write the increasing order of the orbitals.
Starting from the top, the direction of the arrows gives the order of filling of orbitals. See fig.1.

Alternatively, the order of increasing energies of the various orbitals can be calculated on the basis of (`n+ l`) rule.

The energy of an orbital depends upon the sum of values of the principal quantum number (`n`) and the azimuthal quantum number (`l`). This is called (`n+ l`) rule. According to this rule,

In neutral isolated atom, the lower the value of (`n+ l`) for an orbital, lower is its energy. However, if the two different types of orbitals have the same value of (`n+ l`), the orbitals with lower value of `n` has lower energy.

Illlustration of (`n + l`) rule : See fig.2.

- (`n+l`) rule is applicable for multi electronic systems only. For uni-electronic system like `H`, order of energy of orbitals is not "significantly influenced" by `l`.

The atom is built up by filling electrons in various orbitals according to the following rules :

`text(Aufbau Principle)` : This principle states that the electrons are added one by one to the various orbitals in order of their increasing energy starting with the orbital of lowest energy. The increasing order of energy of various orbital is,

`2s`, `2p`, `3s`, `3p`, `4s`, `3d`, `4p`, `5s`, `4d`, `5p`, `6s`, `4f`, `5d`, `6p`, `5f`, `6d`, `7p` ........................

How to remember such a big sequence? To make it simple we are giving you the method to write the increasing order of the orbitals.
Starting from the top, the direction of the arrows gives the order of filling of orbitals. See fig.1.

Alternatively, the order of increasing energies of the various orbitals can be calculated on the basis of (`n+ l`) rule.

The energy of an orbital depends upon the sum of values of the principal quantum number (`n`) and the azimuthal quantum number (`l`). This is called (`n+ l`) rule. According to this rule,

In neutral isolated atom, the lower the value of (`n+ l`) for an orbital, lower is its energy. However, if the two different types of orbitals have the same value of (`n+ l`), the orbitals with lower value of `n` has lower energy.

Illlustration of (`n + l`) rule : See fig.2.

- (`n+l`) rule is applicable for multi electronic systems only. For uni-electronic system like `H`, order of energy of orbitals is not "significantly influenced" by `l`.

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