**Electron configuration of an atom** represents that how the electrons are distributed in its atom among the orbits (shells) and sub shells. The **electron configuration of an atom** is very important as it helps to predict the chemical, electrical and magnetic behavior of substance. Based on electron configuration of atom, we can predict that whether two substances will chemically react or not and if they react, we can also predict what kind of reaction is likely to be happen and how strong it will be.

The **electron configuration of an atom**, describes the arrangement of electrons in space around the nucleus. The electrons are distributed over different energy level. These energy levels are called the shell or orbits.

Basically the distribution of electrons over various shells (energy levels) is such that, the overall energy of all electrons of an atom remains minimum for stability of the atom.

The electron distribution over various energy level is governed by the following rules –

- The maximum number of electrons in any main energy level (shell) is given by, ‘2n
^{2}’, where, n is an integer and representing the “principal quantum number”. For different main energy levels the value of ‘n’ and maximum number of electrons are given in table below-Sl. No. Energy level or Orbit (shell) Principal quantum number ‘n’ Maximum Number of electrons (2n ^{2})1 K 1 2×1 ^{2}= 22 L 2 2×2 ^{2}= 83 M 3 2×3 ^{2}= 184 N 4 2×4 ^{2}= 32

- The each main shell (energy level) is subdivided into sub shells. These sub shell are called orbitals. These sub shells /orbitals are designated by s, p, d, f etc. with corresponding orbital quantum number, l = 0, 1, 2, 3, 4,…..(n-1) etc. The number of sub shells in any main shell is equal to “principal quantum number” ‘n’.

- The maximum electron capacity of sub shells is governed formula of 2(2l + 1). The capacity of sub shells are given in table below-
Sl. No. Subshell Quantum number

(l)Electron capacity of sub shell2(2l+1) 1 s 1 2(2 × 0+1) = 2 2 p 2 2(2 × 1+1) = 6 3 d 3 2(2 × 2+1) = 10 4 f 4 2(2 × 3+1) = 14

- Aufbau Principle

“Aufbau” is a German word whose meaning is “building up”. Therefore, “Aufbau Principle” is also called “building up principle”. According to this principle, the electrons occupy the orbitals in the order of increasing energy. The increasing energy order of various orbitals and order of occupation is as below –

1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p

To simplify the understanding of order of increasing energy, “Madelung’s rule” is used, which is given below-

Based on above rules, the **electron configuration of atom** of some elements is shown in table below-

Element | Chemical Symbol | Atomic No. | Distribution of electrons over orbits (shells), 2n^{2} | Electron configuration of atom | |||

K(n=1) | L(n=2) | M(n=3) | N(n=4) | ||||

Hydrogen | H | 1 | 1 | 0 | 0 | 0 | 1s^{1} |

Helium | He | 2 | 2 | 0 | 0 | 0 | 1s^{2} |

Lithium | Li | 3 | 2 | 1 | 0 | 0 | 1s^{2} 2s^{1} |

Beryllium | Be | 4 | 2 | 2 | 0 | 0 | 1s^{2} 2s^{2} |

Boron | B | 5 | 2 | 3 | 0 | 0 | 1s^{2} 2s^{2} 2p^{1} |

Carbon | C | 6 | 2 | 4 | 0 | 0 | 1s^{2} 2s^{2} 2p^{2} |

Nitrogen | N | 7 | 2 | 5 | 0 | 0 | 1s^{2} 2s^{2} 2p^{3} |

Oxygen | O | 8 | 2 | 6 | 0 | 0 | 1s^{2} 2s^{2} 2p^{4} |

Fluorine | F | 9 | 2 | 7 | 0 | 0 | 1s^{2} 2s^{2} 2p^{5} |

Neon | N | 10 | 2 | 8 | 0 | 0 | 1s^{2} 2s^{2} 2p^{6} |

Sodium | Na | 11 | 2 | 8 | 1 | 0 | 1s^{2} 2s^{2} 2p^{6} 3s^{1} |

Magnesium | Mg | 12 | 2 | 8 | 2 | 0 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} |

Aluminum | Al | 13 | 2 | 8 | 3 | 0 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{1} |

Silicon | Si | 14 | 2 | 8 | 4 | 0 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{2} |

Phosphorus | P | 15 | 2 | 8 | 5 | 0 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{3} |

Sulphur | S | 16 | 2 | 8 | 6 | 0 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{4} |

Chlorine | Cl | 17 | 2 | 8 | 7 | 0 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{5} |

Argon | Ar | 18 | 2 | 8 | 8 | 0 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} |

Potassium | K | 19 | 2 | 8 | 8 | 1 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1}Or [Ar]4s ^{1} |

Calcium | Ca | 20 | 2 | 8 | 8 | 2 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2}Or [Ar]4s ^{2} |

Scandium | Sc | 21 | 2 | 8 | 8 | 2 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{1} 4s^{2}Or [Ar]3d ^{1} 4s^{2} |

Where, [Ar] = 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} represents the electron configuration of Argon which is having completely filled 3-shells. It is used to simplify the electron configuration of elements of higher atomic number.