The **electron configuration** of an element describes how electrons are distributed in their atomic orbitals. Electron configurations of atoms follow a standard notation in which all electron-containing atomic subshells are placed in a sequence. For example, the electron configuration of sodium is **1s ^{2}2s^{2}2p^{6}3s^{1}**.

Electron Configurations are useful for:

- Determining the valency of an element.
- Predicting the properties of a group of elements.
- Interpreting atomic spectra.

This notation for the distribution of electrons in the atomic orbitals of atoms came into practice shortly after the Bohr model of the atom was presented by Ernest Rutherford and Niels Bohr in the year 1913.

### Writing Electron Configurations

### Shells

The maximum number of electrons that can be accommodated in a shell is based on the principal quantum number (n). It is represented by the formula 2n^{2}, where ‘n’ is the shell number. The shells, values of n, and the total number of electrons that can be accommodated are tabulated below.

Shell and ‘n’ value |
Max. Electrons in the Electron Configuration |

K shell, n=1 | 2*1^{2} = 2 |

L shell, n=2 | 2*2^{2} = 8 |

M shell, n=3 | 2*3^{2} = 18 |

N shell, n=4 | 2*4^{2} = 32 |

### Subshells

- The subshells into which electrons are distributed are based on the azimuthal quantum number It is denoted by ‘l’.
- This quantum number is dependent on the value of the principal quantum number, n. Therefore, when n has a value of 4, four different subshells are possible.
- When n=4. The subshells correspond to l=0, l=1, l=2, and l=3 and are named the s, p, d, and f subshells, respectively.
- The maximum number of electrons that can be accommodated by a subshell is given by the formula 2*(2l + 1).
- Therefore, the s, p, d, and f subshells can accommodate a maximum of 2, 6, 10, and 14 electrons, respectively.

All the possible subshells for values of n up to 4 are tabulated below.

Principle Quantum Number Value |
Value of Azimuthal Quantum Number |
Resulting Subshell in the Electron Configuration |

n=1 | l=0 | 1s |

n=2 | l=0 | 2s |

l=1 | 2p | |

n=3 | l=0 | 3s |

l=1 | 3p | |

l=2 | 3d | |

n=4 | l=0 | 4s |

l=1 | 4p | |

l=2 | 4d | |

l=3 | 4f |

Thus, it can be understood that the 1p, 2d, and 3f orbitals do not exist because the value of the azimuthal quantum number is always less than that of the principal quantum number.

### Notation

- The electron configuration of an atom is written with the help of subshell labels.
- These labels contain the shell number (given by the principal quantum number), the subshell name (given by the azimuthal quantum number), and the total number of electrons in the subshell in superscript.
- For example, if two electrons are filled in the ‘s’ subshell of the first shell, the resulting notation is ‘1s
^{2}’. - With the help of these subshell labels, the electron configuration of magnesium (atomic number 12) can be written as 1s
^{2}2s^{2}2p^{6}3s^{2}.