## PERIODICITY OF ELEMENTS

**Definition of Periodicity of Elements or Chemical Periodicity**

**In chemistry periodicity of elements means the recurrence of similar properties of the elements after certain regular intervals when they are arranged in the order of increasing atomic numbers. **

**Cause of Periodicity of Elements**

The **cause of periodicity** of the properties of elements is the **repetition of similar electronic configuration** of their atoms in the outermost energy shell (or **valence shell**) after certain regular intervals. To **explain the cause of periodicity of elements** we are taking two examples.

**Electronic configuration of Alkali Metals: **

The chemical properties of all alkali metals resemble each other. This is due to the fact that chemical properties of elements depend primarily on the arrangement of electrons in the outermost shell (**valence shell**). The **electronic configuration of the atoms of alkali metal** group shows that all the alkali metals have one electron in their outermost s-orbital. So, their electronic configuration may be represented as (noble gas) ns^{1}.

**Table of electronic configurations of Alkali Metals**

Element |
Atomic No. |
Electronic configuration |

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

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

K | 19 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{ 6} 4s^{1} |

Rb | 37 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10}4s^{2}4p^{6}5s^{1} |

Cs | 55 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6}3d^{10}4s^{2}4p^{6}4d^{10}5s^{2}5p^{6}6s^{1} |

Fr | 87 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10}4s^{2}4p^{6}4d^{10}5s^{2}5p^{6}6s^{2}4f^{14}5d^{10}6p^{6}7s^{1} |

**Electronic configuration of Halogens: **

Similarly, all the members of the halogen family have 7 electrons in the valence shell i.e. two electrons in the s-orbital and five electrons in the p-orbital of the valence shell. The general **electronic configuration of halogens** may be expressed as ns^{2} np^{5}. Thus, all the halogens have similar chemical properties due to same outer-shell electronic configuration.

**Table of Electronic Configurations of Halogens**

Element |
Atomic No. |
Electronic configuration |

F | 9 | 1s^{2} 2s^{2} 2p^{5} |

Cl | 17 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{5} |

Br | 35 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10}4s^{2}4p^{5} |

I | 53 | 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10}4s^{2}4p^{6}4d^{10}5s^{2}5p^{5} |

**Test Your Understanding and Answer These Questions:**

- What is Periodicity of elements?
- What is the cause of periodicity of elements in modern periodic table?

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