Modern Periodic Table


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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) ns1.

Table of electronic configurations of Alkali Metals

Element Atomic No. Electronic configuration
Li 3 1s2 2s1
Na 11 1s2 2s2 2p6 3s1
K 19 1s2 2s2 2p6 3s2 3p 6 4s1
Rb 37 1s2 2s2 2p6 3s2 3p6 3d104s24p65s1
Cs 55 1s2 2s2 2p6 3s2 3p63d104s24p64d105s25p66s1
Fr 87 1s2 2s2 2p6 3s2 3p6 3d104s24p64d105s25p66s24f145d106p67s1

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 ns2 np5. 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 1s2 2s2 2p5
Cl 17 1s2 2s2 2p6 3s2 3p5
Br 35 1s2 2s2 2p6 3s2 3p6 3d104s24p5
I 53 1s2 2s2 2p6 3s2 3p6 3d104s24p64d105s25p5

Test Your Understanding and Answer These Questions:

  1. What is Periodicity of elements?

  2. What is the cause of periodicity of elements in modern periodic table?

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