Electrons in valence shell (valence electrons) are responsible for the formation of bonds between atoms.

In previous lessons you studied about exothermic and endothermic reactions. You also studied about reactivity of elements in periodic table.

Some elements are more reactive and some are less reactive;

  • Why there is absorption of energy in certain chemical reactions andrelease of energy in other reactions?

  • Where the absorbed energy goes?
  • Is there any relation to energy and bond formation between atoms?

  • What could be the reason for the change in reactivity of elements?

    • Lewis symbols (or) Lewis dot structures:

    Periodic Classification and arrangement of elements in the periodic table according to the electron configuration gave a new thought aboutchemical bond.

    The discovery of noble gases and the understanding of their electronic configurations helped us in explaining the formation of chemical bonds among the atoms of the elements. Noble gases which belong to zero group (18 group or VIII A) are typical gases with almost negligible chemical activity when compared to other elements. They undergo few or no chemical changes. They are more stable and do not form molecules by allowing their atoms to combine among themselves or with the atoms of other elements.

  • What could be the reason for this?

    • Let us investigate.

    Consult the periodic table given in the previous chapter and observethe following table:

    Look at the second column and third column. It is clear that all thenoble gases have eight electrons in the outermost shell, except Helium (He).


    page no:152

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