11th Class Chemistry Classification of Elements and Periodicity in Properties

  • question_answer 101)   Justify the given statement with suitable examples?"the properties of the elements are a periodic function of their atomic numbers."


      The real meaning of the word periodic in any classification of elements is that when the elements are arranged in order of their increasing atomic numbers in the same period or a group, there is gradual change (i.e., increase or decrease) in a particular property. In a period, the gradual change is due to the gradual change in atomic number or electronic configuration from member to member. In a group, the chemical properties of the elements remain nearly the same due to same valence shell configuration which occurs after the difference of 2 or 8 or 18 or 32 in the atomic number. There is a gradual change in the physical properties due to change in the size of atoms. Examples: (i) Metallic nature decreases gradually in a period. (ii) Electronegativity increases gradually in a period. (iii) Atomic radii decrease gradually in a period. (iv) The valency increases from 1 to 7 in the case of representative elements in a period if oxygen is taken as a standard. 3rd period:   Na Mg    Al   Si   P   S   Cl Valency:        1     2       3     4   5   6     7 (v) In first group, the ionisation enthalpy decreases from top to bottom. (vi) In carbon group, the metallic nature increases from top to bottom. (vii) In halogen group, the electronegativity decreases from top to bottom.

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