Answer:
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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