i. When elements are arranged in increasing order of their atomic number, elements repeat its property after certain elements, this repetition of the property is called periodicity of properties.
ii. Elements of the same group have the same properties because of:
1. A same number of valence electrons in the outermost shell.
2. Same valency
Examples: 1) In group 1 of alkali metals they all (Li, Na, K, Rb, Cs, Fr) have similar property due to the same number of valence electrons (equal to 1) and valency (equal to 1).
2) In group 17 of alkali metals, they all (F, Cl, Br, I ) have similar property due to the same number of valence electrons (equal to 7) and valency (equal to 1).
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