(a) Reccurence of similar properties of elements after certain fixed interval of atomic number.
(b) They have similar properties because:
(i) They all have same number of valence electrons.
(ii) They all have same valency.
© The tendency to gain electrons increases due to increase in the nuclear charge. Due to this, atomic size decreases and a large number of valence electrons are held towards the nucleus and tendency to gain electrons increases.