(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.
(c) 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.