(a) Repetition of properties after a certain interval is called periodicity of properties. If elements are arranged in increasing order of their atomic number in the periodic table, then elements repeat its properties after a definite interval.This repetition of properties is known as periodicity of properties.
(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.
