What is meant by periodicity in properties of elements with reference to the periodic table?

(i) When the elements having similar chemical properties are repeated at a regular interval, this is known as periodicity of elements. In modern periodic table (which is based on atomic number), we see that the elements having similar chemical properties are repeated at a certain interval of atomic number. 

(ii) This is because the elements of same group have same number of valence electrons or they have similar electronic configuration. For example, all alkali metals (like sodium, potassium, etc.) have 1 valence electron. 

(iii) The tendency to gain electrons across a period from left to rightincreases. This is because the effective nuclear charge increases and consequently it becomes easier for the atoms to gain electrons.

The repetition of same properties after definite interval is called periodicity in properties.

(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.