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

(b) Why do all the tendency to gain electrons change as we go from left to right across a period? Why?

(a) it basically means repetition in the chemical properties of the elements at regular intervals when arranged according to increasing atomic number.

(b) You will see that the atomic radius decreases in moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

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