Question

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

Answer 1

Today, the most acceptable periodic table which is accepted is called the Mendeleev’s Periodic Law in 1869. When he started studying the properties of elements, at that time, only 63 elements were known.
Then he took each element and its oxides and hydrides and studied their properties. He arranged all the elements with similar properties below each other in a vertical line. When he arranged the elements, he observed that elements got arranged in order of their increasing atomic weights and the physical and chemical properties were repeating themselves after a period of time.
However, that changed later when Moseley studied it and it got changed from atomic weights to atomic number.

Complete step by step answer:
The Law states that when the elements are arranged in order of their increasing atomic number, then properties which are similar to different elements are repeated after certain intervals. This Is called the periodicity of the elements.
The elements in the same group also have the same properties because they have the same number of electrons in their outermost shell.
If we take the group 1 which belongs to the alkali metals includes, Li, Na, K, Rb, Cs, Fr then they all have 1 electron in their outermost shell that too in s-orbital.
Then, the physical and chemical properties of the elements which belong to this group have the same physical and chemical properties.

Note: The tendency to gain electrons alongside a period which is from left to right increases because the atomic radius decreases and effective nuclear charge also decreases.