Question

What is the cause of periodicity in properties of the elements? Explain with two examples.

Answer 1

The cause of periodicity in properties is the repetition of similar outer electronic configuration after certain regular intervals.
For example, all the elements of group LA i.e., alkali metals, have similar outer electronic configuration as ns1.
Where n refer to the number of outermost principal shell.
In a similar manner all the halogens i.e., elements of group VILA have similar other electronic configuration i.e., ns2 ns5 and hence possess similar properties.

Answer 2

their atomic numbr and electronic configuration