There are numerous physical properties of elements such as melting points, boiling points, heats of fusion and vaporisation, energy of atomisation, etc. which show periodic variations.
The cause of periodicity in properties is the repetition of similar outer electronic configurations after certain regular intervals. e.g., all the elements of 1st group (alkali metals) have similar outer electronic configuration, i.e., ns1.
Therefore, due to similar outermost shell electronic configuration all alkali metals have similar properties. e.g., sodium and potassium both are soft and reactive metals. They all form basic oxides and their basic character increases down the group. They all form unipositive ion by the lose of one electron.
Similarly, all the elements of 17th group (halogens) have similar outermost shell electronic configuration, i.e., ns2np5 and thus possess similar properties.
