What is Newlands' law of octaves? Explain with an example.

According to the Newlands' law of octaves, when elements are arranged in the order of increasing atomic masses, the properties of the eighth element (starting from a given element) are a repetition of the properties of the first element.

For example: Lithium is the first element having valence electron 1 and we find the eighth element from it is sodium having similar properties of lithium.