According to the Newlands’ law of octaves, when elements are arranged in the order of their increasing atomic masses, the properties of the eighth element (starting from a given element) occur in a repetition of the properties of the first element. Thus, the 1st and the 8th element share similar properties.
For example: Lithium is the first element having one valence electron and sodium which is the 8th element from lithium show similar properties as lithium.