what is a traids?

According to Dobernier's law of triads when elements are arranged in the order of increasing atomic masses, groups of three elements( known as triads) having similar chemical properties are obtained. The atomic mass of middle element of the triad being equal to the arithmetic mean of the atomic masses of the other two elements.

For e.g. Lithium, sodium and potassium forms a triad in which the arithmetic mean of the two elements lithium(7) and potassium(39) comes out to be equal to that of sodium (7+39)/2=23