State and explain Dobereiner's "Triad" .

Explanation

Some elements such as chlorine, bromine and iodine with similar chemical properties into the group of three elements called as triads. In triads, the atomic weight of the middle elements nearly equal to the arithmetic mean of the atomic weights of the remaining two elements . Any one example
`{:("S.No.","Elements in the Triad","Atomic weight of middle element","Average atomic weight of the remaining elements"),(" 1","Li,Na,K"," 23"," "(7+30)/(2)=23),(" 2","Cl,Br,1"," 80"," "(35.5+127)/(2)=81.25),(" 3","Ca,Sr,Ba"," 88"," "(40+137)/(2)=88.5):}`