What was john dobereiner’s contribution to the development of the periodic table

John Dobereiner proposed the idea of similarity in the properties of elements which can be placed together.

He proposed the idea of classification so that we can study the properties of a group of elements together

According to him, three elements can be grouped based on their similarities in properties. This group of three elements was termed as "Triads"

For example :

Li, Na, K

Now the significance of the triad was that the atomic weight of middle element is average of atomic weights of the side atoms

So atomic weight of Na = 23 is average of atomic weights of Lithium and Potassium

7 + 39/2 = 46/2= 23

The other triads were

Ca Sr Ba

Cl Br I

S Se Te

explanation:

metals have excess of electrons whereas non-metals have deficiency of electrons. thus, in order to gain stability metals easily donate their electrons to the non-metals.

for example, electronic configuration of sodium is and electronic configuration of phosphorous is .

when a phosphorous atom and sodium atom combine together then sodium will donate is extra one electron to the phosphorous atom. as a result there will be formation of ionic bond between sodium and phosphorous.

the reaction will be as follows.

there, are three sodium sodium atoms and each of them will donate its one extra electron to phosphorous atom. thus, the sub-shell of phosphorous gets completely filled.  

thus, we can conclude that electrons are transferred from atoms of sodium to atoms of phosphorous. this transfer makes the sodium atoms donate one electron and the phosphorous atoms to gain one electron. as a result, the sodium and phosphorus atoms strongly attract each other.