The Begininig of the Periodic Table

The law of triads was proposed by Johann Dobereiner, a German chemist, in 1829.

He arranged elements into four to five groups. Each group consisted of three elements resembling in properties. Since elements were classified into sets of three, we call the law as the law of triads. The first triad was recognized in 1817. It consisted of calcium, strontium, and barium. Other triads were found in the following years.

Dobereiner's triads Triad Elements Symbol Atomic weight Average Position in the periodic table Triad 1 Lithium Li 6.9 - Group 1 Sodium Na 23.0 23.0 Potassium K 39.1 - Triad 2 Calcium Ca 40.1 - Group 2 Strontium Sr 87.6 88.7 Barium Ba 137.3 - Triad 3 Chlorine Cl 35.5 - Group 17 Bromine Br 79.9 81.2 Iodine I 126.9 - Triad 4 Sulfur S 32.1 - Group 16 Selenium Se 79.0 79.9 Tellurium Te 127.6 -

From Above, the atomic weight of the second element in a triad approximately equals the average of the atomic weight of other two. For example, the atomic weight of lithium and potassium is 6.9 and 39.1. The average of it gives 23.0, which is the atomic weight of sodium.

Mendeleev arranged the elements in order of increasing relative atomic mass. When he did this he noted that the chemical properties of the elements and their compounds showed a periodic trend.

He then arranged the elements by putting those with similar properties below each other into groups. To make his classification work Mendeleev made a few changes to his order:

he left gaps for yet to be discovered elements he switched the order of a few elements to keep the groups consistent

Mendeleev left gaps in his table to place elements not known at the time. By looking at the chemical properties and physical properties of the elements next to a gap, he could also predict the properties of these undiscovered elements.

For example, Mendeleev predicted the existence of 'eka-silicon', which would fit into a gap next to silicon. The element germanium was discovered later. Its properties were found to be similar to the predicted ones and confirmed Mendeleev's periodic table.