|Some of the first elemental similarities were noted by a German chemist
named Döbereiner in 1829. His observations began with bromine which had just been
discovered. He noticed that the properties of bromine were similar to chlorine and iodine.
Not only were they similar but various properties of bromine, including the atomic weight,
fell midway between the properties of chlorine and iodine. Not only were there
similarities in the properties but also there was a pattern or trend within the group of
regularly increasing atomic weights.
|He noticed a couple other groups of elements with patterns
like this. They were calcium, barium, and strontium; also sulfur, selenium, and tellurium.
He described these groups as being triads, groups of three elements that had similar
properties. Not much was made of this because the triads only covered one-sixth of the
known elements. Most chemists of the time considered them to be inconsequential