Pioneering Periodic Arrangements of the ElementsLaws of Triads
and of Octaves Early in the 19th cent., a number of chemists had
noticed certain relationships between the properties of elements
and their atomic weight. In 1829 J. W. Döbereiner stated that
there existed some three-element groups, or triads, in which the
atomic weight of the middle element was the average of the other
two and the properties of this element lay between those of the
other two. For example, calcium, strontium, and barium form a
triad; lithium, sodium, and potassium, another. The English
chemist J. A. Newlands found (1863–65) that if the elements are
listed according to atomic weight starting with the second, the
8th element following any given element has similar chemical
properties, and so does the 16th. This became known as the law
of octaves. About the same time, A. E. de Chancourtois arranged
the elements according to increasing atomic weight in the form
of a vertical helix with eight elements in a turn, so that
elements having