Early classifications and Mendeleev's table

More than 100 elements are known, each with its own properties. Studying them one by one would be endless. But many elements behave alike, so grouping similar ones together makes their chemistry much easier to study and remember. Scientists tried this for decades:

• Döbereiner (1829) found groups of three similar elements, called triads, in which the middle element's atomic weight is about halfway between the other two. In lithium, sodium and potassium, sodium's 23 is about halfway between 7 and 39. But only a few triads could be found.
• Newlands (1865) arranged elements by increasing atomic weight and noticed that every eighth element was like the first, as in music, where after sa re ga ma pa dha ni the eighth note is sa again. He called it the law of octaves. But it worked only up to calcium.
• Mendeleev and Lothar Meyer (1869), working separately, found that properties repeat regularly: the properties of elements are a periodic function of their atomic weights.

Mendeleev's table was the most successful, because he was willing to break his own order when the chemistry demanded it:

• He put elements with similar properties together, even if that meant placing a slightly heavier element before a lighter one.
• He left gaps for elements that had not yet been discovered.

Better still, he predicted the properties of the missing elements. He called two of them eka-aluminium and eka-silicon ('eka' is Sanskrit for 'one': one place below aluminium or silicon). When gallium and germanium were discovered, their properties matched his predictions very closely. That convinced chemists his table was right.