By the early nineteenth century chemists had discovered many elements, and each behaved differently. To make sense of this growing list, scientists looked for a pattern that would let them group similar elements together. The story of those early attempts is the story of how the periodic table was born.
Dobereiner's triads (1817)
The German chemist Johann Dobereiner arranged elements with similar chemical properties into groups of three, called triads. His key observation was that the atomic mass of the middle element was very close to the average of the other two. For the triad lithium, sodium and potassium the masses are 7, 23 and 39; the average of 7 and 39 is 23, which is exactly the mass of sodium. Calcium, strontium and barium form another triad. The idea was clever, but Dobereiner could find only three or four triads among all the known elements, so it failed as a general system.
Newlands' law of octaves (1866)
The English scientist John Newlands arranged the known elements in increasing order of atomic mass and noticed that every eighth element had properties similar to the first, like the eighth note in a musical octave. He called this the Law of Octaves. It worked well for the lighter elements up to calcium but broke down afterwards. Newlands assumed no new elements would be found, and to make things fit he placed two elements in the same slot and even grouped unlike elements together (such as cobalt and nickel with halogens). His law was not accepted at the time.
Mendeleev's periodic table (1869)
The Russian chemist Dmitri Mendeleev made the real breakthrough. He arranged 63 known elements in order of increasing atomic mass and grouped them by similar chemical properties, especially the formulae of their oxides and hydrides. This gave his Periodic Law: the properties of elements are a periodic function of their atomic masses.
- Gaps and predictions: Mendeleev boldly left empty boxes for elements not yet discovered and predicted their properties, naming them eka-boron, eka-aluminium and eka-silicon. When scandium, gallium and germanium were later found, their properties matched almost exactly — a stunning success.
- Noble gases: when helium, neon and argon were discovered later, they fitted neatly into a new group without disturbing the table.
Limitations of Mendeleev's table
- No fixed position could be given to hydrogen — it resembles both the alkali metals (Group I) and the halogens (Group VII).
- Isotopes of the same element have different atomic masses, yet they had to share one position.
- In a few pairs the order of increasing mass was reversed to keep similar elements together (for example argon before potassium, and cobalt before nickel).