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Atomic Weights

Updated on December 2, 2016

From the middle of the 19th century, scientists were hard at work on experiments designed to reveal the composition, formulas and chemical properties of their new-found compounds. They tried, like stamp collectors, to arrange the elements in a kind of album. In the same way as stamp collectors might arrange their stamps according to value or nationality, so chemists grouped the known elements according to certain common chemical properties.

For example, the elements potassium, sodium, calcium and magnesium, as a group, behaved in much the same way; while bromine, chlorine and iodine belonged to another group which again had many properties common to each other.

As a kind of 'value' chemists used the weights of the atoms of each element as another method of arrangement. Strictly speaking this 'atomic weight' was not a measure of actual weight, but a number representing how much heavier the atoms of one element were than those of another.

Since hydrogen was the lightest known element this was used as a standard, and by careful experiments other elements were compared with it. Oxygen turned out to have an atomic weight of 16, while lead was found to be 82 times as heavy as hydrogen. A very large number of these atomic weights appeared to be whole numbers.

This led scientists to think that hydrogen might be the basic element from which all others could be built.

But several of the elements refused to fit into this neat pattern, even when oxygen was taken as the standard unit, and the idea of a hydrogen atom building block had to be abandoned.

Nevertheless, using this atomic weight as a value, an English chemist, John Newlands (1837-98), arranged the elements in ascending order of atomic weight and discovered a curious, regular pattern. With one or two exceptions, every eighth element in his list was one of a group which had common properties. The exceptions to this pattern left his table incomplete, but the 'octave law' (law of eight) did encourage other scientists to search for a more complete arrangement.

Quite independently of each other a German chemist, Lothar von Meyer (1830-95), and a Russian, Dmitri Mendeleyev (1834-1907), produced a better collection of the elements which not only took into account the octave law, but purposely introduced gaps into the list. It was suggested that the gaps represented elements as yet undiscovered, which would eventually complete the collection. This album, or Periodic Table as it came to be called, had later to be changed in the light of new knowledge, but it enabled a large number of the apparently unrelated facts of chemistry to be united. Once the position of an element in the Periodic Table had been established by means of its atomic weight, its chemical properties could be predicted with reasonable certainty. This was a very great help not only to the research chemist, but also to the coming generations of students of chemistry who now had a framework on which to build their knowledge without having to memorize many unrelated facts.

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