Chemistry Module 11, Atoms in the Periodic Table
A. History of the Periodic Table
The early years of the 19th century witnessed a rapid development in chemistry. The
art of distinguishing similarities and differences among atoms prompted scientists to devise
a way of arranging the elements. Relationships were discerned more readily among the
compounds than among the elements; thus, the classification of elements lagged many
years behind the classification of compounds. In fact, no general agreement had been
reached among chemists as to the classification of elements for nearly half a century after
the systems of classification of compounds had been established.
It was in 1817 when Johann Wolfgang Döbereiner showed that the atomic weight
of strontium lies midway between those of calcium and barium. Some years later he showed
that other such “triads” exist (chlorine, bromine, and iodine [halogens] and lithium, sodium,
and potassium [alkali metals]). He also showed that similar relationships extended further
than the triads of elements, fluorine being added to the halogens, and magnesium to the
Key to answers on page 27. - 6 -alkaline-earth metals. Oxygen, sulfur, selenium, and tellurium were classed as one family, and nitrogen, phosphorus, arsenic, antimony, and bismuth as another family of elements.
Chemistry Module 11, Atoms in the Periodic TableThe early years of the 19th century witnessed a rapid development in chemistry. The
art of distinguishing similarities and differences among atoms prompted scientists to devise
a way of arranging the elements. Relationships were discerned more readily among the
compounds than among the elements; thus, the classification of elements lagged many
years behind the classification of compounds. In fact, no general agreement had been
reached among chemists as to the classification of elements for nearly half a century after
the systems of classification of compounds had been established.
It was in 1817 when Johann Wolfgang Döbereiner showed that the atomic weight
of strontium lies midway between those of calcium and barium. Some years later he showed
that other such “triads” exist (chlorine, bromine, and iodine [halogens] and lithium, sodium,
and potassium [alkali metals]). He also showed that similar relationships extended further
than the triads of elements, fluorine being added to the halogens, and magnesium to the
Key to answers on page 27. - 6 -alkaline-earth metals. Oxygen, sulfur, selenium, and tellurium were classed as one family, and nitrogen, phosphorus, arsenic, antimony, and bismuth as another family of elements.