Atoms and elements

Atoms, elements and compoundsMixturesDevelopment of the model of the atomStructure of the atomIsotopes and relative atomic massElectronic configurationThe modern periodic tableDevelopment of the periodic tableMetals and non-metalsGroup 1Group 7Group 0Transition metals

In the early days of chemistry, before the discovery of protons, neutrons, and electrons, scientists attempted to classify elements by arranging them in order of their atomic weights. This method was not perfect, and the early periodic tables were often incomplete and sometimes placed elements in the wrong groups if the strict order of atomic weights was followed.

Dmitri Mendeleev's contribution

In 1869, Dmitri Mendeleev solved some of the problems with these early tables by:

Leaving gaps for elements that had not yet been discovered. Mendeleev predicted the existence of these elements based on the patterns he observed in the known elements.
In some cases, Mendeleev rearranged elements slightly out of the order of atomic weights. He did this to ensure elements with similar properties were placed in the same groups, even if their atomic weights did not strictly follow the pattern.

Discovery of predicted elements

Mendeleev’s predictions were later confirmed when new elements were discovered that fit the properties he had predicted.

The discovery of isotopes in the 20th century explained why some elements did not follow the strict order of atomic weights. Isotopes have different atomic weights but the same chemical properties, allowing for a better understanding of why some elements had previously been placed incorrectly.

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