In the early 1800s, periodic tables were not complete and some elements were placed in the wrong group. This is because elements were placed in the order of atomic mass and not by its properties.
In 1869, Dmitri Mendeleev overcame some of these problems by taking 50 elements and arranging them into his “table of elements” with quite a few gaps.
Mendeleev put these elements mainly in order of atomic mass. However, he switched them depending on their properties.
Gaps were left in the table so that elements with similar properties stayed in the same groups. Some of these gaps indicated the existence of undiscovered elements. This allowed Mendeleev to predict what their properties might be. When the elements were found and fitted well, this helped a lot with his table of elements, especially with its chemical and physical properties.
The discovery of isotopes in the early 20th century meant that Mendeleev was right not to place elements in order of atomic mass but also by their properties. Isotopes (of an element) are atoms that have the same number of protons but different number of neutrons. In other words, isotopes (of an element) have the same properties but different atomic masses.
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