In the early 1800s elements were arranged by atomic mass

• Until quite recently, there were two obvious ways to categorise elements:

  • Their physical and chemical properties

  • Their relative atomic mass

• Remember, scientists had no idea of atomic structure or of protons, neutrons or electrons, so there were no such thing as atomic number to them.(It was only in the 20th century after protons and electrons were discovered that it was realised the elements were best arranged in order of atomic number)

• Back then, the only thing they could measure was relative atomic mass, and so the known elements were arranged in order of atomic mass. When this was done, a periodic pattern was noticed in the properties of the elements. This is where the name ‘periodic table’ comes from

• Early periodic tables were not complete and some elements were placed in the wrong group. This is because elements were placed in the order of relative atomic mass and did not take into account their properties.

Dmitri Mendeleev left gaps and predicted new elements

• In 1869, Dmitri Mendeleev overcame some of the problems of early periodic tables by taking 50 known elements and arranging them into his table of elements - with various gaps as shown

• Mendeleev put the elements mainly in order of atomic mass but did switch that order if the properties meant it should be changed. An example of this can be seen with Te and I - iodine actually has a smaller relative atomic mass but is placed after tellurium as it has similar properties to the elements in that group.

• Gaps were left in the table to make sure that elements with similar properties stayed in the same groups. Some of these gaps indicated the existence of undiscovered elements and allowed Mendeleev to predict what their properties might be. When they were found and they fitted the pattern it helped confirm Mendeleev’s ideas.

  • For example, Mendeleev made really good predictions about the chemical and physical properties of an element he called ekasilicon which we know today as germanium

The discovery of isotopes in the early 20th century confirmed that Mendeleev was correct to not place elements in a strict order of atomic mass but to also take account of their properties. Isotopes of the same elements have different atomic masses but have the same chemical properties so occupy the same position on the periodic table.