The Evolution of Mendeleev’s Periodic Table

Mendeleev’s periodic table was a powerful tool for the scientific classification and discovery of elements. However, his use of atomic mass rather than atomic number to order elements meant that it was never quite correct.

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It’s not surprising that Mendeleev made this mistake: subatomic particles, such as electrons and protons, had not yet been discovered when he designed his periodic table. Therefore, the concept of atomic numbers, which are determined by the number of protons in an atom’s nucleus, was not known at the time.

Inconsistencies in Mendeleev’s Table

We now know that relative atomic mass can vary quite drastically between elements, given the relative abundance of different isotopes. For Mendeleev’s periodic table, this led to some inconsistencies between relative atomic mass and chemical properties.

For example, let’s look at Mendeleev’s struggle to place Iodine (I) in the periodic table:

• Chemical Properties: Iodine shares similar chemical properties with chlorine and bromine, suggesting they belong in the same group (column).

• Relative Atomic Mass Confusion: Iodine’s lower relative atomic mass compared to tellurium (Te) suggested a different placement.

• Mendeleev’s Decision: Prioritising chemical properties over atomic mass, Mendeleev placed iodine in the same column as chlorine and bromine, despite its lower atomic mass.

The Discovery of Subatomic Particles

The discovery of subatomic particles, particularly the fact that all atoms of an element have the same number of protons, changed the face of chemistry forever. In 1914, the physicist Henry Moseley demonstrated that the properties of elements depend more on their atomic number than on their relative atomic mass. This led to a new rearrangement of the periodic table according to elements’ atomic numbers, thereby correcting the biggest error in Mendeleev’s table.

For example, the issue that Mendeleev encountered with placing iodine was simply resolved. Even though iodine has a lower atomic mass than tellurium, it has a greater atomic number – meaning it can now be placed in the correct group (column) on the periodic table and the sequence of atomic numbers remains in order.

This version of the periodic table is still the one we still use today. As new elements have been discovered since then, they have been added to this table. The periodic table is now one of the most fundamental tools in chemistry.