4.1.2.2 - Development of the periodic table

How were elements arranged before the discovery of protons, neutrons and electrons?

Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.

(What we refer to today as their relative atomic mass)

Describe the early periodic tables.

The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.

Who overcame these issues with the periodic table?

Dimitri Mendeleev in 1869

How did Mendeleev improve the periodic table?

• left gaps for elements he thought has not yet been discovered

• in some place he changed the order based on atomic weights

What did Mendeleev put the elements in order of?

In order of atomic weight

But he did switch the order depending on the properties

Why did Mendeleev leave gaps?

Ensured elements with similar properties remained in the same groups

These gaps also indicated the existence of undiscovered elements allowing Mendeleev to predict their properties.

What was discovered to explain why the order based on atomic weights was not always correct?

Isotopes