# Lesson 2.3 - Developing The Periodic Table

## Initial beliefs

Before subatomic particles were discovered, scientists ordered the elements by their **atomic weight**.

These early periodic tables were incomplete and many elements were in the wrong place.

This was because of the atomic weight order.

## Mendeleev's periodic table

Mendeleev left gaps in his periodic table for undiscovered elements.

However, he also used the atomic weight of elements to order them, meaning his periodic tables were still not very accurate.

## Isotopes

As more elements were discovered, they were placed in the gaps of Mendeleev's periodic table.

Eventually, **isotopes** were discovered, which explained why grouping by atomic weight was not always correct.
