Development of the periodic table

What you need to know

• Early attempts arranged elements in order of atomic weight because subatomic particles had not yet been discovered.
• Early periodic tables were incomplete, and strict atomic-weight order sometimes put elements in inappropriate groups.
• Mendeleev left gaps for elements he thought were undiscovered and changed the order in some places.
• Later discoveries filled Mendeleev's predicted gaps, while knowledge of isotopes explained problems with atomic-weight ordering.

The big picture

Before subatomic particles were discovered, scientists tried to classify elements by arranging them in order of atomic weight. Early tables were incomplete, and strict atomic-weight order sometimes placed elements in inappropriate groups. Mendeleev improved the arrangement by leaving gaps for undiscovered elements and changing the order in some places. Later discoveries filled his predicted gaps, and knowledge of isotopes explained why atomic-weight order was not always correct.

How the periodic table developed

Follow the changes from early atomic-weight ordering to the explanation provided by isotopes.

From problem to improved table

Trace how each development responded to what scientists knew at the time.

1. 1

   Start with atomic weights

   Before subatomic particles were discovered, atomic weight provided a way to put elements into an order.

2. 2

   Notice the problems

   Strict ordering did not always work well because early tables were incomplete and some elements fell into inappropriate groups.

3. 3

   Leave room for discoveries

   Mendeleev left gaps because he thought some elements had not yet been discovered.

4. 4

   Adjust the order

   He changed the order in some places rather than keeping a strict atomic-weight sequence when that caused problems.

5. 5

   Gaps are filled

   Elements discovered later occupied the gaps Mendeleev had predicted.

6. 6

   Isotopes explain the mismatch

   Knowledge of isotopes later explained why atomic-weight order was not always correct.

Key ideas in the development

Keep each term tied to its role in the story of the table.

Predict, then check

Think about what Mendeleev should do when strict atomic-weight order gives an unsuitable grouping.

If strict atomic-weight order places an element in an inappropriate group, which response matches Mendeleev's approach?

Memory hook

Weight → problems → gaps → discoveries → isotopes: follow the fixes that improved the table.

Check yourself

Why did Mendeleev sometimes depart from strict atomic-weight order?