GCSE · Chemistry · AQA · Spec 8462

Development of the periodic table

How the periodic table developed

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

Early tables and Mendeleev's approach

Early periodic tablesvsMendeleev's approach

Focus on how Mendeleev responded to the weaknesses of strict atomic-weight ordering.

Focus

Atomic-weight order

Early periodic tables

Elements were arranged in order of atomic weight.

Mendeleev's approach

Atomic weight was used, but the order was changed in some places.

The insight

Mendeleev did not force every element to remain in strict atomic-weight order when that produced problems.

Incomplete table

Early periodic tables

The tables were incomplete.

Mendeleev's approach

Gaps were deliberately left for elements thought to be undiscovered.

Grouping problem

Early periodic tables

Some elements were placed in inappropriate groups when strict atomic-weight order was followed.

Mendeleev's approach

Changing the order in some places helped overcome this problem.

What happened later

Early periodic tables

The early problems remained unexplained at the time.

Mendeleev's approach

Later discoveries filled predicted gaps, and isotope knowledge explained why atomic-weight order was not always correct.

From problem to improved table

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

  1. Start with atomic weightsBefore subatomic particles were discovered, atomic weight provided a way to put elements into an order.
  2. Notice the problemsStrict ordering did not always work well because early tables were incomplete and some elements fell into inappropriate groups.
  3. Leave room for discoveriesMendeleev left gaps because he thought some elements had not yet been discovered.
  4. Adjust the orderHe changed the order in some places rather than keeping a strict atomic-weight sequence when that caused problems.
  5. Gaps are filledElements discovered later occupied the gaps Mendeleev had predicted.
  6. Isotopes explain the mismatchKnowledge 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?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

See how problems in early arrangements led to a better way of organising the elements.

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