AQA Chemistry 5.1.2.2
Development of the periodic table
Before anyone knew about protons, chemists ordered the elements by weight, and it didn't quite work. Make Mendeleev's moves yourself, then see what finally explained them.
Combined Science · Higher tier
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A sticker album with one missing.
You are filling a sticker album. One sticker in a set hasn't come out yet. You can either leave its space empty, or shuffle the other stickers along to fill the gap.
If you had to guess, does leaving the space make the pattern of the album more believable, or less?
Make Mendeleev's moves
Order by weight. Hit a clash. Decide.
Tellurium and iodine
By atomic weight, iodine comes before tellurium. That would put iodine in the group with sulfur and selenium, and tellurium with chlorine and bromine. But iodine reacts like chlorine and bromine.
The steps, in order
From atomic weights to atomic numbers.
STEP 1
Ordered by atomic weight
Before protons, neutrons and electrons were discovered, early tables put the elements in order of atomic weight. They were incomplete, and some elements were in the wrong groups.
STEP 2
Newlands' octaves
Every eighth element seemed similar. But the pattern only worked for the first elements, he forced elements into groups where they didn't fit, and he left no gaps. His idea was rejected.
STEP 3
Mendeleev's gaps and swaps
He also used atomic weight, but changed the order of some pairs, such as tellurium and iodine, to keep similar elements together, and left gaps for elements he thought had not been discovered.
STEP 4
Predictions tested
Elements were later discovered to fill the gaps, such as germanium (his eka-silicon), and their properties closely matched his predictions. Passing that test is why his table was accepted.
STEP 5
Isotopes explain the swaps
Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct. Today's table is in order of atomic number.
Why weight misleads · isotopes
Fewer protons, but heavier on average.
Atoms of the same element can have different numbers of neutrons: these are isotopes. Relative atomic mass is an average that takes account of how abundant each isotope is.
Tellurium has 52 protons and iodine 53, but much of natural tellurium is made of its heavier isotopes, so its relative atomic mass (127.6) is higher than iodine's (126.9). Ordered by atomic number, tellurium comes first, just where Mendeleev put it. Argon, found later, and potassium show the same thing.
Think again · what Mendeleev used
“Mendeleev put the elements in order of atomic number.”
Your call
ChooseIn 1869, what could Mendeleev use to put the elements in order?
Sort
Newlands, Mendeleev or the modern table?
Place each statement.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- The steps in order: weights, gaps and swaps, predictions found, isotopes.
- Explain
- Why the table was accepted: predictions made, tested, and found correct.
- Suggest
- Use protons and relative atomic mass to explain a pair that looks out of order.
Key fact
Mendeleev ordered elements by atomic weight but left gaps and swapped some pairs. Elements found later matched his predictions, so his table was accepted. Isotopes explain why atomic-weight order was not always correct.
Key note · AQA 5.1.2.2
Development of the periodic table
- Before protons were discovered, elements were ordered by atomic weight; early tables were incomplete.
- Newlands’ octaves: worked only for the first elements, forced fits, no gaps; rejected.
- Mendeleev: swapped some pairs (tellurium and iodine) to keep similar elements together.
- He left gaps for undiscovered elements and predicted their properties.
- Germanium (eka-silicon) was found with matching properties: tested predictions led to acceptance.
- Isotopes explain why atomic-weight order was not always right. Modern table: order of atomic number.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Not sure how isotopes explain the swaps?
Relative atomic masses are today's values. The table fragments show only the elements needed for each step.