Mendeleev's breakthrough and modern understanding Revision Notes | GCSE Edexcel Higher Chemistry

1. The Puzzle That Stumped Scientists

By 1860, chemists knew about 60 elements but couldn't make sense of them. Some tried arranging by atomic weight, but the patterns kept breaking down. Then a Russian chemistry professor called Dmitri Mendeleev had a breakthrough that changed everything.

Mendeleev was preparing lecture notes when he noticed something remarkable. If he arranged elements by atomic weight, their properties repeated in a pattern — but only if he was willing to break his own rules.

2. Mendeleev's Bold Moves

Mendeleev made two revolutionary decisions that other scientists wouldn't dare make. First, he left gaps where elements seemed to be missing. Second, he sometimes ignored atomic weight order if it meant keeping similar elements together.

Look at his reasoning: if boron and aluminium were similar, and carbon and silicon were similar, then there must be an element between silicon and tin with predictable properties. He called this missing element 'eka-silicon' — meaning 'like silicon'.

Even more daringly, Mendeleev swapped the positions of some elements. He put tellurium (atomic weight 127.6) before iodine (atomic weight 126.9) because tellurium behaved like sulfur and selenium, while iodine behaved like chlorine and bromine.

3. The Predictions That Made Him Famous

Mendeleev didn't just leave gaps — he described exactly what the missing elements would be like. For eka-silicon, he predicted: atomic weight around 72, grey metallic appearance, density 5.5 g/cm³, and it would form an oxide with formula XO₂.

In 1886, German chemist Clemens Winkler discovered germanium. Its properties matched Mendeleev's predictions almost perfectly: atomic weight 72.6, grey metallic appearance, density 5.3 g/cm³, oxide formula GeO₂.

Worked example: Comparing Mendeleev's Predictions with Reality

Compare Mendeleev's predictions for eka-silicon with the actual properties of germanium

1. Predicted: ~72 Actual: 72.6 — Incredibly close — less than 1% error
2. Predicted: 5.5 g/cm³ Actual: 5.3 g/cm³ — Again, very close match
3. Predicted: XO₂ Actual: GeO₂ — Perfect match — germanium forms GeO₂
4. Predicted: Grey metallic Actual: Grey metallic — Exact match in appearance

Answer: Mendeleev's predictions were remarkably accurate, proving his periodic law was correct

Two more of Mendeleev's predicted elements were discovered: gallium (eka-aluminium) in 1875 and scandium (eka-boron) in 1879. Each discovery confirmed his periodic law and made him internationally famous.

4. The Atomic Weight Problem

Mendeleev's table worked brilliantly, but there was a nagging problem. Sometimes he had to ignore atomic weight order to keep similar elements together. Why did tellurium (127.6) have to come before iodine (126.9)?

The answer came decades later with the discovery of isotopes. Scientists realised that atomic weight wasn't fundamental — atomic number was. Elements should be arranged by number of protons, not total mass.

Tellurium has isotopes that make its average atomic weight higher than iodine's, even though tellurium has fewer protons (52 vs 53). When we arrange by atomic number instead of atomic weight, the order makes perfect sense.

5. From Mendeleev to Modern

Today's periodic table follows Mendeleev's basic pattern but uses atomic number instead of atomic weight. We've filled all his gaps and discovered elements he never imagined — the noble gases, which he couldn't predict because they don't react.

The modern table has 118 elements arranged in 18 groups and 7 periods. Elements in the same group have the same number of outer electrons, explaining why Mendeleev's property patterns worked so well.

6. Exam Success Strategy

Exam questions on this topic often ask you to describe the development of the periodic table. You need to explain Mendeleev's methods and why they worked, plus how modern understanding improved on his ideas.

Remember the key phrase: 'Mendeleev overcame problems by leaving gaps and changing order based on properties.' This shows you understand his revolutionary approach to organising the elements.