1.14 - Mendeleev's predictions
Mendeleev used gaps in a repeating pattern to predict elements that had not yet been discovered. You will follow the reasoning from a gap to a prediction, then see how later discoveries tested it.
From gaps to predictions
Mendeleev arranged known elements so that chemically similar elements and their compounds formed a repeating pattern. Sometimes forcing a known element into the next space would break that pattern. He left a gap instead and proposed that an undiscovered element would eventually fill it.
He could then use the surrounding pattern to predict more than the element's existence:
| Evidence | Possible prediction |
|---|---|
| Chemical behaviour of elements in the same group | Similar reactions for the missing element |
| Compounds formed by similar elements | Similar types of compound |
| Trends in measured properties near the gap | Approximate physical properties, such as density |
These were estimates based on evidence, not exact values read from a missing box. He did not use atomic number: its modern meaning was established later.
For example, if elements above and below a gap form similar oxygen compounds, a reasonable prediction is that the missing element will form a similar oxygen compound. A regular density trend can support an estimated density, but does not guarantee an exact value.
Testing the predictions
Gallium, scandium and germanium were later discovered and fitted gaps in Mendeleev's table. Their measured properties were close to the predicted ones. This supported his arrangement because the predictions had been made before the discoveries.
A prediction becomes testable when it states what scientists should find. Merely saying “more elements exist” is less informative than predicting an element's position, properties and compounds. Agreement supports a model; a major disagreement would be a reason to check the evidence and reconsider it.