Mendeleev's Masterpiece
Arrange elements by mass and predict missing ones.
What this lesson covers
Why it matters
Mendeleev laid the elements out by increasing atomic mass — but where the next element didn't fit the pattern, he left an empty seat, predicting an undiscovered element (he called one 'eka-aluminium') would fill it.
Predict first
Why did Mendeleev leave gaps in his table instead of forcing elements to fit?
Mendeleev ordered elements by increasing atomic mass. 'Eka-aluminium' was his name for the missing element below aluminium — later discovered as gallium (mass ≈ 70), slotting exactly between zinc and arsenic.
- He predicted undiscovered elements would fill them, and even forecast their properties — correct
- He ran out of cards while building the table
- He thought those positions were impossible
What you do
Arrange these by increasing atomic mass. Mendeleev's predicted 'eka-aluminium' must slot into its gap.
Check yourself
Mendeleev's predicted 'eka-aluminium' was later discovered and named…
Gallium matched Mendeleev's eka-aluminium predictions; germanium matched his eka-silicon.
Mendeleev arranged his periodic table primarily in order of…
Mendeleev used atomic mass; the switch to atomic number came later with Moseley.
- gallium — correct
- aluminium
- germanium
- increasing atomic mass — correct
- increasing atomic number
- alphabetical order of names