The periodic table · Model
Mendeleev and the table that predicted
Mendeleev described an element nobody had ever seen, down to the density of the metal and the formula of its oxide, and fifteen years later somebody dug it out of a silver mine. How do you describe something that has not been found?
Start here
Sixty-three elements, written on cards, laid out on a desk in order of mass. Every eighth card, the properties come round again.
Lay the cards in a long line and something odd shows up: soft reactive metal, then several ordinary metals, then a violent gas — and then a soft reactive metal again. The pattern repeats. Cut the line into rows so the repeats fall underneath each other and you have a table where every column is a family.
Except the pattern breaks in places. What should you do when an element does not fit the column it lands in?
Leave a gap. Dmitri Mendeleev's decision was that the pattern mattered more than the list of known elements: if nothing known fitted the square, the square belonged to something not yet discovered. He left several empty, described what each missing element would be like, and waited. Within seventeen years three of them had been found — and they matched.
Mendeleev arranged the elements in order of atomic mass and started a new row whenever the properties began repeating. Elements with similar behaviour ended up in the same column, which he called a group.
Two decisions made his table different from everyone else's. He left gaps for elements not yet found, and he swapped a few pairs out of mass order when their properties demanded it. Both looked like cheating at the time. Both turned out to be right.
Your turn · fill the gap
Here is the square Mendeleev left empty. Its neighbours are all you get.
0 of 3 predicted
Si
Silicon
mass 28 · density 2.3
Ga
Gallium
mass 70 · density 5.9
As
Arsenic
mass 75 · density 5.7
Sn
Tin
mass 119 · density 7.3
Silicon above has a mass of 28 and tin below has a mass of 119. What mass would you predict for the missing element?
About 72 — roughly halfway between its neighbours above and below, which is how every other column in the table behaves. Mendeleev predicted 72. The measured value is 72.6.
Gallium to the left has a density of 5.9 and arsenic to the right has 5.7. What density would you predict?
About 5.5 — between the two neighbours in the same row. Mendeleev predicted 5.5 exactly. Germanium turned out to be 5.32.
Silicon forms an oxide with the formula SiO2 and tin forms SnO2. What formula would you predict for the missing element's oxide?
XO2. Elements in the same group combine in the same ratios — that is the most useful thing a group tells you. Germanium oxide is GeO2, exactly as the column requires.
| Property | Mendeleev, 1871 | Measured, 1886 |
|---|---|---|
| Atomic mass | 72 | 72.6 |
| Density | 5.5 g/cm³ | 5.32 g/cm³ |
| Appearance | dark grey solid | greyish-white, shiny, brittle |
| Formula of oxide | XO2 | GeO2 |
Nobody had seen this element. Mendeleev described it from an empty square, and a German chemist dug it out of a silver mine fifteen years later and found the description fitted. That is the moment the table stopped being a filing system and became a theory.
Key fact
Mendeleev ordered the elements by mass, started a new row where the properties repeated, and left gaps for elements nobody had found. The table was accepted because those gaps were filled by exactly what he described.
Three decisions
Would you have made the same calls?
0 of 3 decided
Each of these was a real objection raised against the table at the time. Commit before you read.
No element known in 1869 fitted the square below silicon. Mendeleev left it empty rather than moving the next element up. Was that justified?
Justified, and it was the boldest thing in the table. Moving the next element up would have hidden the problem and broken the column below it. Leaving a gap turned an inconvenience into a prediction — and a prediction can be checked, which is what happened seventeen years later.
Tellurium has a greater atomic mass than iodine, but iodine behaves like the other elements in the column tellurium would land in. Mendeleev swapped them. Was that justified?
Justified. He trusted the chemistry over the measurement, and assumed the masses had been measured wrong. The masses were right — but the table was still correct, because the true order is by number of protons, not by mass. Tellurium has 52 and iodine 53. He got the right answer for a reason he could not have known.
A critic pointed out that the table contained squares with nothing in them, and called it incomplete. Was that a fair criticism?
Fair at the time and wrong in the end. An empty square is only a weakness if it stays empty. Every gap Mendeleev left was filled within thirty years by an element with the properties he had written down, and the gaps became the strongest evidence that the arrangement was real rather than convenient.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Mendeleev's table was accepted because it was tidy.”
Several chemists had noticed the repeating pattern before him. Commit before you read on.
Tidiness convinces nobody. John Newlands had published a repeating pattern five years earlier and was laughed at for it — one chemist asked whether he had tried arranging the elements alphabetically. What Mendeleev's table did that the others did not was make predictions that could have been wrong.
He named three missing elements and gave their masses, densities and the formulae of their compounds. Gallium turned up in 1875, scandium in 1879, germanium in 1886, and each one matched. An arrangement that only organises what you already know is a filing cabinet. One that tells you what you will find is a theory.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
How did Mendeleev order the elements in his table?
Rung 2 · The one that catches people
Why is leaving a gap in a table stronger evidence than filling every square?
Rung 3 · Explain
Explain how Mendeleev was able to describe germanium fifteen years before it was discovered, and why chemists took the table seriously once it was found.
Rung 4 · Take it somewhere new
A student arranges twenty unknown substances into a table by colour, and every column looks neat. Explain what would have to happen before that table could be called scientifically useful.
Key note
Mendeleev arranged the known elements in order of atomic mass and began a new row each time the properties repeated, so that similar elements fell into the same column. He left gaps for undiscovered elements and predicted their properties, and he swapped pairs that were out of order. When gallium, scandium and germanium were found and matched his descriptions, the table was accepted.
Going further
Mendeleev never knew why his table worked. Atoms were still thought to be indivisible, and the reason elements repeat every so often — the arrangement of electrons around the nucleus — was fifty years away. The modern table is ordered by atomic number, the number of protons, rather than mass, and that single change quietly repairs every pair Mendeleev had to swap by hand. Tellurium and iodine sit in the right order because tellurium has 52 protons and iodine 53, whatever their masses do.
One whole column was missing from his table and he never suspected it. The noble gases had not been discovered in 1869, because they react with nothing and so leave no trace in any compound. When argon was isolated in 1894 it fitted nowhere at all — until someone realised the table needed a new group on the end. A theory that can absorb an entire unexpected family without collapsing is a strong one.
Before this lesson
Next in this unit
At GCSE this becomes
- Ordering by atomic number, electron configuration as the reason for the repeat, and why the noble gases were missed.
Where to next
Ask Mr Badmus AI
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