What it does
In 1869 Dmitri Mendeleev arranged the 63 known elements by atomic mass and noticed that their properties repeated in a pattern. Where the pattern needed an element that nobody had found, he left a gap, and he was bold enough to predict what the missing element would be like by looking at its neighbours. When gallium, scandium and germanium were discovered, his predictions turned out remarkably close, and the periodic table was accepted.
This tool lets you repeat his trick. Hide an element, and it shows the four elements around it in the table: above, below, left and right. Their average gives a prediction of the hidden element’s atomic mass or density. Reveal it to see how close you came. A table below lists his three famous predictions beside what was later found.
It follows the school topics on the genesis of periodic classification, Mendeleev’s periodic table and his predictions (Class 11 Chemistry, classification of elements and periodicity; Class 10 Science in the older books). For the modern table see interactive periodic table, for the trends that make the averaging work periodic trends, and for naming brand-new elements new element names.
How to use it
- Pick the element to hide: Ga, Ge, As, Se, Sn or Sb.
- Choose whether to predict its atomic mass or its density.
- Read the four neighbours and their average, then tap Reveal the real element to compare.
- Look at the table of Mendeleev’s own predictions.
Worked example
Open the page: germanium is hidden. Above it is silicon (28.085 u), below it tin (118.71 u), to the left gallium (69.723 u) and to the right arsenic (74.922 u). Their average is (28.085 + 118.71 + 69.723 + 74.922) ÷ 4 = 72.86 u. Reveal: germanium’s real atomic mass is 72.63 u, so the prediction is only 0.32 % too high. Mendeleev himself predicted 72 for eka-silicon.
Switch to density: the neighbours give (2.33 + 7.29 + 5.91 + 5.73) ÷ 4 = 5.315 g/cm³ against a real 5.32 g/cm³. Hide tin instead and the mass prediction comes out 8.75 % too high, because lead below it is much heavier: fourteen f-block elements are squeezed in between.
Tips
- Averages work best in the middle of a block where properties change smoothly.
- Density depends on how atoms pack in a solid, so it is harder to predict than mass.
- Mendeleev also swapped a few elements (such as tellurium and iodine) to keep families together, trusting properties over mass.
FAQ
What does “eka” mean? It is Sanskrit for “one”: eka-silicon is one place below silicon in the same family.
Was Mendeleev right about everything? No. Some predicted elements never existed, and the noble gases, unknown in 1869, needed a whole new column.
Is anything stored? No. It all runs in your browser.
Where the numbers come from
- Standard atomic weights: IUPAC (CIAAW), from the site's element list. Densities: rounded reference values near room temperature, general facts found in any data book.
- Mendeleev's 1871 predictions for eka-aluminium, eka-boron and eka-silicon and the discoveries of gallium (1875), scandium (1879) and germanium (1886): history of chemistry, as told in textbooks.
- Checked by hand: germanium's neighbours Si, Sn, Ga and As average 72.86 u (real 72.63 u) and 5.315 g/cm³ (real 5.32 g/cm³); every hidden element has four neighbours with data.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.