Mendeleev and the History of the Periodic Table
Mendeleev ordered elements by mass and left gaps for undiscovered ones.
About this topic
Early chemists searched for order among the elements. Dobereiner noticed triads of similar elements, Newlands proposed the law of octaves in 1865, and in 1869 Dmitri Mendeleev arranged the known elements by atomic mass and by chemical properties, leaving gaps for undiscovered elements. His table's power was in prediction.
Mendeleev placed elements with similar properties in the same column, even if that meant ignoring strict order of mass, and left gaps, predicting the properties of eka-aluminium and eka-silicon. When gallium (1875) and germanium (1886) were discovered with properties closely matching his predictions, his table gained wide acceptance. For example, he predicted that eka-silicon would have an atomic mass near 72 and a density of 5.5 g/cm3, and germanium has 72.6 and 5.35.
The table had flaws: tellurium and iodine had to be reversed to fit their chemistry. In 1913 Henry Moseley, using X-ray spectra, showed that the true ordering principle is atomic number, the number of protons, which resolved these cases. The noble gases were added in the 1890s as a new group, and Seaborg later moved the actinides below the main table. Today, elements up to 118 are known, with the latest discovered in laboratories.
A misconception is that Mendeleev's table was the first or the only one; many contributed, but he made the bold predictions. Another is that the modern table is finished; new superheavy elements are still being sought.
Key ideas
- Predicted gallium and germanium
- Moseley later ordered by atomic number
- The modern law is periodic in Z
Quick check
Use cases: where Mendeleev and the History of the Periodic Table helps
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Frequently asked questions
What is Mendeleev and the History of the Periodic Table?
Mendeleev ordered elements by mass and left gaps for undiscovered ones.
What will I learn in Mendeleev and the History of the Periodic Table?
Predicted gallium and germanium. Moseley later ordered by atomic number. The modern law is periodic in Z.
Is Mendeleev and the History of the Periodic Table part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Mendeleev and the History of the Periodic Table lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Mendeleev and the History of the Periodic Table?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
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