1Ordering the elements
In 1869 Dmitri Mendeleev put the elements in order of atomic mass: how heavy their atoms are.
Mendeleevatomic mass
Lesson 20.2 notes
Mendeleev’s periodic table
How Mendeleev sorted the elements, left gaps, and predicted elements nobody had found.
2Similar elements together
He arranged them so that elements with similar properties were in the same group, like lithium, sodium and potassium.
3Gaps
Mendeleev left gaps for elements nobody had found yet, and predicted what they would be like.
prediction
4Predicting eka-silicon
Its density should be between those of silicon (2.3 g/cm³) and tin (7.3 g/cm³): about 5 g/cm³.
5Predictions come true
Germanium was found in 1886 and matched his predictions. Gallium and scandium did too, so scientists accepted his table.
6Properties first
Iodine's atoms are lighter than tellurium's, but Mendeleev put iodine after tellurium, with chlorine and bromine, because it behaves like them.
7Today's table
Today the elements are in order of atomic number, the number of protons. This puts tellurium (52) before iodine (53), as Mendeleev did.
atomic numberproton
Explore: Predict a missing element
Use the elements above and below a gap to predict its density or mass. Then see what Mendeleev predicted and what was measured.
Common mistake: Mendeleev used atomic mass, not atomic number
Atomic number hadn't been discovered in 1869. Mendeleev used atomic mass, and swapped a few elements to keep similar ones together. Today's table uses atomic number.
Remember
- Mendeleev (1869) ordered the elements by atomic mass and put similar elements in the same group.
- He left gaps for undiscovered elements and predicted their properties. Gallium and germanium matched.
- Today's table is in order of atomic number: the number of protons.
Going further at GCSE
GCSE: Development of the periodic table → GCSE: The periodic table →