Periodic Table
The tabular arrangement of the chemical elements by increasing atomic number, grouping those with similar properties in vertical columns.
Unit
Dimensionless
In depth
The modern table has 18 groups and 7 periods, with the lanthanides and actinides drawn separately. Its structure mirrors the filling of electron subshells exactly: s-block on the left, p-block on the right, d-block in the middle and f-block below. Mendeleev's original version famously left gaps for undiscovered elements.
Examples in the real world
Mendeleev predicted the properties of eka-silicon in 1871; germanium, discovered in 1886, matched them closely.