Diagonal Relationship
The similarity in properties between an element of period 2 and the element one place to its lower right in period 3.
Unit
Dimensionless
In depth
Moving right increases charge and moving down increases size, and the two effects roughly cancel to leave similar charge densities and polarising powers. The result is genuinely parallel chemistry despite different group membership, most clearly among the first three pairs.
Examples in the real world
Lithium resembles magnesium in forming a nitride and a sparingly soluble carbonate; beryllium resembles aluminium in amphoterism.