The diagonal relationship is a term used in chemistry to describe the similarities in properties between certain pairs of diagonally adjacent elements in the second and third periods of the periodic table. The pairs of elements that exhibit diagonal relationships are:
- Lithium (Li) and Magnesium (Mg)
- Beryllium (Be) and Aluminum (Al)
- Boron (B) and Silicon (Si)
The reasons for the existence of diagonal relationships are not fully understood, but charge density is a factor. For example, Li+ is a small cation with a +1 charge and Mg2+ is somewhat larger with a +2 charge, so the ionic potential of each of the two ions is roughly the same. It is found that the chemistry of a period 2 element often has similarities to the chemistry of the period 3 element one column to the right of it in the periodic table.
