What is the diagonal relationship? Give one example.

Hint: Firstly we should know the periodic properties and terms related to periodic table. Diagonal relationships exist among elements present in periodic table diagonally situated, especially second and third period elements.
Such elements do not have similar property to their respective group, rather has similarity with the element diagonally present.

Complete step by step answer:
In the periodic table, we observe that there are some elements of the second period which have similarity in properties to third period elements which are present diagonally to them, meaning on the right side below (down) the next element. This similarity of properties is called Diagonal relationship.
The similar properties include size, Electronegativity, properties of compounds like oxides, halides and hydrides. It may also have metallic, non-metallic or metalloid nature similar.
The few elements do not show property of their family (Group), but behaves in properties of diagonal present elements. The examples and certain properties are mentioned below.
The common examples are Lithium and Magnesium, Beryllium and Aluminium, Boron and Silicon.
2 elements can be used as semiconductors are Boron (2nd period element) and Silicon (3rd period element), which are diagonally present.
The oxides of Lithium (2nd period) and Magnesium (3rd period) are normal oxides, while other elements in alkali metals form peroxides and superoxides. Lithium Carbonate and Magnesium carbonate are both unstable, and give out Carbon dioxide.

Additional information: Moving rightward and descending the periodic table have opposite effects on atomic radii of isolated atoms. Moving rightward across the period decreases the atomic radii of atoms, while moving down the group will increase the atomic radii.
Similarly, on moving rightward in a period, the elements become progressively more covalent, less basic and more electronegative, whereas on moving down a group the elements become more ionic, more basic and less electronegative.
Thus, on both descending a period and crossing a group by one element, the changes "cancel" each other out, and elements with similar properties which have similar chemistry are often found.

Note:
The diagonal relationship is due to similar size of ions of second period element and third period element. Due to comparable ionic radius, other properties like polarizing power, electronegativity, etc will also exhibit similar nature. These elements lie in a diagonal position in the periodic table. For this we need good knowledge of the position of elements in periodic tables.