What is diagonal relationship ? In what ways lithium shows similarities to magnesium in its ( chemical behaviour)?

Some elements of certain groups in the second j period resemble with the certain elements of the next higher group in the third period. This is called | diagonal relationship.
Similarities between lithium and magnesium:
(i) Both Li and Mg form a nitride ${{Li}{3}}$N and ${{Mg}{3}}$${{N}{2}}$ by direct combination with nitrogen.
(ii) The oxides, ${{Li}
{2}}$0 and MgO do not combine with excess oxygen to form any superoxide.