At the beginning of periods 2 and 3 of the Periodic Table, there are several cases where an element at the top of one group has some similarities with an element in the next group.

For example, boron is a non-metal with some properties rather like silicon. Unlike the rest of Group 2, beryllium has some properties resembling aluminium. And lithium has some properties which differ from the other elements in Group 1, and in some ways resembles magnesium.

There is said to be a diagonal relationship between these elements.

There are several reasons for this, but each depends on the way atomic properties like electronegativity vary around the Periodic Table.