Both Lithium and magnesium have small size and high charge density.
The electronegativities of is and is 1.2.
They are low and almost same. Their ionic radii are similar.
Thus, they show similarities which is called diagonal relationship between first element of a group with the second element in the next higher group.
Final answer:
Both Lithium and magnesium have small size and high charge density. The electronegativities of is and is 1.2. They are low and almost same. Their ionic radii are similar.