Complex sentence: Lithium and magnesium have a diagonal relationship: because of this, lithium has some similarities to magnesium, such as its formation of a stable nitride, a property common among all the alkaline earth metals (magnesium's group) but unique among the alkali metals.
Split into simpler sentences: Lithium and magnesium have a diagonal relationship: because of this, lithium has some similarities to magnesium. For example, lithium forms a stable nitride, a property common among all the alkaline earth metals (magnesium's group) but unique among the alkali metals.