Here's why:

* Alkali metals are in Group 1 of the periodic table. They are highly reactive metals with one valence electron. Examples include lithium (Li), sodium (Na), and potassium (K).

* Alkaline earth metals are in Group 2 of the periodic table. They are also reactive metals, but with two valence electrons. Examples include beryllium (Be), magnesium (Mg), and calcium (Ca).

Helium (He) is a noble gas, located in Group 18 of the periodic table. Noble gases are unreactive due to having a full outer shell of electrons.