* Alkali Metals (Group 1): Alkali metals are highly reactive due to having only one valence electron. This lone electron is easily lost, making them excellent reducing agents (they readily donate electrons). They are not considered stable in their elemental form.

* Alkaline Earth Metals (Group 2): Alkaline earth metals are also reactive, but slightly less so than alkali metals. They have two valence electrons and readily form +2 ions. They are not considered stable in their elemental form.

* Halogens (Group 17): Halogens are highly reactive nonmetals. They have seven valence electrons and tend to gain one electron to form a -1 ion, becoming stable halides. They are not stable in their elemental form.

* Noble Gases (Group 18): Noble gases are extremely stable. They have a full outer shell of electrons (eight electrons, except for helium with two), making them very unreactive. They exist as stable monatomic gases.

Summary:

* Stable: Noble gases (Group 18)

* Unstable: Alkali metals (Group 1), Alkaline earth metals (Group 2), Halogens (Group 17)