Final answer:
The highly reactive metal used to treat manic-depression is lithium, which is an alkali metal along with sodium, potassium, rubidium, cesium, and francium.
Step-by-step explanation:
The highly reactive metal used to treat manic-depression is lithium. Lithium is part of the Group 1 elements known as the alkali metals, which also include sodium, potassium, rubidium, cesium, and francium. These metals are known for being soft, shiny, and highly reactive. Lithium, in particular, has significant applications beyond its use in mental health treatment; it's also used in greases, batteries, and has various industrial purposes. Despite its reactivity, lithium is relatively rare in nature compared to the more abundant sodium and potassium, which are respectively the seventh and eighth most abundant elements in Earth's crust.