Answer:
Rb is an alkali metal.
Step-by-step explanation:
The elements characterized by having a single electron in its outermost orbital (s) are called alkali metals or simply alkaline metals.
Due to the presence of this electron, they are chemically very reactive because they have a tendency to lose this electron. In other words, their high reactivity is due to the fact that they form chemical bonds with many types of substances and elements and do so easily. This is due to its high electronegativity.
Like all metals, they are very good thermal and electrical conductors. But in comparison, alkali metals are soft and have a low melting point due to their metallic bond. This link is quite weak because it is the external electrons that are responsible for the bond and in these metals there is only one electron per atom.
The alkali metals are located in group 1 of the periodic table and are: lithium (Li), sodium(Na), potassium(K), rubidium(Rb), cesium(Cs) and francium(Fr).