Here's why:

* Alkali Metals are located in Group 1 of the periodic table. They are highly reactive metals that readily lose one electron to form a +1 cation.

* Sodium (Na) and Calcium (Ca) both fit this description. They are both shiny, soft, and have low melting points. They react vigorously with water and air.

It's important to note that while calcium is in group 2 (alkaline earth metals), it shares many similarities with the alkali metals due to its tendency to lose two electrons and form a +2 cation. However, it is more reactive than the other alkaline earth metals and shares several properties with alkali metals.