Here's why:

* Electron Configuration: Strontium is in group 2 of the periodic table, also known as the alkaline earth metals. Its electron configuration is [Kr] 5s².

* Valence Electrons: Valence electrons are the electrons in the outermost shell of an atom, which are involved in chemical bonding. The 5s² part of the configuration tells us strontium has two electrons in its outermost shell.