* Barium (Ba) is an alkaline earth metal, located in Group 2 of the periodic table. This means it has two valence electrons (electrons in its outermost shell).

* Oxygen (O) is a nonmetal in Group 16, meaning it has six valence electrons. It needs two more electrons to achieve a stable octet (8 electrons in its outer shell).

The Reaction:

When barium and oxygen react, barium loses its two valence electrons to become a positively charged ion (cation):

* Ba → Ba²⁺ + 2e⁻

Oxygen gains these two electrons, becoming a negatively charged ion (anion):

* O + 2e⁻ → O²⁻

The resulting ions, Ba²⁺ and O²⁻, have opposite charges and attract each other electrostatically, forming the ionic compound barium oxide (BaO).

Key takeaway: Barium loses two electrons to form a +2 cation, while oxygen gains two electrons to form a -2 anion. This is a classic example of ionic bonding.