Final answer:
The most stable ion of barium is Ba²+, as it loses two electrons, and the most stable ion of oxygen is O²-, as it gains two electrons to achieve a full octet.
Step-by-step explanation:
To predict the charge on the most stable ion of barium (Ba), we look at its position in the periodic table. Barium is in group 2, which means it has two electrons in its outer shell. To achieve stability, barium tends to lose these two electrons to form a Ba²+ ion. The most stable ion of oxygen, on the other hand, is achieved by gaining electrons. Oxygen has six electrons in its outer shell and needs two more to complete its octet, leading to the formation of an O²- ion. When these ions form an ionic bond, they do so in a one-to-one ratio to balance out their charges, resulting in the ionic compound BaO.