Final answer:
The correct answer is a. Ba and O because they have a significant difference in electronegativity, necessary for forming monatomic ions. Barium has a low electronegativity, and oxygen has a high electronegativity, leading to the formation of Ba²+ and O²⁸ ions.
a is correct
Step-by-step explanation:
The question 'On the basis of electronegativity, which of these pairs of elements is most likely to form monatomic ions?' is asking us to determine which pair of elements shows the greatest difference in electronegativity, as this is a strong indicator that they will form ions. Electronegativity reflects an atom's ability to attract and hold onto electrons.
Among the given pairs:
Ba and O have a significant difference in electronegativity, with oxygen being much more electronegative than barium. This large difference means that oxygen will attract electrons much more strongly than barium, leading to the formation of the ions ²⁸O and Ba²+.
Cs and Rb are both alkali metals and have similar electronegativities, so they are unlikely to form ions with each other.
O and F are both highly electronegative elements and have relatively small differences in electronegativity, so they are more likely to share electrons (form a covalent bond) than form ions.
Na and K are also alkali metals with similar electronegativities and are unlikely to form ions with each other.
Therefore, a. Ba and O is the correct answer. Barium and oxygen are most likely to form monatomic ions due to their significant electronegativity difference.