To determine which pair of ions can form an ionic bond, it's essential to understand the fundamental principle that ionic bonds occur between ions of opposite charges, i.e., one cation (positively charged) and one anion (negatively charged).
Let's analyze each given option:
1. [tex]$Cu ^{+}$[/tex] and [tex]$Ag ^{+}$[/tex]: Both are positively charged ions (cations). Since both carry the same positive charge, they will repel each other and cannot form an ionic bond.
2. [tex]$S ^{2-}$[/tex] and [tex]$O ^{2-}$[/tex]: Both are negatively charged ions (anions). Similar to the first option, these ions will repel each other because they have like charges and thus cannot form an ionic bond.
3. [tex]$Br ^{-}$[/tex] and [tex]$At ^{-}$[/tex]: Both of these ions are negatively charged (anions) and belong to the halogen group. Again, since they both have the same negative charge, they will repel each other and cannot form an ionic bond.
4. [tex]$Li ^{+}$[/tex] and [tex]$Br ^{-}$[/tex]: The lithium ion ([tex]$Li ^{+}$[/tex]) is a positively charged ion (cation), while the bromide ion ([tex]$Br ^{-}$[/tex]) is a negatively charged ion (anion). Because they have unlike charges (opposite charges), they will attract each other, allowing them to form a stable ionic bond.
Given the detailed analysis, the pair that can form an ionic bond because they have unlike charges is:
[tex]$Li ^{+}$[/tex] and [tex]$Br ^{-}$[/tex]
Thus, the correct answer is the fourth option.
