To determine which of the given options can easily form an ionic bond with a cation, let's analyze each choice:
1. Sr (Strontium): Strontium is an alkaline earth metal which typically forms a cation itself, Sr^2+, by losing two electrons. As it forms positive ions (cations), it would not easily form an ionic bond with another cation, since ionic bonds generally form between positive and negative ions.
2. Ne (Neon): Neon is a noble gas with a completely filled outer electron shell. It is chemically inert and does not usually form bonds, including ionic bonds. Noble gases are known for their lack of reactivity, so Neon would not form an ionic bond with a cation.
3. [tex]$NH_4^+$[/tex] (Ammonium ion): This is already a cation. Since ionic bonds typically form between a cation and an anion, [tex]$NH_4^+$[/tex] would repel other cations instead of forming a bond with them. Therefore, [tex]$NH_4^+$[/tex] will not easily form an ionic bond with another cation.
4. [tex]$PO_4^{3-}$[/tex] (Phosphate ion): This is an anion with a -3 charge. Anions are negatively charged ions which attract and can easily bond with positively charged ions (cations). Therefore, the phosphate ion can easily form an ionic bond with a cation.
Given these considerations, the substance that can easily form an ionic bond with a cation is the phosphate ion, [tex]$PO_4^{3-}$[/tex].
Thus, the answer is:
4. [tex]$PO_4^{3-}$[/tex]
