Final answer:
A. PO₄³⁻. Covalent bonding involves the sharing of electron pairs between atoms. In the given options, A. PO₄³⁻ (phosphate ion) displays covalent bonding, whereas the others do not feature electron sharing.
Step-by-step explanation:
In the context of the student's question about which substance involves the sharing of electrons between two atoms, we need to identify the type of bond present in each option. The sharing of electrons is characteristic of covalent bonding, where atoms share electron pairs. Among the choices provided:
- A. PO₄³⁻ features covalent bonding within the phosphate ion, where phosphorus shares electrons with oxygen atoms.
- B. Ag (silver) is a metal and typically does not share electrons through covalent bonds.
- C. NaI, consisting of sodium and iodine, forms an ionic bond wherein an electron is transferred from sodium to iodine.
- D. He (helium) is a noble gas that does not typically form bonds because it has a complete valence shell.
Therefore, the substance where the sharing of electrons between two atoms occurs is A. PO₄³⁻.