The formula of mercury (l) dioxonitrate (III) is

Analyze the question: We need the formula of mercury(I) dioxonitrate(III). The name has two parts: the cation and the anion.

Explain the concept:

• Mercury(I) is unusual. Mercury in the +1 state exists as the diatomic ion Hg₂²⁺ (two Hg⁺ ions joined by a metal-metal bond), so its charge is 2+ overall.
• Dioxonitrate(III) is the nitrite ion, NO₂⁻. "Dioxo" means two oxygen atoms, and (III) is the oxidation state of nitrogen. Compare this with trioxonitrate(V), which is nitrate, NO₃⁻.

Solution:

1. Cation: Hg₂²⁺
2. Anion: NO₂⁻
3. Balance the charges: one Hg₂²⁺ (2+) needs two NO₂⁻ (2−).
4. Formula: Hg₂(NO₂)₂

Check the options:

• A. HgNO₃ is mercury(I) trioxonitrate(V), which is the wrong anion.
• B. Hg₂NO₂ has unbalanced charges.
• C. Hg₂(NO₂)₂ has the correct ions and balanced charges.
• D. Hg(NO₃)₂ is mercury(II) trioxonitrate(V).

Tip: In the older Nigerian naming system, the number of "oxo" oxygens tells you the anion, and the roman numeral is the oxidation state of the central atom. "Dioxonitrate(III)" gives NO₂⁻, and "trioxonitrate(V)" gives NO₃⁻. Also remember that mercury(I) is always Hg₂²⁺, never Hg⁺ on its own.