Nonmetal families, halogens and noble-gas chemistry

A nonmetal family contains many different entities

Nitrogen in NH₃ has assignment −3 using H=+1, while nitrate NO₃⁻ gives N +5 using O=−2. Phosphate PO₄³⁻ gives P +5. Sulfur in H₂S is −2 and in sulfate SO₄²⁻ is +6 under the stated conventions. The element name alone does not specify oxidation state, formula or reactivity.

Bonding and phase also matter. N₂’s strong triple bond helps explain low reactivity in many ordinary settings, but it is not a numerical activation barrier or an absolute exclusion of activated pathways. Ordinary sulfur can contain S₈ rings, while other sulfur phases differ. Gas-phase molecular PCl₅ and an ionic solid representation involving PCl₄⁺ and PCl₆⁻ have different structures despite the same overall empirical composition.

Subscripts and charges distinguish oxygen species

Monatomic oxide O²⁻ has one oxygen assigned −2. Peroxide O₂²⁻ has two oxygens averaging −1, and superoxide O₂⁻ has two averaging −1/2. Elemental O₂ gives zero, while OF₂ assigns O +2 when F is −1. Always preserve atom subscripts and the whole charge rather than relying on ambiguous plain-text O2− notation.

Peroxide has 2× 6+2=14 valence electrons; superoxide has 2× 6+1=13. Oxidation averages and electron counts are formal constraints rather than complete molecular-orbital or reactivity descriptions. A salt formula MO₂ can represent different oxygen species depending on the metal assignment and structure; O=−2 is not a universal shortcut.

Halogen and halide enter different electron ledgers

The selected ordinary aqueous net Cl₂ + 2 Br⁻ → 2 Cl⁻ + Br₂ expresses chlorine reduction and bromide oxidation. Each Cl goes from zero to −1; each Br goes from −1 to zero. The net preserves both elements and total charge −2. Actual extent and rate still depend on the conditions.

For a purely stipulated complete-conversion inventory, start with 0.150 mol Cl₂ and 0.200 mol Br⁻. Bromide limits the net extent to 0.100 mol, giving 0.200 mol Cl⁻, 0.100 mol Br₂ and residual Cl₂ 0.0500 mol. The formal electron exchange is 0.200 mol. This is a composition puzzle rather than measured equilibrium, kinetics or a reaction procedure.

Low ordinary reactivity does not forbid every compound

Selected KrF₂ and XeF₂ Lewis ledgers have 22 valence electrons, two central bonds and three central lone pairs. XeF₄ has 36 electrons, four central bonds and two central lone pairs. The central formal charges are zero in these selected drawings, while central oxidation-state assignments are +2, +2 and +4 respectively when F is −1.

A qualitative electron-region model gives linear XeF₂ and square-planar XeF₄ molecular shapes. More than an octet in a Lewis drawing does not prove literal sp³d or sp³d² hybridization, nor does it directly measure a central atomic population. Compound persistence also depends on environment; different noble-gas fluorides do not share one universal thermal-stability claim.

Unusual conditions can reveal other noble-gas chemistry

In the original 2000 argon study, HArF was assigned in solid argon using infrared and isotope evidence with computational support. Its absorber persisted below about 27 K in those experiments. Secondary reactions above that range prevented the authors from establishing an intrinsic thermal-stability limit. This condition-specific persistence is not ordinary ambient stability or proof of global thermodynamic preference.

The original 2017 sodium–helium study reported experimental Na₂He in a high-pressure regime above about 113 GPa, while its calculation estimated thermodynamic stability above 160 GPa. The material is described as an electride with localized interstitial electron pairs; this does not establish ordinary localized Na–He covalent pair bonds. Keep the experimental and computational pressure findings distinct.

These examples reject an absolute assertion that the lighter noble gases cannot enter compounds. They do not demonstrate ordinary helium or argon reactivity under ambient conditions, a general preparation method or the same kind of bonding in every noble-gas compound. Phase, pressure, temperature and the evidence for persistence versus thermodynamic stability belong with the claim.

Worked examples

Try it yourself

1. Is O always −2 in these examples?

   Show the answer and reasoning

   No.

   Peroxide, superoxide, elemental oxygen and OF₂ have other selected assignments.

2. For stipulated complete Cl₂+2Br⁻→2Cl⁻+Br₂, start with 0.0400 mol Cl₂ and 0.120 mol Br⁻. Find products and the leftover reactant.

   Show the answer and reasoning

   Cl⁻: 0.0800 mol, Br₂: 0.0400 mol; Br⁻: 0.0400 mol remains.

   Cl₂ limits extent to 0.0400 mol, consuming 0.0800 mol Br⁻. This is a theoretical inventory, not a measured equilibrium.

3. For selected XeF₄ with four ordinary bonds and F assigned −1, find total valence electrons, central lone pairs and Xe oxidation state.

   Show the answer and reasoning

   36 electrons; two Xe lone pairs; Xe+4.

   8+4× 7=36. Four bonds and three lone pairs on each F use 32, leaving four electrons at Xe; x−4=0 gives the assigned state.

4. Does the reported HArF behavior establish its intrinsic thermal limit at 27 K?

   Show the answer and reasoning

   No.

   Secondary reactions prevented that conclusion in the inspected study.

What to carry forward

• Compare specified species and phases.
• Keep oxygen subscripts and whole charges unambiguous.
• Apply electron and material balances within their declared scope.
• Use noble-gas compound evidence with its actual conditions.
• Separate persistence, calculation and thermodynamic stability claims.