Structure and General Properties of the Nonmetals

Core Concepts

Position, electronegativity, and the electron-gaining habit

Nonmetals sit to the right of the metalloid staircase, where ionization energies and electronegativities are highest. Fluorine (χ = 3.98, Pauling scale) is the most electronegative element; oxygen (3.44), chlorine (3.16), and nitrogen (3.04) follow. Holding valence electrons tightly, nonmetals rarely form simple cations. Instead they:

• gain electrons to complete an octet, forming anions in ionic compounds: 2Na + Cl₂ → 2NaCl (Cl gains one electron → Cl⁻);
• share electrons in covalent bonds: in H₂O, NH₃, CH₄, and in the giant structures of C, P, S, and Se.

Hydrogen is the exception — it can lose (H⁺ in acids), gain (H⁻ in ionic hydrides like NaH), or share (in covalent compounds).

Molecular nonmetals: why so many are gases

Elements that achieve an octet by forming small molecules — H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂, and the noble gases — are held in condensed phases only by weak London dispersion forces. The trend is regular: at room temperature H₂, N₂, O₂, F₂, and Cl₂ are gases; Br₂ is a liquid; I₂ is a solid. Bond strength matters too: N₂'s triple bond (N≡N, ~945 kJ/mol) makes it famously inert, while O₂'s double bond (~498 kJ/mol) is reactive enough to support combustion and respiration.

Network and molecular-crystal nonmetals

• Carbon is the master of allotropes: diamond (tetrahedral network — hardest natural substance, insulator), graphite (fused-hexagon layers with a delocalized π system — soft, slippery, conductive in-plane), plus graphene, fullerenes (C₆₀ cages), and nanotubes.
• Phosphorus exists as white (P₄ tetrahedra — waxy, toxic, ignites in air, stored under water), red (polymeric chains, safer), and black (layered, most stable). The strained 60° angles of the P₄ tetrahedron explain white phosphorus's high reactivity.
• Sulfur forms S₈ crown rings; rhombic and monoclinic allotropes differ only in ring packing, and both melt to a liquid that turns viscous on heating as rings open into chains.
• Selenium exists in gray (photoconductive — used in photocopiers and solar cells) and red (molecular) forms.

Oxidation states and the halogens' reactivity order

Nonmetals display a wide range of oxidation states: nitrogen spans −3 (NH₃) to +5 (HNO₃); sulfur spans −2 (H₂S) to +6 (H₂SO₄); chlorine spans −1 to +7. The halogens are the most reactive nonmetals, with reactivity decreasing down the group (F₂ > Cl₂ > Br₂ > I₂); a more reactive halogen displaces a less reactive one: Cl₂ + 2NaBr → 2NaCl + Br₂. The noble gases, by contrast, have filled octets and are essentially unreactive — though the heavier ones (especially Xe) form compounds with fluorine and oxygen (XeF₂, XeO₃).

Reactivity patterns: oxides and ionic compounds

Nonmetal chemistry is summarized by reactions with oxygen and metals:

• Nonmetals burn in oxygen to form acidic oxides (anhydrides of acids): C + O₂ → CO₂, S₈ + 8O₂ → 8SO₂, 4P + 5O₂ → 2P₂O₅. In water these give carbonic, sulfurous/sulfuric, and phosphoric acids — the chemistry behind acid rain.
• Nonmetals react with active metals to form ionic compounds: 2Na + Cl₂ → 2NaCl, 3Mg + N₂ → Mg₃N₂.
• Strong oxidizing nonmetals (O₂, F₂, Cl₂) drive combustion, rusting, bleaching, and disinfection. Ozone is a far stronger oxidant than O₂ — protecting us (stratospheric UV) and harming us (lung irritant in smog).

Trends to remember

• Electronegativity and ionization energy rise across a period toward the halogens; noble gases have the highest ionization energies of all.
• Atomic radius decreases across a period; fluorine is small, which is part of why HF is a weak acid despite F's record electronegativity.
• Down a group, nonmetallic character decreases: carbon is a nonmetal, silicon a metalloid, tin and lead metals.

Common Confusions