18.4 Structure and General Properties of the Nonmetals

Structure and Bonding of Nonmetals

Nonmetals form covalent bonds by sharing electrons, producing a wide range of structures from simple diatomic molecules to complex networks like diamond. Their high electronegativities and flexible bonding arrangements give nonmetals diverse physical and chemical properties that set them apart from metals.

Structure and Bonding of Nonmetals

Nonmetals achieve stable electron configurations by sharing electrons through covalent bonds rather than transferring them. Because nonmetals have high electronegativity, they attract shared electrons strongly, which is why they bond covalently with each other instead of forming ionic bonds.

Diatomic molecules are the simplest covalent structures: two atoms of the same element bonded together. Seven elements exist as diatomic molecules under standard conditions: $H_2$, $N_2$, $O_2$, $F_2$, $Cl_2$, $Br_2$, and $I_2$. A helpful mnemonic is "HOFBrINCl" (pronounced "hofbrinkle").

Allotropes are different structural forms of the same element in the same physical state. The same atoms arranged differently can produce dramatically different properties:

• Carbon allotropes:
  • Diamond has every carbon atom covalently bonded to four others in a rigid 3D network. This makes it extremely hard with a very high melting point.
  • Graphite consists of layers of carbon atoms in hexagonal sheets. Within each layer, bonding is strong, but the layers are held together by weak van der Waals forces, so they slide over each other easily. That's why graphite works as a lubricant and in pencils.
  • Fullerenes (like $C_{60}$, buckminsterfullerene) are hollow, cage-like molecules of carbon atoms arranged in pentagons and hexagons.
  • Graphene is a single layer of graphite with remarkable strength and electrical conductivity.
• Phosphorus allotropes:
  • White phosphorus ($P_4$) consists of tetrahedral molecules. It's highly reactive, spontaneously flammable in air, and toxic.
  • Red phosphorus ($P_n$) has a polymeric chain structure. It's much more stable and less reactive than white phosphorus.
• Sulfur allotropes:
  • Rhombic sulfur and monoclinic sulfur are both made of crown-shaped $S_8$ rings. The difference is in how those rings pack into crystals. Rhombic is the more stable form at room temperature.

Lewis structures are used to represent the arrangement of valence electrons in these molecules, and the resulting molecular geometry directly affects a molecule's shape, polarity, and reactivity.

Physical Properties of Nonmetals

Nonmetals span all three states of matter at room temperature:

• Gases: $H_2$, $N_2$, $O_2$, $F_2$, $Cl_2$, and the noble gases
• Liquid: $Br_2$ (the only nonmetal that's liquid at room temperature)
• Solids: Carbon, phosphorus, sulfur, selenium, and $I_2$

What determines whether a nonmetal is a gas, liquid, or solid? It comes down to intermolecular forces, which are generally much weaker in nonmetals than the bonding forces in metals.

• Van der Waals (London dispersion) forces are the primary intermolecular forces in most nonmetals. These arise from temporary dipoles created by electron movement. Their strength increases with molecular size and surface area, which is why $I_2$ (large, heavy molecules) is a solid while $F_2$ (small, light molecules) is a gas.
• Hydrogen bonds are stronger than van der Waals forces and occur when hydrogen is bonded to a highly electronegative atom (N, O, or F). Water ($H_2O$), ammonia ($NH_3$), and hydrogen fluoride ($HF$) all exhibit hydrogen bonding, which gives them higher boiling points than you'd expect based on molecular size alone.

Nonmetals generally have lower melting and boiling points than metals because of these weaker intermolecular forces. The big exception is diamond, which has an extremely high melting point (~3,550°C) because you have to break strong covalent bonds throughout its entire network structure, not just intermolecular forces.

Chemical Reactivity of Nonmetals

Oxidation states describe the degree of oxidation of an atom in a compound. Nonmetals tend to show negative oxidation states when bonded to metals (e.g., Cl is $-1$ in $NaCl$) and positive oxidation states when bonded to more electronegative nonmetals (e.g., S is $+6$ in $SO_3$).

Acid formation is a major part of nonmetal chemistry:

• Hydrogen halides ($HF$, $HCl$, $HBr$, $HI$) dissolve in water to form hydrohalic acids. All of these are strong acids that fully dissociate in water except $HF$, which is a weak acid because of the unusually strong H–F bond.
• Oxoacids form when a nonmetal bonds with both oxygen and hydrogen ($H_2SO_4$, $HNO_3$, $H_3PO_4$). Their acid strength increases with the electronegativity of the central atom and with the number of oxygen atoms bonded to it.

Some nonmetals can also produce bases. For example, $NH_3$ acts as a weak base in water, accepting a proton to form $NH_4^+$ and $OH^-$.

Nonmetals also play key roles in redox reactions:

• Oxidizing agents readily accept electrons. Fluorine ($F_2$) is the strongest oxidizing agent of all elements, and $Cl_2$ and $O_2$ are also powerful oxidizers.
• Reducing agents readily donate electrons. Hydrogen ($H_2$) and carbon ($C$) commonly act as reducing agents, which is why carbon is used to extract metals from their ores.

Bonding and Periodic Trends

The type of bond that forms depends on the elements involved:

• Ionic bonds form between metals and nonmetals through electron transfer.
• Metallic bonds hold metal atoms together in a "sea of electrons."
• Covalent bonds form between nonmetals through electron sharing.

Several periodic trends directly affect nonmetal properties:

• Electronegativity increases across a period (left to right) and decreases down a group. This is why fluorine, in the upper right of the periodic table, is the most electronegative element.
• Atomic radius decreases across a period and increases down a group. Smaller atoms with high electronegativity tend to form stronger covalent bonds.
• Ionization energy increases across a period and decreases down a group. High ionization energy means an atom holds onto its electrons tightly, which is characteristic of nonmetals.

These trends explain why nonmetals on the upper right of the periodic table (excluding noble gases) tend to be the most reactive nonmetals, while those closer to the metal-nonmetal boundary (like selenium or tellurium) show more moderate behavior.