18.4 Structure and General Properties of the Nonmetals

Nonmetals form covalent bonds by sharing electrons, creating diverse structures like diatomic molecules and allotropes. These elements exhibit unique properties due to their high electronegativity and ability to form various molecular geometries, influencing their reactivity and behavior.

Physical properties of nonmetals vary widely, with states ranging from gases to solids at room temperature. Intermolecular forces, primarily van der Waals and hydrogen bonds, play a crucial role in determining their melting and boiling points, as well as their overall chemical reactivity.

Structure and Bonding of Nonmetals

Structure and bonding of nonmetals

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• Nonmetals form covalent bonds by sharing electrons to achieve stable electron configurations
  • High electronegativity allows nonmetals to attract shared electrons in covalent bonds
• Diatomic molecules composed of two atoms of the same element bonded together ($H_2$, $N_2$, $O_2$, $F_2$, $Cl_2$, $Br_2$, $I_2$)
  • Held together by strong covalent bonds
• Allotropes are different structural forms of the same element in the same physical state
  • Carbon allotropes: graphite, diamond, fullerenes ($C_{60}$ buckminsterfullerene), graphene
    • Graphite has a layered structure with weak intermolecular forces between layers
    • Diamond has a rigid, three-dimensional structure with strong covalent bonds in all directions
    • Fullerenes are molecules composed of carbon atoms arranged in a hollow sphere or tube
    • Graphene is a single layer of graphite with unique electronic properties
  • Phosphorus allotropes: white phosphorus ($P_4$), red phosphorus ($P_n$)
    • White phosphorus is highly reactive, unstable, and toxic; composed of tetrahedral $P_4$ molecules
    • Red phosphorus is more stable and less reactive; has a polymeric structure with chains of phosphorus atoms
  • Sulfur allotropes: rhombic sulfur ($S_8$), monoclinic sulfur ($S_8$)
    • Both composed of crown-shaped $S_8$ molecules with different crystal structures
• Lewis structures are used to represent the arrangement of valence electrons in molecules and ions
• Molecular geometry influences the shape and properties of molecules, affecting their reactivity and behavior

Physical properties of nonmetals

• Nonmetals can exist in all three states of matter at room temperature
  • Gases ($H_2$, $N_2$, $O_2$, $F_2$, $Cl_2$), liquids ($Br_2$), solids (carbon, phosphorus, sulfur, $I_2$)
• Intermolecular forces in nonmetals are generally weaker than those in metals
  • Van der Waals forces (dispersion forces) are the primary intermolecular forces in nonmetals
    • Arise from temporary dipoles created by the movement of electrons
    • Strength increases with increasing molecular size and surface area
  • Some nonmetals can form hydrogen bonds, which are stronger than van der Waals forces
    • Occur when a hydrogen atom is bonded to a highly electronegative atom (N, O, or F)
    • Examples: $NH_3$, $H_2O$, $HF$
• Nonmetals generally have lower melting and boiling points compared to metals due to weaker intermolecular forces
  • Exception: diamond has a high melting point due to its strong, three-dimensional covalent bonding

Chemical reactivity of nonmetals

• Nonmetals can have various oxidation states in compounds
  • Oxidation state represents the degree of oxidation of an atom in a compound
  • Tend to have negative oxidation states when bonded to metals and positive oxidation states when bonded to other nonmetals ($NaCl$: Cl has -1; $SO_3$: S has +6)
• Many nonmetals can form acids when dissolved in water or reacted with water
  • Hydrogen halides ($HF$, $HCl$, $HBr$, $HI$) form hydrohalic acids in water
    • These acids are strong acids that fully dissociate in water
  • Oxoacids are formed when nonmetals bond with oxygen and hydrogen ($H_2SO_4$, $HNO_3$, $H_3PO_4$)
    • Strength depends on the electronegativity of the central nonmetal atom and the number of oxygen atoms
• Some nonmetals can also form bases when reacted with water
  • $NH_3$ forms $NH_4OH$, a weak base, when dissolved in water
• Nonmetals can act as oxidizing or reducing agents in chemical reactions
  • Oxidizing agents readily accept electrons ($F_2$, $Cl_2$, $O_2$)
  • Reducing agents readily donate electrons ($H_2$, $C$)
• Chemical reactivity of nonmetals is influenced by their electron configuration and ability to form covalent bonds

Bonding and Periodic Trends

• Ionic bonds form between metals and nonmetals through the transfer of electrons
• Metallic bonds occur in metals, while nonmetals primarily form covalent bonds
• Periodic trends affect the properties of nonmetals:
  • Electronegativity generally increases across a period and decreases down a group
  • Atomic radius decreases across a period and increases down a group
  • Ionization energy increases across a period and decreases down a group