Chemical Bonds: Formation, Types, and Properties
Chemical Bonds
Chemical bonds form between atoms to achieve greater stability, often attaining a noble gas electron configuration. Bonds can be homoatomic (same atom) or heteroatomic (different atoms).
Ionic Bonds
Ionic bonds form between metals and nonmetals. Nonmetals gain electrons to form anions, achieving the nearest noble gas configuration. Metals lose electrons, forming cations. Stoichiometry depends on anion and cation charges.
Covalent Bonds
Covalent bonds form between nonmetals. Atoms share electrons to form homoatomic (elements) or heteroatomic (compounds) substances.
Octet Theory and Lewis Structures
The octet theory states that atoms share electrons to achieve eight valence electrons. Shared electrons belong to both atoms. Dative covalent bonds occur when one atom donates both electrons in a shared pair.
Formation of Ionic Solids
Cations and anions attract each other, forming a solid lattice structure. The coordination number (CN) is the number of oppositely charged ions surrounding a given ion, depending on the radius ratio.
Lattice Energy
Lattice energy (LE) is the energy released when one mole of an ionic crystal forms from its ions. LE is proportional to the product of ion charges and inversely proportional to interionic distance. Radius increases down and left in the periodic table.
General Properties of Ionic Compounds
- Crystalline solids
- Hard and brittle
- Conduct electricity when molten or in solution
- Generally soluble in water
- High melting and boiling points
Polyatomic Ions
Polyatomic ions are groups of covalently bonded atoms with a net charge (cation or anion).
Valence Shell Electron Pair Repulsion (VSEPR) Theory
Electron pairs around a central atom repel each other, determining molecular geometry. Repulsion strength order: lone pair-lone pair > lone pair-bond > bond-bond.
Polarity of Covalent Bonds
Electrons in a covalent bond are shared equally if atoms have the same electronegativity. Unequal sharing creates a polar covalent bond. Electronegativity increases up and to the right in the periodic table.
Molecular Polarity
Diatomic molecules are polar if the bond is polar. For polyatomic molecules, polarity depends on bond polarity and molecular geometry. Nonpolar molecules have either nonpolar bonds or symmetrical arrangements of polar bonds.
Intermolecular Forces
Intermolecular forces are weaker than ionic or covalent bonds but explain the condensed states of molecular substances. Van der Waals forces are attractive and repulsive forces between molecules.
Types of Van der Waals Forces
- Dipole-dipole forces: Between polar molecules.
- Dispersion forces: Present in all substances, important for nonpolar molecules.
- Hydrogen bonds: Strong dipole-dipole interaction between molecules with H bonded to F, O, or N.
Solids
Covalent Network Solids
Examples: SiO2, C, Si, SiC. High melting and boiling points, insulators, insoluble.
Covalent Molecular Solids
Lower melting and boiling points, softer, more soluble (especially if polar or containing hydrogen bonds).
Metallic Bonding
Metals have a characteristic luster, are generally grayish, conduct heat and electricity well, and are ductile and malleable. Metallic properties persist in solid and liquid states.