Chemical Bonding: Forces, Structures, and Properties
Chemical Bonds
A chemical bond is the union between atoms, molecules, or ions. Lewis explained that atoms lose or share electrons to achieve a noble gas electron configuration, with 8 electrons in the valence layer.
Forces in Bond Formation
When atoms approach to form a bond, forces appear due to the charges in the atom:
- Attractive forces: Occur between the nucleus of each atom and the electron cloud of the other.
- Repulsive forces: Exist between the nuclei and between the electron shells.
A bond forms when the attractive forces are balanced by the repulsive forces, resulting in a minimum potential energy at a specific distance (bond distance).
Bond Structures
Links originate two types of structures:
- Molecular: Clusters of atoms with a fixed composition and specific positions, but not ordered.
- Glass: Clusters of atoms with an indeterminate, non-ordered arrangement.
Octet Rule
Atoms tend to have 8 electrons in their valence layer by acquiring the electronic structure of the nearest noble gas. Exceptions include:
- The first four elements, which tend to acquire helium’s structure with only 2 electrons (incomplete octets).
- Elements from the third period onwards, which can have expanded octets due to empty d orbitals.
- Molecules with odd valence electrons, where some atoms will not complete the octet.
Ionic Bonding
In an ionic bond, a metal and a nonmetal with large electronegativity differences bind. There is a transfer of electrons, resulting in a static electric attraction.
Ionic Compounds
Ionic compounds are crystalline solids under ambient conditions, forming crystal lattices where cations and anions are arranged in a repeating pattern.
Factors Determining Lattice Type
- Electrical factors: The net charge is zero for electrical neutrality.
- Geometric factors: Ions are distributed as compactly as possible, with the coordination number being the maximum possible.
Energy Issues
Born-Haber cycles show the energies involved in forming an ionic crystal.
- Formation energy: Energy absorbed or released when forming one mole of a product from its elements.
- Dissociation energy: Energy required to separate one mole of a substance into its constituents.
- Lattice energy: Energy absorbed or released when cations and anions approach to form one mole of an ionic compound.
Properties of Ionic Compounds
- Crystalline solids with high melting and boiling points due to high lattice energy.
- Do not conduct electricity in solid state, but do in liquid state.
- Hard, rigid, and brittle due to strong electrostatic forces.
- Soluble in polar solvents due to solvation process.