Chemical Bonding Fundamentals and Material Characteristics

Introduction to Chemical Bonding

Chemical bonding is the force that binds atoms, ions, or molecules together to form various substances. When two molecules are linked by intermolecular forces, these are also considered types of attractive forces.

The coordination number refers to the number of ions surrounding a central ion in a crystal lattice.

Types of Chemical Bonds

Ionic Bonds

Ionic bonds result from the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). Each positive ion is typically surrounded by the largest possible number of negative ions, and vice versa, forming a crystal lattice.

Lattice Energy

Lattice energy (E_lattice) is the energy released when one mole of a solid ionic compound is formed from its gaseous ions.

Born-Haber Cycle: Indirect and Direct Paths

The Born-Haber Cycle is a thermochemical cycle that allows for the indirect calculation of the lattice energy of an ionic compound. It is based on the principle that the formation of an ionic compound can be achieved in two ways:

• Indirect Path: The elements react to form a solid ionic crystal through a series of steps.
• Direct Path: The elements are converted into gaseous ions, which then condense to form the solid ionic compound.

Steps involved in the direct path (example for LiF):

• Vaporization of one mole of Lithium (Li) atoms.
• Dissociation of half a mole of Fluorine (F₂) molecules.
• Ionization of one mole of Lithium (Li) atoms (formation of Li⁺).
• Electron affinity of one mole of Fluorine (F) atoms (formation of F^-).
• Condensation of gaseous Li⁺ and F^- ions to form one mole of solid LiF.

Covalent Bonds

Covalent bonds are formed by the union of two atoms that share one or more pairs of electrons.

Lewis Model

In the Lewis Model, symbols representing elements are surrounded by dots that simulate the valence electrons (electrons in the outermost shell). These dots are used to show how electrons are shared or transferred during bond formation.

Valence Bond Theory

According to the Valence Bond Theory, when two atoms are joined by covalent bonding, their atomic orbitals overlap and mix. The stronger the overlap, the stronger the bond.

The number of covalent bonds an atom can form is often related to its number of unpaired valence electrons.

Bonding Parameters and Properties

• Bond Enthalpy

  Bond enthalpy (E_bond) is the energy required to dissociate one mole of a specific type of bond and obtain the constituent elements in their gaseous state.

• Bond Length

  Bond length is the average distance between the nuclei of two atoms joined by a covalent bond.

• Dipole Moment

  The dipole moment measures the polarity of a bond or molecule. It indicates how much the electrical charge is unevenly distributed.

Polar and Nonpolar Bonds

• Polar Bonds

  In polar bonds, one atom is more electronegative than the other, causing the shared negative electrical charge to shift towards the more electronegative atom, creating partial positive and negative poles.

• Nonpolar Bonds

  In nonpolar bonds, electrons are shared equally between atoms, typically when the atoms have similar or identical electronegativity.

Metallic Bonds

Metallic bonds occur when metal atoms come together and are arranged to form a crystal lattice. The valence electrons are delocalized and form a"sea of electron" that surrounds a lattice of positive metal ions.

Intermolecular Forces

Intermolecular forces are attractive forces that exist between molecules.

Van der Waals Forces

Van der Waals forces are weak intermolecular forces, including:

• Dipole-dipole forces: Occur between polar molecules.
• Dipole-induced dipole forces: Occur between a polar molecule and a nonpolar molecule.
• London Dispersion Forces (induced dipole-induced dipole): Occur between all molecules, arising from temporary fluctuations in electron distribution that create instantaneous dipoles.

Hydrogen Bonds

Hydrogen bonds are a special type of strong dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom like Fluorine (F), Oxygen (O), or Nitrogen (N).

Properties of Substances Based on Bonding

Ionic Substances

• Composed of positive and negative ions in a crystal lattice.
• Generally hard and brittle.
• Have high melting points (typically 600-3000 °C).
• Are non-conductive in solid state but conduct electricity when molten or dissolved in water.

Covalent Molecular Substances

• Composed of discrete molecules.
• Held together by Van der Waals forces and sometimes Hydrogen bonds.
• Tend to be very soft.
• Have low melting points (e.g., -272 to 400 °C).
• Are typically non-conductive.

Covalent Network Solids (Atomic)

• Composed of atoms covalently bonded in a continuous network.
• Examples include diamond and silicon dioxide.
• Are typically very hard.
• Have very high melting points (e.g., 1200 to 3600 °C).
• Are generally non-conductive (with some exceptions like graphite).

Metallic Substances

• Involve a lattice of positive metal ions surrounded by a delocalized electron cloud.
• Held together by metallic bonds.
• Vary from soft to very hard.
• Have a wide range of melting points (e.g., -39 °C for Mercury to 3400 °C for Tungsten).
• Are excellent electrical and thermal conductors.