Chemical Bonds and Crystal Structure Fundamentals

Chemical Bond Fundamentals

Chemical Link: A set of forces that hold atoms or molecules together when they form crystals, as well as the forces holding molecules when present in solid or liquid states.

Bond Energy

Bond Energy: $E_b$ is the energy evolved in the formation of a link and results from a balance between the energy of attraction and repulsion. The attractive forces are due to the attractions that occur between the electron cloud of one atom and the nucleus of another atom, while the repulsive forces ($E_b$) are due to repulsions that occur between nuclei or between the electron clouds of the two atoms.

Interatomic Distance

Distance Liaison: The distance between the atoms when they form the link. At closer distances, the repulsion forces are greatly increased, and the system becomes unstable.

Electronegativity and Bond Type

Electronegativity: Determines the behavior of the atom and the type of link formed.

• If both atoms have very different electronegativities, one atom will lose an electron, forming a negative ion (anion). This results in an Ionic bond.
• If both atoms have a great tendency to attract electrons, they link by sharing their valence electrons, forming a Covalent bond.
• If the two atoms have very little tendency to attract electrons, both will give up their valence electrons, forming a structure in which cations are stabilized by free electrons, giving rise to Metallic bonding.

Ionic Bonding Details

Ionic Bond: This is formed when elements with very different electronegativities combine. The most electronegative element (non-metal) forms negative ions, and the least electronegative element (metal) forms positive ions. The binding occurs through electrostatic attraction between ions with opposite charges.

Lattice Energy

Lattice Energy: This is the energy required to separate one mole of an ionic compound so that its constituent ions are at an infinite distance where they cease to interact.

Structure of Ionic Crystals

Structure of Ionic Crystals: The way in which the constituent ions arrange themselves in an ionic crystal lattice depends crucially on the proportion of cations and anions and their respective sizes.

Coordination Number

Coordination Number: The number of ions of one type surrounding a central ion of the opposite sign in its nearest field. Its value is a fundamental characteristic of the type of network formed.

Factors Affecting Ionic Bond Strength

Factors affecting the strength of ionic bonding:

• Ionic Charges: An ionic compound is more stable the lower the charge on its ions.
• Size of Ions: An ionic compound is more stable the more compact the lattice is and the more similar the sizes of the anion and cation.
• Electronegativity Difference: An ionic compound will be more stable the larger the electronegativity difference between the constituent atoms.