Ionic, Covalent, and Metallic Bonds: Properties
Ionic Bonds
Ionic bond: A transfer of electrons between a metal and a nonmetal, both achieving a stable configuration. It consists of the union of atoms with opposite electrical charges, held together by forces of electrostatic attraction. This attraction occurs in three dimensions, forming a compact, three-dimensional, neutral lattice of alternating cations and anions.
Properties of Ionic Compounds
- Solid substances at room temperature, existing in a crystal lattice.
- Very high melting and boiling points, due to strong electrostatic forces.
- To melt or dissolve, the crystal lattice must be broken, which is very stable due to the electrostatic attraction between ions of opposite sign. The higher the absolute value of the lattice energy (UR), the higher the melting and boiling points.
- Low compressibility, making them hard and brittle. They are hard because of the intense electrostatic attractions; to scratch them, bonds must be broken, overcoming the electrostatic attraction. Therefore, they will be harder if the electrostatic attraction is stronger (+ Stable, + UR, + Hard). They are brittle because sliding layers upon each other causes ions with the same sign to face each other, resulting in electrostatic repulsion and breaking the network.
- Soluble in polar solvents (like water). When introduced into the solvent, the small charges are immediately surrounded by it, breaking the lattice. They will be more soluble at lower values of UR.
- Electrical and thermal conductivity is not present in their natural state. In solids, cations and ions are maintained in a fixed position and cannot move. They only conduct electricity when melted or in solution.
Covalent Bonds
Covalent bonds: A bond between atoms justified by electron sharing. They have less energy than two isolated atoms; their formation releases energy.
Polarity
- Nonpolar bond: If the atoms are the same or have similar electronegativities, the electron pair is in the middle of both.
- Polar bond: If electronegativities are different.
Properties of Covalent Substances
Atomic Substances
- At room temperature, they are crystalline solids with high melting points. To melt them, the lattice of covalent bonds would have to be broken.
- Just driving the C (graphite), as there is electronic mobility through the links between layers.
- Not soluble in water, being compact.
Molecular Substances
- At room temperature, they can be solids, liquids, or gases.
- Low melting and boiling points, as to melt them, only the intermolecular forces need to be broken, never the covalent bond.
- Do not conduct electricity, except for very polar aqueous solutions.
- Nonpolar molecular compounds (like gasoline) are soluble in apolar substances (like benzene).
- Polar covalent compounds dissolve in polar solvents (like water or ethyl alcohol).
Metallic Bonds
Metallic Bond: Compact packaging.
Properties of Metallic Substances
- Metallic crystal lattice, where cations are perfectly ordered in space and electrons move freely throughout the crystal. The crystal structures are mainly cubic or hexagonal.
- High electrical and thermal conductivity, due to the high mobility of valence electrons.
- High deformability, as any plane can be moved without ever facing cations, making them ductile and malleable.
- Higher melting and boiling points, as the atoms are bound very strongly, requiring a lot of energy to separate them.
- High density; the mass is very large compared to the volume.
- Electron emission or photoelectric effect.
Intermolecular Forces
- Van der Waals forces: Present in polar molecules like HCl, PH3, H2S, and CO.
- Hydrogen bond: Occurs in polar molecules with a hydrogen atom and a small, highly electronegative atom (N, O, F). It is the strongest type of intermolecular bond.
Born-Haber Cycle
The Born-Haber Cycle describes, from an energy standpoint, ionic bond formation and all the intermediate steps needed to form one mole of an ionic compound from its elements in their most stable state (lowest energy).
Lattice Energy
Lattice energy is the energy released in the process of ionic bond formation from ions in a gaseous state. It is what justifies this process thermodynamically.
Summary of Bond Types
- Ionic bond: Non-metals reach a noble gas configuration, and metals achieve it by gaining or losing electrons in their valence level.
- Covalent bond: Atoms reach the electronic octet by sharing electron pairs. Occurs between non-metals and non-metals with hydrogen.
- Metallic bond: Atoms achieve noble gas configuration by transferring their valence electrons and becoming cations, forming a lattice.