Ionic Bonding

Ionic Bonds

The ionic bond is one of three (only two are discussed at this site) intramolecular bonds. They generally deal with a combination of metals (e.g. K, Na, Al, Fe), which are the elements on the left side of the periodic table, and nonmetals (e.g. O, F, S, Cl), which are located on the right side of the periodic table. This is due to the fact that metals have low ionization energies, which allows them to lose electrons quite easily, and that nonmetals have a high tendency to gain one or more electrons, also known as a high affinity for electrons. Therefore, the bonding electrons are strongly pulled towards the anion, which is why the anion takes on a negative charge and the cation a positive one. The fact that the bonding electrons are pulled very strongly towards one side of the compounds is why the electronegativity of ionic compounds are very high.

Monoatomic and Polyatomic compounds

Two type of ionic compounds exist. The first type deals strictly with monoatomic anions, which are just ionized elements. One example of such a compound is Al2O3. In this case, even though 2 moles of aluminum and 3 moles of oxygen are combined, it is still monoatomic due to the fact that oxygen is the lone anion with a charge of -2. Polyatomic ionic compounds (say that ten times fast) deal with compounds that contain larger, more complex anions, such as SO4- or NO3-. One example of a polyatomic compound is KMnO4. In this case, K is the cation with a charge of +1, and MnO4 is the anion with a charge of -1.

Crystal Lattice

All of the above ionic compounds, both monoatomic and polyatomic, form crystal lattices of alternating cations and anions in a 3D structure, where in the case of NaCl, is similar to a Rubik's Cube(r) (or (c) or whatever), with the corners of each individual cube containing alternating cations or anions. However, different compounds have different structures, which result in different densities. In addition, all crystal lattices are constructed of a unit cell that is different for certain types of compounds, which is the basic structure that is repeated throughout the lattice. That's about all you need to know about crystal lattices.

Enthalpy of Dissociation

The strength of attraction between a cation and an anion is the enthalpy of dissociation, which is the energy required to decompose an ionic compounds to its gaseous ions. The stronger the attraction between the two ions, the higher the enthalpy of dissociation. The enthalpy of dissociation is proportional to the charge of the cation multiplied by the charge of the anion divided by the distance between the two ions.

The complete equation for finding the energy is:

One trend that is observed between ion radius and enthalpy of dissociation is that the larger the radius of the ion (cation or anion), the smaller the enthalpy of dissociation. This is due to the increase in the distance between the two ions, which decreases energy (see either equation). In addition to the size of the ions, the charge of the ions are equally important. Usually, the charge of the ions determine the energy, since the two values are multiplied togethere, whereas the distance is only a divisor. On a side note, the enthalpy of dissociation is a good determiner of comparing the melting temperatures of different ionic compounds because the melting point occurs when the structure of the ionic compound collapses.

When dissolving an ionic solid, the stronger the enthalpy of dissociation, the less soluble it will be in solvent. This is due to the stronger "force" that the solvent requires to pull the ions apart.