7.2: Contrasting Ionic Compounds and Covalent Compounds
- Page ID
- 366535
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- Describe ionic compounds as an extended three dimensional array, lattice structures, versus discrete molecules in covalent compounds.
- Explain that the formula of an ionic compound specifies the ratio of cations to anions, while the molecular formula specifies that number of atoms in the molecule.
We have seen that ionic and molecular compounds differ greatly from each other on the particle level. These structural differences lead to very different physical properties on the macroscopic level.
Physical Properties of Molecular Compounds
The physical state and properties of a particular compound depend in large part on the type of chemical bonding it displays. Covalent compounds, sometimes called molecular compounds, display a wide range of physical properties due to the many different sizes, shapes, and compositions of molecules. The melting and boiling points of covalent compounds are generally quite low compared to those of ionic compounds. This is because melting of ionic compounds involves breaking ionic bonds whereas the melting of covalent compounds involves disrupting the weak forces between molecules. When covalent compounds melt, the covalent bonds in the molecules are not broken.
Since covalent compounds are composed of neutral molecules, their electrical conductivity is generally quite poor, whether in the solid or liquid state. Ionic compounds do not conduct electricity in the solid state because of their rigid structure, but conduct well when either molten or dissolved into a solution. The water solubility of covalent compounds is variable and depends on the structure of the molecule. Many, but not all, ionic compounds are quite soluble in water. The table below summarizes some of the differences between ionic and covalent compounds.
Also note that the chemical formula of a covalent compound represents something slightly different than the chemical formula of an ionic compound. The chemical formula of a covalent compound represents the identity and actual number of atoms that compose a molecule. The chemical formula of an ionic compound, on the other hand, represents the simplist whole number ratio of anions to cations in the crystal.
| Property | Ionic Compounds | Covalent Compounds |
| Type of elements | Metal and nonmetal | Nonmetals only |
| Bonding | Ionic - attraction between anions and cations | Covalent - sharing of pair(s) of electrons between atoms |
| Representative unit | Formula unit | Molecule |
| What the formula represents | The ratio of cations to anions | The type and number of atoms in the molecule |
| Physical state at room temperature | Solid | Gas, liquid, or solid |
| Water solubility | Usually high | Variable |
| Melting and boiling temperatures | Generally high | Generally low |
| Electrical conductivity | Good when molten or in solution | Poor |
| State when dissolved in water | Separates into ions | Remains molecules |
Table 5.8.1: Comparison of Ionic and Molecular Compounds