Ionic compounds are compounds that contain ions—one or more cations (positive ions) and one or more anions (negative ions), and are held together by ionic (electrovalent) bonds. Ionic bonds typically occur between a metal and a non-metal.
- Ionic compounds are usually hard solids, and they tend to have high melting and high boiling points.
- Ionic compounds can dissolve in water, but they're less soluble in organic solvents like alcohol, because organic solvents are non-polar and therefore cannot break the ionic bonds.
Covalent compounds are compounds that are formed from covalent bonds, which occur when one or two atoms share electrons, specifically pairs of electrons. Covalent bonds most commonly occur between two or more non-metals or similar metals.
1. Covalent compounds are mostly liquids or gasses, but they can also be soft solids, and they tend to have low melting and boiling points.
2. They are soluble in most organic solvents, but they don't usually dissolve in water, as water is a polar solvent and covalent compounds are mainly non-polar. Some covalent compounds, however, can dissolve in water and when they do, they break into uncharged molecules; sugar, for example, is a covalent compound that can dissolve in water.
A polar bond is a covalent bond in which electrons are unevenly shared between two or more atoms that have different electronegativities. A polar molecule is a molecule in which one of its poles is more positive, and the other pole is more negative, while a non-polar molecule has no positive or negative poles, as there are no polar bonds. However, a molecule that has a symmetrical shape or has polar bonds that are evenly distributed can be non-polar, as the bond dipoles cancel each other out. For example, carbon dioxide (CO2) has two polar C-O bonds, but due to its linear molecular geometry, it is considered a non-polar molecule. You can read more about the polarity of CO2 here.