The terms "ionic" and "covalent" are used to refer to the types of bonds within compounds. An understanding of basic chemistry is essential to comprehend the significant differences between these two types of bonds. Atoms in molecules are bonded by the way in which they share electrons, and these may be ionic bonds or covalent bonds.
In a covalent bond, atoms have the ability to combine, which they do by sharing electrons. They may share a pair of electrons or several pairs depending on the elements involved. In the case of two atoms of fluorine that combine to form a molecule, the bond is covalent. By sharing a pair of electrons, the molecule achieves a state in which each atom has an octet of valence electrons. When two atoms of oxygen form a molecule, they form a covalent bond in which they share two pairs of electrons.
The difference between covalent and ionic bonds explains why water-based solutions of ionic compounds are conductors, whereas water-based solutions of covalent compounds do not conduct electricity. When a salt is dissolved in water, it releases ions into the solution. The ions flow through the solution and produce an electric current. Covalent compounds, on the other hand, release neutral molecules into the solution. These neutral particles are unable to conduct electricity.
The manner and extent to which bonded atoms share a pair of electrons differs in ionic and covalent bonds. Any bond requires a sharing of electrons. The bond is ionic if one of the atoms more strongly attracts electrons toward itself than the other atom. When the atoms in a bond have equal ability to attract electrons, the bond is covalent.
Metals and nonmetals often react with each other to form salts or ionic compounds. Nonmetals also combine with other nonmetals to form covalent compounds. This general rule has limitations in that the type of bonding in compounds cannot be neatly divided into ionic and covalent in many instances because they have bonding that lies somewhere in between. Another difference is that ionic compounds tend to have high boiling points as well as high melting points. For this reason, at room temperature, ionic compounds are in the solid state. Covalent compounds tend to be in the gaseous state at room temperature.