Ionic compounds form when electrons transfer from one atom to another. Covalent compounds form when atoms share electrons, resulting in no net loss or gain of electrons as seen in ionic compounds.
When two or more different chemical elements combine, they form a compound. How those atoms combine or bond together depends on the element and its characteristics. The bond that holds the atoms together determines the type of compound it is, and these can be either ionic or covalent.
Bonding Basics Atoms are classified and characterized by the number of electrons they contain. Each element has a different number of electrons housed in the orbitals going around the nucleus. The outer orbital is known as the valence orbital, and these electrons are the ones used in the bonding process. Each atom seeks to have a full outer orbital of electrons in order to be stable. For most atoms, this outer orbital holds eight electrons. The octet rule refers to the requirement of electrons to gain, lose or share electrons in order to fill the outer orbital. When atoms gain or donate electrons to another element, they form ionic compounds. When atoms share electrons with another element, they form covalent compounds.
Ionic compounds form between metals and nonmetals. Metals typically have few valence electrons in the outer orbital while nonmetals have many. The metal acts as a donor and transfers its valence electrons to the nonmetal, thus satisfying the octet rule necessary to form a stable atom. This results in a positively charged metal atom and a negatively charged nonmetal atom, known as ions. Because of the opposite charges, the two ions are attracted to one another and stay close together, thus forming an ionic compound. More than one electron can be transferred in this process. Because the charged atoms are floating freely, they are attracted to other atoms with the opposite charge, not just the single atom that they donated or received electrons from.
The characteristics of ionic compounds are that they are crystalline solids with high melting and boiling points. They conduct electricity when melted, and they are soluble in water but not non-polar liquids. One of the most common ionic compounds is sodium chloride (NaCl), also known as table salt.
In contrast, covalent compounds typically occur between nonmetals or two atoms of the same or similar elements. Atoms with similar electronegativities, which can be determined from the periodic table, tend to attract one another and form covalent bonds. Rather than transferring the valence electrons completely, the atoms share the outer electrons to satisfy the octet rule. The shared electrons can move back and forth between the bonded atoms. Neither atom is gaining or losing an electron in this type of bond, so no ions are formed. Because of this, the atoms in the compound are not attracted to other atoms nearby like those seen in ionic compounds.
Covalent compounds can be solids, liquids or gases. They tend to be poor electrical conductors and have low melting and boiling points. They tend to be soluble in non-polar liquids but not water. Some common examples of covalent compounds are methane (CH4) and carbon dioxide (CO2).