A covalent bond is a bond formed when two atoms share a pair of electrons. A neutral group of atoms bonded together covalently is called a molecule, and a substance which is made up of molecules is called a molecular substance. Covalent molecules make up many common substances, including plastics, paper, and human tissue.

Another type of bond that occurs when electrons are transferred between atoms is called an ionic bond(or electrovalent bond). Compounds held together by ionic bonds are called ionic compounds. Molecular compounds have lower melting and boiling points than do ionic compounds. Molecular compounds often occur as individual molecules, whereas ionic compounds occur as a crystal lattice. This crystal lattice structure gives the ionic compounds their higher melting and boiling points.

One characteristic that both ionic and covalent compounds share is that they both adhere to the octet rule. The octet rule...