For covalent compounds, the representative particles are molecules. Molecules are electrically neutral groups of atoms that are held together by covalent bonds. Covalent bonds are formed when atoms share electrons in order to achieve a stable electron configuration. Molecules can be composed of two or more atoms of the same element (e.g., H2, O2) or different elements (e.g., H2O, CO2). The properties of a covalent compound are determined by the atoms that make up the molecule and the way in which the atoms are bonded together.