Molecular compounds are held together on an atomic level by chemical bonds. Three types of chemical bonds include ionic bonds, metallic bonds, and covalent bonds. The types of chemical bond influence the physical properties of the molecular compounds they form.
Molecular compounds held together by ionic bonds are salts. An ionic bond is a chemical bond between two oppositely charged ions. Typically, metals lose valence electrons (loose electrons in their outer shell of orbiting electrons) to become positively charged cations, whereas the nonmetal accepts electrons to become negatively charged anions. For example, common salt (NaCl) has ionic bonds between sodium (Na+) has a positive charge and chlorine (Cl-) has a negative charge. Salts readily dissolve in water as their charged ions are attracted to parts of water molecules that can also have positive and negative charges. As water evaporates, the ions dissolved in water will precipitate again as salts. Natural salts like halite (NaCl) and gypsum (CaSO4) are generally soft minerals and can dissolve in water.
Figure 1.21. Salt crystals are held together by ionic bonds. Salt compounds dissolve in and precipitate from water.
Metals are held together by metallic bonds. Compounds with metallic bonds transmit electricity. With metallic bonds, the valence electrons disassociated from orbiting a single atom and become more of a cloud electrons that surround the positively charged nuclei of interacting metallic ions. Metalloids are intermediate between those of metals and solid nonmetals. Although most elements are metals (all those on the left and center parts of the Periodic Table), only a few elements occur naturally in metallic form including gold, platinum, copper, iron, and mercury (in liquid form). Some minerals are metalloid compounds including pyrite (FeS2), magnetite (Fe3O4), and galena (PbS).
Molecular compounds held together by covalent bonds are non-metallic compounds. Covalent bonds occur when two or more atoms share orbiting electrons, creating more stability in the valence shell of electrons between the bonding elements. These materials can form crystal complexes and do not transmit electricity and tend to be harder, more durable compounds. For instance, most gem minerals are non-metallic compounds with covalent bonds. The mineral quartz (SiO2) is a non-metallic crystalline compound (see Figure 1.24).
Van der Waals forces (bonds) are weak, nonspecific forces between molecules and include attractions and repulsions between atoms, molecules, and surfaces. Van der Waals forces are responsible for friction and what makes water sticky.