Covalent compounds are a second kind of chemical compound. Unlike ionic compounds, which are made with ionic bonds or bond between differently charged ions, covalent compounds and covalent bonds do not use charges. Instead of giving and taking electrons, covalent bonds form when two atoms share electrons. Because they are sharing the electrons, both atoms can count the shared electrons towards their ideal of eight electrons. Covalent bonds are weaker than ionic bonds, but they make up for it with the fact that any two atoms can have up to three covalent bonds with each other at a time. A single bond is when each atom shares one electron, making a single bond. When atoms share two electrons each, this is called a double bond because two bonds are formed. And when atoms share three electrons each it is called a triple bond because three bonds are formed. Just like you might expect, single bonds are the weakest, and triple bonds are the strongest. This behavior makes for some interesting properties:
  • Covalent compounds happen between two non-metals or between a non-metal and a semi-metal.
  • Most covalent compounds have much lower melting and boiling points than ionic compounds. Think about the gases in the air--all of them are formed with covalent bonds.
  • Diatomic elements are bound together by covalent bonds.
  • Double and triple bonds are very strong, meaning that substances such as carbon dioxide and nitrogen gas are very stable. It is because of the strength of these bonds that fire and explosives exist. When nitrogen gas or carbon dioxide are formed, lots of energy is released in the form of heat and light (and sometimes pressure).
  • Materials and objects made with covalent compounds are usually strong in tension and stretch rather well. All oil and wood products are made of covalent compounds such as plastics, paper, cloth, rope, etc.
  • The element carbon almost always forms covalent compounds. This has led to an entire branch of chemistry call organic chemistry
  • Polyatomic ions are covalent compounds that have an ionic charge as a unit.
  • Some covalent compounds can link together like a crystal and act as one very large molecule. These are called network solids and are very hard and fairly brittle. Diamond and silicon are well-known examples of these.
  • Another kind of covalent compounds is called amorphous solids. They are like network solids, but they don't exist in a crystal form, instead the atoms and bonds are all over the place very chaotically. Glass is the best known example of an amorphous solid.