A molecule of aspirin

So what constitutes a chemical bond? A bond is formed when electrons from two atoms interact with each other and their atoms become joined. The electrons that interact with each other are VALENCE ELECTRONS, the ones that reside in the outermost electron shell of an atom.

As you sit reading this sentence, billions of atoms surround you. When any two of them approach each other, their outermost electrons come into close contact. Recall that electrons are negatively charged particles, so a chance meeting between atoms usually results in the two repelling each other. Occasionally, however, atoms will stick together by forming a chemical bond.

There are two main types of bonding discussed here. A COVALENT BOND results when two atoms "share" valence electrons between them. An IONIC BOND occurs when one atom gains a valence electron from a different atom, forming a negative ion (ANION) and a positive ion (CATION), respectively. These oppositely charged ions are attracted to each other, forming an ionic bond.

Table salt (NaCl)

Why are chemical bonds important? The type of chemical bond that occurs in a molecule or substance in part defines its properties. For example, consider sodium chloride (NaCl) and hydrogen chloride(HCl). Both substances contain chlorine, but NaCl is the white solid crystalline substance sprinkled on French fries, and HCl is a foul smelling gas.(note: when this gas is dissolved in water, it forms a solution known as hydrochloric acid. This is the acid that your stomach uses to digest food.)

How can this be if both materials have chlorine in them? The chemical bonding that takes place in NaCl is different than that in HCl. This gives NaCl and HCl very different structures, appearances, and properties.

What other differences are noticeable among molecules that result from different types of chemical bonding? Think about what happens when a bunch of sodium and chlorine ions join together to form rock salt. If we hit this with a hammer, it shatters into tiny pieces. It does this because the bonds between the atoms in rock salt are ionic. The particles are arranged in such a way that they line up along rows of positive and negative charge. Under enough stress, the salt crystals break along those lines into much smaller pieces.

The bonds that hold the carbon and hydrogen atoms in rubber together, on the other hand, are not ionic but covalent. Each carbon atom shares four of its outermost electrons with its immediate neighbors. Under stress, the bonds stretch, then snap back as each atom pulls on the shared electrons. And that's the way the ball bounces.

These silver atoms are joined by metallic bonds.

There is a third type of bonding, called METALLIC BONDING. As the name implies, metallic bonding usually occurs in metals, such as copper. A piece of copper metal has a certain arrangement of copper atoms. The valence electrons of these atoms are free to move about the piece of metal and are attracted to the positive cores of copper, thus holding the atoms together.

Essential to understanding all types of chemical bonding is realizing that all bonds use electron "glue." Every substance is made up of atoms, and all atoms are surrounded by the charged particles called electrons. Electrons in the outermost shell can and do bond with those of other atoms to create everything from quartz and quarters to plastic and pennies. In large part, the difference between materials as diverse as diamonds and pencils is how they're glued together.

Next up: a closer look at the various types of ties that bind the universe.