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Tech Topic: Interatomic Bonding

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Note: The following discussion is a simplified treatment, and as such is not 100% accurate. For more detailed coverage of interatomic bonding, consult the Molecular Bonding or Molecular Orbitals chapter of any good organic chemistry text. The theories that explain how atoms bond together to form molecules are a cornerstone of the study of chemistry.

Most simply, interatomic bonds are the sharing of electrons between two atoms. A single bond between two atoms is made up of two shared electrons (small particles that carry a negative charge of one unit). These electrons may be shared evenly, or they may be shared unevenly, with one atom pulling the electrons away from the other. Exactly how a given pair of atoms will share electrons depends on the electronegativity of each atom. Electronegativity is just a measure of how strongly an atom will pull on an electron. Think of the bond as a game of tug-of-war with two opponents, where electronegativity represents the physical strength of the opponents.

If two atoms of similar electronegativity bond together, then the electrons will be shared about evenly- this is known as a covalent bond. If two atoms of very different electronegativity bond, then the atom with the higher electronegativity will actually strip an electron away from the other atom, going from a neutral charge to a charge of -1. Since the other atom has lost an electron, it goes from a neutral charge to a charge of +1. Since opposites attract, the two atoms are held together by electrostatic attraction. This is known as an ionic bond. An atom with a non-zero charge is referred to as an ion. The positively charged atom is referred to as a cation, and the negatively charged atom is known as an anion.

Of these two major types of bonds, we are concerned most with covalent bonds. Covalent bonds are further classified into two categories. In covalent bonds where the electronegativity differences are considerable (although still not large enough for the bond to be ionic), one atom will exert more pull on the electrons. Because the electrons stay closer to this atom, it effectively has a small (less than a full unit) negative charge. The other atom effectively has a small positive charge. This unevenness of charge distribution is referred to as a dipole. A covalent bond that has a significant dipole is known as a polar bond.

Covalent bonds in which the electronegativity differences are very small, and thus in which the electrons are shared very evenly, are known as nonpolar bonds. The difference between a polar and nonpolar bond is a difference of degree; any bond between two different types of atoms will have some degree of polarity; if the bond has a lot of polarity it is considered to be a polar bond, and if it does not have a lot of polarity it is considered a nonpolar bond.

Bonds between two carbon atoms are intuitively covalent and 100% nonpolar: one carbon atom will pull on an electron exactly as hard as any other carbon atom, so the electron tug-of-war is an even match. Thus, there is absolutely no dipole, and the bond is truly nonpolar.

There can be more than one covalent bond between two atoms, and carbon is well known for forming multiple bonds with itself. A double bond between two atoms is really two single bonds (simplification!), so there are four electrons shared between the two atoms. Similarly, a triple bond is really three single bonds (another simplification!), so there are six electrons shared between the two atoms.

In reality, the two bonds composing a double bond are not exactly the same; one is higher in energy than the other, and is thus less stable and more prone to react with other chemicals. Similarly, all three bonds in a triple bond are different and have different energy values.

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