The connections between the atoms in a compound are called chemicalbonds. Atoms form bonds by sharing their electrons with each other,relying on the power of electric charge to keep themselves attached.Molecules and compounds can also bond with each other. Important bondsbetween atoms are covalent and ionic bonds. Bonds between molecules orcompounds are called dipole-dipole bonds.
Covalent bonds
Bonds formed through the more or less equal sharing of electrons between atoms are known as covalent bonds.
If the electrons in a covalent bond are shared equally, the resulting bond is called a
nonpolar covalent bond.When one atom pulls the shared electrons toward itself a little moretightly than the other, the resulting covalent bond is said to be a
polar bond.In a polar bond, the atom that pulls electrons toward itself gains aslight negative charge (because electrons have a negative charge).Since the other atom partially loses an electron, it gains a slightpositive charge. For example, the atoms in water form polar bondsbecause oxygen, which has eight protons in its nucleus, has a
greaterpull on electrons than hydrogen, which has only one proton.
Ionic Bonds
Polar covalent bonds involve the unequalsharing of electrons. This inequality is brought to an extreme in abonding arrangement called an ionic bond. In an ionic bond, one atompulls the shared electrons away from the other atom entirely. Ionicbonds are stronger than polar bonds.
One example of ionic bonding is the reaction between sodium (Na) and chlorine (Cl) to form table salt (NaCl).The chlorine atom steals an electron from the sodium atom. Because itloses an electron, the sodium atom develops a charge of +1. Thechlorine atom has a charge of –1, since it gained an electron.
Dipole-Dipole Bonds
As seen in polar covalent compounds, due tothe unequal sharing of electrons, some molecules have a slightlypositive and a slightly negative end to them, or a dipole (di-pole =two magnetic poles). These compounds can form weak bonds with oneanother without combining together completely to create new compounds.This type of bonding, known as dipole-dipole interaction, takes placeswhen the positively charged end of one polar covalent compound (
d+) comes in contact with the negatively charged end of another polar covalent compound (
d–):
Dipole-dipole interactions are much weakerthan the bonds within molecules, but they play a very important role inthe chemistry of life. Perhaps the most important dipole-dipole bond inbiochemistry (and on the
SAT II
Biology) is the dipole-dipoleinteraction between positively charged hydrogen molecules andnegatively charged oxygen molecules. This reaction is so important, itgets its own special name:
hydrogen bond. These bonds accountfor many of the exceptional properties of water and have importanteffects on the structure of proteins and DNA.
