Ionic Bonds

The Formation of NaCl: Sodium Chloride

An ionic bond forms when two atoms transfer electrons to fill their outermost energy level and then find themselves attracted to each other because of the resulting electrostatic charge differences. A perfect example of this process is the formation of a sodium chloride crystal. Chlorine only requires one additional electron to fill its outermost shell, while sodium needs to give one up to be left with its last level complete. Sometimes when these two atoms come in contact, they transfer an electron to satisfy these complementary requirements. After the transfer, though, chlorine has one more electron than proton, giving it an overall negative charge, and sodium is in the opposite position with a sum positive charge. Opposite charges attract, so the two particles stick together in an ionic bond.

Ionic bonds are generally fairly weak, and can be broken easily when subjected to heat or submerged in water.

Covalent Bonds
Covalent bonds form when two atoms that want to fill their outermost energy level come in contact, but don't have enough energy to completely capture or release an electron. Instead, these atoms share electrons. By sharing electrons, atoms form powerful bonds with each other and become molecules. Because some atoms, such as carbon, can form multiple covalent bonds simultaneously and also create strong double or even triple bonds, long chains of atoms can combine to make huge, complex molecules. A single DNA molecule, for example, can sometimes have up to fifty billion atoms.

Hydrogen Bonds
Hydrogen bonds are the very weak bonds between two or more polar molecules. A polar molecule is a molecule that has a slight positive charge at one end and a slight negative charge on the other (giving it poles, like the north and south pole of the earth). Water (H2O) is an example of such a molecule. Because of the way the electrons are arranged on the oxygen atom, water's two slightly positive hydrogen atoms tend to stay on one side of the molecule. This imbalance creates a positive pole near the hydrogen atoms and a negative one on the opposite side. A hydrogen bond forms, then, when one of these poles is attracted to an opposite pole on another molecule.

Perhaps the most famous example of hydrogen bonding is in DNA: this is the force that connects two strands together into a double helix. It is the perfect type of bonding for this application- it is strong enough to keep the structure together, yet weak enough for an organism to disconnect the two strands in order to access the delicate genetic data stored within.