Life cannot be understood without a basic grasp of chemistry. Living creatures are made of chemical elements held together by chemical bonds. They rely on chemical reactions to grow, breathe, move, eat, react, and reproduce. When they die, chemical reactions are needed to recycle their chemical components back into the world of the living.

Atoms
Atoms are the tiny building blocks of matter. They are composed of protons, neutrons and electrons. Each of these particles has unique characteristics that together determine the behavior of the atom.

Protons are located in the nucleus, or center, of an atom. They carry a positive charge and cannot easily be separated from the other atomic particles. As the only subatomic particle to remain exclusively with the atom, the number of protons determines an atom’s atomic number.

Neutrons are also found in the nucleus of the atom, but unlike protons they carry no charge and can be added or removed from the nucleus without great difficulty. The number of protons plus the number of neutrons gives the atomic mass. Each proton and neutron has a mass of 1AMU (atomic mass unit), so an atom with 2 protons and 2 neutrons has an atomic mass of 4, and an atomic number of 2.

Electrons are not found in the nucleus but instead orbit around the protons and neutrons. Electrons carry a negative charge and can be shared, given up, or accepted by atoms. The electrons determine how reactive an atom is.

Electrons move around atoms in energy levels called shells. The shell closest to the nucleus can hold a maximum of 2 electrons. Shells farther out from the nucleus can hold more electrons. In general, the important organic molecules are most stable and least reactive when the outermost shells of their atoms contain 8 electrons, a tendency known as the octet rule.

Elements
Atoms are identified and described by their atomic number. Pure substances made up entirely of atoms with the same atomic number are called elements.

There are approximately 117 known elements, however the bulk of living tissue is made up of just a fraction of these: hydrogen, carbon, nitrogen, oxygen, phosphorous, calcium, iron, magnesium, potassium, iodine, sulfur, chlorine, and sodium.

All of the naturally occurring elements as well as the handful of manmade ones can be arranged on a chart known as the Periodic Table. Each element is represented by a symbol (usually but not always the first letter of its name) of the table, a symbol which is used in chemical formulas. Carbon is represented by the letter C and hydrogen by H, but potassium is abbreviated with a K.

Also included in the periodic table is the full name, the atomic number and the atomic mass of each element. The layout of the table is based on atomic number and the number of electrons in the outer shell. This arrangement creates clusters of elements that have similar properties, and helps explain the behavior of elements during biochemical reactions.

Bonding
Individual atoms are held together to form elements and compounds by bonds. Most bonds involve the sharing or transfer of electrons, so as to satisfy the octet rule.

Some elements are inert – their atoms have full electron shells, which makes them unlikely to react or combine with other elements. Most elements have unfilled electron shells, which makes them candidates for bonding.

There are three primary types of chemical bonds: ionic, covalent and hydrogen. These bond types are distinguished by the distinctive behavior of the electrons involved.

Ionic Bonds

In an ionic bond, electrons are either given up or accepted in order to fill an outermost shell. Consider sodium chloride, composed of sodium with an atomic number of 11, and chlorine with an atomic number of 17. The outermost shell of sodium has only 1 electron. It is easier to give up that single electron and become positively charged than it is to gain 7 more electrons to fill up the shell.

Chlorine, on the other hand, has 7 electrons in its outer shell. It can fill the shell by picking up the cast-off electron from sodium, which also gives the chlorine a negative charge. The positive charge on the sodium attracts the negatively charged chlorine and creates a new molecule, sodium chloride (NaCl).

Covalent Bonds

Covalent bonds are strong bonds formed when atoms share electrons. In chlorine gas (Cl2) molecules, a pair of electrons is shared between two chlorine atoms. The shared electrons act together to satisfy the needs of both atoms.

Although molecules held together by covalent bonds have an overall neutral charge, there may be charges on either side of a molecule that balance one other out. Called polar molecules, these kinds of compounds act like bar magnets, positive on one end and negative on the other. A non-polar molecule has no charge variations around its perimeter.

Hydrogen Bonds

Hydrogen bonds are weak bonds formed from the attraction of the positive and negative ends of polar molecules. For example, water (H2O) is a polar covalent molecule with a positive end on the hydrogen side and a negative end on the oxygen side.

The positive hydrogen end of one water molecule is attracted to the negative oxygen end of another water molecule, forming a weak hydrogen bond.