CHAPTER 2

An element is a substance that cannot be broken down into other substances by ordinary chemical means. Ninety-two elements are naturally occurring. Some are commonly involved in many different biological reactions.

Carbon ( C )

Oxygen ( O )

Hydrogen ( H )

Nitrogen ( N )

Phosphorus ( P )

Sulfur ( S )

Calcium ( Ca )

Potassium ( K )

Sodium ( Na )

Chlorine ( Cl )

Many other elements are necessary for life, some in very small quantities (they are referred to as trace elements)

The basic unit of matter is the atom. One refers to atoms of any particular element. (e.g., an atom of Oxygen ). The behavior of atoms can be predicted based on their subatomic structure. We will concern ourselves with 3 types of subatomic particles: electrons, protons, and neutrons. The mass (in Daltons) of each of these particles is: 0,1,1 respectively. The electrical charge each particle possesses is -1, +1, and 0 respectively.

The nucleus of an atom consists of all of its protons and neutrons.

All atoms are electrically neutral, so the number of protons = the number of electrons.

Each element has a unique number of protons in its atom. This is called the atomic number. The mass number = the # of protons + the # of neutrons. The symbol for the element is often subscripted with the atomic number and superscripted with the mass number to convey information about the subatomic structure of the element.

Atoms with the same atomic number, but different numbers of neutrons are isotopes of each other.

Electrons have more potential energy if they are further removed from the nucleus (that is they are in a higher energy shell). Electrons tend to fill energy shells such that the shells with the lowest energy levels are filled before any having higher potential energy. The lowest electron shell holds a maximum of two electrons. All other shells behave in a stable manner if they contain eight electrons. The number of electrons in the outer shell of an atom is referred to as the number of valence electrons. Atoms behave as if they are attempting to stabilize their valence shell.

The attraction of an atom’s nucleus for electrons is referred to as the atom’s electronegativity. When atoms of approximately equal electronegativity are near each other, the electron orbitals may overlap, and the atoms may share one or more pair of electrons. This occurs if the valence electron shell becomes more stable as a result of the sharing. Atoms that share electrons move together as a unit (molecule). This sharing of electrons is referred to as a COVALENT chemical bond. It is symbolized in a structural formula with a solid line -. ( e.g., H-H ). Covalent bonds are very strong.

If the electrons are shared equally the bond is a nonpolar covalent bond. If one atom is more electronegative than the sharing is asymetrical and the bond is polar.

If the differences in electronegativity are very large, one atom may completely pull an electron away from another. This causes the donor to become positively charged and the recipient to become negatively charged. They are each referred to as ions (+ = cation, - = anion) . Cations attract anions and an ionic bond forms. The strength of an ionic bond depends on its environment. In water the bonds are weak, but out of water they are quite strong.

Hydrogen bonds may form where strongly electronegative atoms ( O or N ) covalently bond with Hydrogen. These represent very polar bonds, imparting a fractional negative charge near the Oxygen (or Nitrogen) and a fractional positive charge near the Hydrogen. The fractional charges on within any such molecule ( e.g., water H₂O ) can attract opposite fractionally charged molecules. Hydrogen bonds are very weak, but also very important for biological processes.

When molecules are very near each other, the random movement of electrons sporadically creates partial + and - charges near each other in such a way that they temporarily attract each other. Such attractions are called Van der Waals interactions and are weak.

Nonpolar molecules behave as if they avoid water. They tend to clump up or form layers which expose little of their surface to water. These clumps or layers are said to maintain their integrity through hydrophobic interactions.

The making or breaking of chemical bonds are called chemical reactions. Starting materials are called reactants, these are converted to products. Often reactions can go in either direction and are in equilibrium. The equilibrium is determined in part by the realative concentrations of the reactants and products.