Chemistry
Elements:
All matter, living and nonliving, is composed of chemical elements, substances that cannot be broken down into simpler substances by chemical reactions. There are 90-95 naturally occurring elements, ranging from hydrogen (the lightest) to uranium (the heaviest). In addition to the naturally occurring elements, about 17 elements heavier than uranium have been made in laboratories by bombarding elements with subatomic particles.
Instead of writing out the name of each element, chemists use a system of abbreviations called chemical symbols-usually the first one or two letters of the English or Latin name of the element. For example, O is the symbol for oxygen, C for carbon, Cl for chlorine, and Na for sodium (its Latin name is natrium).
Atoms:
The atom is the smallest subdivision of an element that retains the characteristic chemical properties of that element. Atoms are almost unimaginably small, much smaller than the tiniest particle visible under a light microscope. An atom is composed of smaller components called subatomic particles-protons, neutrons, and electrons. Protons have a positive electrical charge; neutrons are uncharged particles with about the same mass as protons. Protons and neutrons make up almost all of the mass of an atom and are concentrated in the atomic nucleus. Electrons have a negative electrical charge and an extremely small mass (only about 1/1800 of the mass of a proton). The electrons spin in the space surrounding the atomic nucleus.
Each kind of element has a fixed number of protons in the atomic nucleus. This number, called the atomic number, determines the chemical identity of the atom. The total number of protons plus neutrons in the atomic nucleus is termed the atomic mass. For example, the element oxygen has eight protons and eight neutrons in the nucleus; it therefore has an atomic number of 8 and an atomic mass of 16. Where an atom is uncombined, it contains the same number of electrons as protons. Some kinds of chemical combinations and certain other circumstances change the number of electrons, but chemical reactions do not affect anything in the atomic nucleus. Because electrons and protons have equal but opposite charges, an uncombined atom is electrically neutral. The way electrons are arranged around an atomic nucleus is referred to as the atom's electronic configuration. Knowing the locations of electrons enables chemists to predict how atoms can combine to form different types of chemical compounds.
An atom may have several energy levels, or electron shells, where electrons are located. The lowest energy level is the one closest to the nucleus. Only two electrons can occupy this energy level. The second energy level can accommodate a maximum of eight electrons. Although the third and outer shells can each contain more than eight electrons, they are most stable when only eight are present. We may consider the first shell complete when it contains two electrons and every other shell complete when it contains eight electrons.
The energy levels correspond roughly to physical locations of electrons, called orbitals. There may be several orbitals within a given energy level. Electrons are thought to whirl around the nucleus in an unpredictable manner, now close to it, now farther away. Orbitals represent the places where electrons are most probably found.
Isotopes:
Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons. Isotopes, therefore, have different atomic mass numbers. The three isotopes of hydrogen contain zero, one, and two neutrons, respectively. Elements usually occur in nature as mixtures of isotopes.
All isotopes of a given element have essentially the same chemical characteristics. Some isotopes with excess neutrons are unstable and tend to break down, or decay, into a more stable isotope (usually of a different element). Such isotopes are termed radioisotopes, since they emit high-energy radiation when they decay.
Molecules:
Two or more atoms may combine chemically to form a molecule. When two atoms of oxygen combine, for example, a molecule of oxygen is formed. Different kinds of atoms can combine to form chemical compounds. A chemical compound is a substance that consists of two or more different elements combined in a fixed ratio. Water is a chemical compound in which each molecule consists of two atoms of hydrogen combined with one atom of oxygen.
Chemical Bonds:
The chemical properties of an element are determined primarily by the number and arrangement of electrons in the outermost energy level (electron shell). In a few elements, called the noble gases, the outermost shell is filled. These elements are chemically inert, meaning that they will not readily combine with other elements. The electrons in the outermost energy level of an atom are referred to as valence electrons. The valence electrons are chiefly responsible for the chemical activity of an atom. When the outer shell of an atom contains fewer than eight electrons, the atom tends to lose, gain, or share electrons to achieve an outer shell of eight. (The exceptions are zero or two electrons in the case of the lightest elements, hydrogen and helium.)
The elements in a compound are always present in a certain proportion. This reflects the fact that atoms are attached to each other by chemical bonds in a precise way to form a compound. A chemical bond is the attractive force that holds two atoms together. Each bond represents a certain amount of potential chemical energy. The atoms of each element form a specific number of bonds with the atoms of other elements-a number dictated by the number of valence electrons.
Ions:
Some atoms have the ability to gain or lose electrons. Because the number of protons in the nucleus remains unchanged, the loss or gain of electrons produces an atom with a net positive or negative charge. Such electrically charged atoms are termed ions.
Chemical Formula:
A chemical formula is a shorthand method for describing the chemical composition of a molecule. Chemical symbols are used to indicate the types of atoms in the molecule, and subscript numbers are used to indicate the number of each atom present. The chemical formula for molecular oxygen, O2, tells us that each molecule consists of two atoms of oxygen. This formula distinguishes it from another form of oxygen, ozone, which has three oxygen atoms and is written O3. The chemical formula for water, H2O, indicates that each molecule consists of two atoms of hydrogen and one atom of oxygen. Note that when a single atom of one type is present it is not necessary to write 1; it is not necessary to write H2O1.
Acids and Bases:
An acid is a compound that ionizes in solution to yield hydrogen ions (H{pos})-that is, protons-and a negatively charged ion. Acids turn blue litmus paper red and have a sour taste. Hydrochloric acid (HCl) and sulfuric acid (H2SO4) are examples of acids. The strength of an acid depends on the degree to which it ionizes in water, releasing hydrogen ions. Thus, HCl is a very strong acid because most of its molecules dissociate, producing hydrogen and chloride ions. Most bases are substances that yield a hydroxide ion (OH{neg}) and a positively charged ion when dissolved in water. Bases turn red litmus paper blue. Sodium hydroxide (NaOH) and aqueous ammonia (NH4OH) are examples of bases. Bases react with hydrogen ions and remove them from solution.
pH:
Since the concentration of hydrogen or hydroxide ions is usually small, it is convenient to express the degree of acidity or alkalinity in a solution in terms of pH, formally defined as the negative logarithm of the hydrogen ion concentration. The pH scale is logarithmic, extending from 0, the pH of a very strong acid, to 14, the pH of a very strong base. The pH of pure water is 7, neither acidic nor alkaline (basic), but neutral. Even though water does ionize slightly, the concentrations of H{pos} ions and OH{neg} ions are exactly equal; each of them has a concentration of 10{neg}7, which is why we say that water has a pH of 7. Solutions with a pH of less than 7 are acidic and contain more H{pos} ions than OH{neg} ions. Solutions with a pH greater than 7 are alkaline and contain more OH{neg} ions than H{pos} ions.
Because the scale is logarithmic (to base 10), a solution with a pH of 6 has a hydrogen ion concentration that is ten times greater than a solution with a pH of 7, and is much more acidic. A pH of 5 represents another tenfold increase. Therefore, a solution with a pH of 4 is 10 times 10 or 100 times more acidic than a solution with a pH of 6.
The contents of most animal and plant cells are neither strongly acidic nor alkaline but are an essentially neutral mixture of acidic and basic substances. Most life cannot exist if the pH of the cell changes very much.