(For more detail, see Appendix A, Periodic Table of the Elements, page 1027.)

(For more detail, see Appendix A, Periodic Table of the Elements, page 1027.)

average weight of the isotopes present. (See Appendix A, Periodic Table of the Elements, page 1027)

The ways atoms interact with one another are due largely to their numbers of electrons. Because the number of electrons in an atom equals its number of protons, all the isotopes of a particular element have the same number of electrons and chemically react in the same manner. For example, any of the isotopes of oxygen can have the same function in the metabolic reactions of an organism.

Isotopes of an element may be stable, or they may have unstable atomic nuclei that decompose, releasing energy or pieces of themselves until they reach a stable form. Such unstable isotopes are called radioactive, and the energy or atomic fragments they emit are called atomic radiation. Elements that have radioactive isotopes include oxygen, iodine, iron, phosphorus, and cobalt. Some radioactive isotopes are used to detect and treat disease (Clinical Application 2.1).

Atomic radiation includes three common forms called alpha (a), beta (P), and gamma (y). Each kind of radioactive isotope produces one or more of these forms of radiation. Alpha radiation consists of particles from atomic nuclei, each of which includes two protons and two neutrons, that move relatively slowly and cannot easily penetrate matter. Beta radiation consists of much smaller particles (electrons) that travel faster and more deeply penetrate matter. Gamma radiation is similar to X-radiation and is the most penetrating of these forms.

H What is the relationship between matter and elements? ^9 Which elements are most common in the human body?

^9 How are electrons, protons, and neutrons positioned within an atom?

Q What is an isotope? B What is atomic radiation?

Molecules and Compounds

Two or more atoms may combine to form a distinctive kind of particle called a molecule (mol'e-kul). A molecular formula is used to depict the numbers and kinds of atoms in a molecule. Such a formula consists of the symbols of the elements in the molecule with numbers as subscripts to indicate how many atoms of each element are present. For example, the molecular formula for water is H2O, which indicates two atoms of hydrogen and one atom of oxygen in each molecule. The molecular formula for the sugar glucose is C0H12O0, which means there are six atoms of carbon, twelve atoms of hydrogen, and six atoms of oxygen in a glucose molecule.

If atoms of the same element combine, they produce molecules of that element. Gases of hydrogen (H2), oxygen (O2), and nitrogen (N2) consist of such molecules. If atoms of different elements combine, molecules of substances called compounds form. Two atoms of hydrogen, for example, can combine with one atom of oxygen to produce a molecule of the compound water (H2O), as figure 2.2 shows. Table sugar, baking soda, natural gas, beverage alcohol, and most medical drugs are compounds.

A molecule of a compound always contains definite types and numbers of atoms. A molecule of water (H2O), for instance, always contains two hydrogen atoms and one oxygen atom. If two hydrogen atoms combine with two oxygen atoms, the compound formed is not water, but hydrogen peroxide (H2O2).

Bonding of Atoms

Atoms combine with other atoms by forming bonds. When atoms form such bonds, they gain or lose electrons or share electrons.

The electrons of an atom are found in one or more regions of space called shells around the nucleus. The maximum number of electrons that each of the first three

Shier-Butler-Lewis: I I. Levels of Organization I 2. Chemical Basis of Life I I © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition

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