Molecules are formed through interaction of the valence electrons between two or more atoms. These interactions, such as the sharing of electrons, produce chemical bonds (fig. 2.2). The number of bonds that each atom can have is determined by the number of electrons needed to complete the outermost shell. Hydrogen, for example, must obtain only one more electron— and can thus form only one chemical bond—to complete the first shell of two electrons. Carbon, by contrast, must obtain four more electrons—and can thus form four chemical bonds—to complete the second shell of eight electrons (fig. 2.3, left).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.