Elements can have more than one atomic form. Isotopes of the same element all have the same, definitive, number of protons, but differ in the number of neutrons in the atomic nucleus.
In nature, many elements exist as several isotopes. The isotopes of hydrogen shown in Figure 2.4 have special names, but the isotopes of most elements do not have distinct names. For example, the natural isotopes of carbon are 12C, 13C, and 14C (spoken of as carbon-12, carbon-13, and carbon-14). Most carbon atoms are 12C, about 1.1 percent are 13C, and a tiny fraction are 14C. An element's atomic mass, or atomic weight,* is the average of the mass numbers of a representative sample of atoms of the element, with all isotopes in their
*The concepts of "weight" and "mass" are not identical. Weight is the measure of the Earth's gravitational attraction for mass; on another planet, the same quantity of mass would have a different weight. On Earth, however, the term "weight" is often used as a measure of mass, and in biology one encounters the terms "weight" and "atomic weight" more frequently than "mass" and "atomic mass." Therefore, we will use "weight" for the remainder of this book.
1 proton 0 neutrons
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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.