Ions

A single atom is electrically neutral since it contains equal numbers of negative electrons and positive protons. If, however, an atom gains or loses one or more electrons, it acquires a net electric charge and becomes an ion. For example, when a sodium atom (Na), which has 11 electrons, loses 1 electron, it becomes a sodium ion (Na+) with a net positive charge; it still has 11 protons, but it now has only 10 electrons. On the other hand, a chlorine atom (Cl), which has 17 electrons, can gain an electron and become a chloride ion (CP) with a net negative charge—it now has 18 electrons but only 17 protons. Some atoms can gain or lose more than 1 electron to become ions with two or even three units of net electric charge (for example, calcium Ca2+).

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Chemical Composition of the Body CHAPTER TWO

Chemical Composition of the Body CHAPTER TWO

Covalent Bond Nitrogen

Methane (CH4)

Water (H2O)

FIGURE 2-2

Geometric configuration of covalent bonds around the carbon, nitrogen, and oxygen atoms bonded to hydrogen atoms.

Methane (CH4)

Water (H2O)

FIGURE 2-2

Geometric configuration of covalent bonds around the carbon, nitrogen, and oxygen atoms bonded to hydrogen atoms.

Hydrogen atoms and most mineral and trace element atoms readily form ions. Table 2-3 lists the ionic forms of some of these elements. Ions that have a net positive charge are called cations, while those that have a net negative charge are called anions. Because of their ability to conduct electricity when dissolved in water, the ionic forms of the seven mineral elements are collectively referred to as electrolytes.

The process of ion formation, known as ionization, can occur in single atoms or in atoms that are cova-lently linked in molecules. Within molecules two commonly encountered groups of atoms that undergo ion-ization are the carboxyl group (—COOH) and the amino group (—NH2). The shorthand formula when indicating only a portion of a molecule can be written as R—COOH or R—NH2, where R signifies the remaining portion of the molecule. The carboxyl group ionizes when the oxygen linked to the hydrogen captures the hydrogen's only electron to form a carboxyl ion (R—COO-) and releases a hydrogen ion (H+):

The amino group can bind a hydrogen ion to form an ionized amino group (R—NH3+):

The ionization of each of the above groups can be reversed, as indicated by the double arrows; the ionized carboxyl group can combine with a hydrogen ion to form an un-ionized carboxyl group, and the ionized amino group can lose a hydrogen ion and become an un-ionized amino group.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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