3.13: Ions and Ionic Bonds

Ions

When an atom fills its outer electron shell by accepting or donating electrons, it becomes an ion. Because electrons are negatively charged, atoms become negatively charged ions when they take on extra electrons (have more electrons than protons). Atoms become positively charged ions when they give up electrons (have fewer electrons than protons).

Atoms that have a nearly complete outer electron shell (see chlorine atom in Figure 3.1) tend to get electrons from another atom. Atoms that have just begun a new electron shell (see sodium atom in Figure 3.1) tend to give up those electrons to another atom. Chlorine has 7 electrons in its outer shell, and needs only 1 more to fill it. Sodium has only 1 in its outer shell, so sodium readily gives up its single outer-shell electron to chlorine. Both sodium and chloride are now stable—each now has a complete outer electron shell.*

Sodium and chloride now have an electrical charge: sodium has given an electron (negative charge) to chloride. Sodium, with 11 protons (positive charges), now has only 10 electrons (negative charges). It thus has a net positive charge of +1 and is called a sodium ion (Na+). Chlorine, with 17 protons, now has 18 electrons. It thus has a net negative charge of -1 and is called a chloride ion (Cl-).

Ionic Bonds

An ionic bond is formed by the attraction of a positively charged ion to a negatively charged ion (opposites attract). A positively charged sodium ion is strongly attracted to a negatively charged chloride ion to form sodium chloride (table salt). An ionic bond is like a bond between a magnet and nails; the nails can be pulled away by another magnet. Ions held together by ionic bonds tend to separate when dissolved in water.†

Figure 3.3: Phenylketonuria (PKU)—One Chemical Reaction Can’t Occur

Salt (sodium chloride) separates into sodium ions and chloride ions when dissolved in the water in our bodily fluids. This separation into ions is very important in the body, e.g., the transmission of nerve impulses depends on sodium and potassium ions. (These ions are also called electrolytes because they conduct electricity.)

We’ll see in upcoming chapters that calcium ions are needed for blood to clot, and a nerve cell can transmit nerve impulses because its cell membrane is polarized with positive ions lined up on one side and negative ions lined up on the other. We’ll also see how the similarity of ions can fool the body. For example, calcium ions (required by the body) and lead ions (toxic to the body) each have two positive charges. Lead ions are toxic because they interfere with the essential jobs of calcium ions.

^{*See bottom section of Figure 3.1 and compare it to sodium and chlorine atoms in the top section. Note also that sodium ion’s electron shells are now the same—and as stable—as neon’s.
†A water molecule, though overall neutral in charge, has a slight negative charge on one side and a slight positive charge on the other. This is because water’s 2 hydrogen atoms (with their positive protons) are on one side of a water molecule (see Figure 3.1). Thus, when sodium chloride is dissolved in water, the charges on the water molecules tend to pull the sodium and chloride ions away from each other.}