3.1: Ions - Losing and Gaining Electrons

Learning Objectives

• Define the two types of ions.

Most atoms do not have eight electrons in their valence electron shell. Some atoms have only a few electrons in their outer shell, while some atoms lack only one or two electrons to have an octet. In cases where an atom has three or fewer valence electrons, the atom may lose those valence electrons quite easily until what remains is a lower shell that contains an octet. Atoms that lose electrons acquire a positive charge as a result because they are left with fewer negatively charged electrons to balance the positive charges of the protons in the nucleus. Positively charged ions are called cations. Most metals become cations when they make ionic compounds.

Cations

A neutral sodium atom is likely to achieve an octet in its outermost shell by losing its one valence electron.

\[\ce{Na \rightarrow Na^{+} + e^{-}}\]

The cation produced in this way, Na⁺, is called the sodium ion to distinguish it from the element. The outermost shell of the sodium ion is the second electron shell, which has eight electrons in it. The octet rule has been satisfied. Figure \(\PageIndex{1}\) is a graphical depiction of this process.

Figure \(\PageIndex{1}\): The Formation of a Sodium Ion. On the left, a sodium atom has 11 electrons. On the right, the sodium ion only has 10 electrons and a 1+ charge.

Anions

Some atoms have nearly eight electrons in their valence shell and can gain additional valence electrons until they have an octet. When these atoms gain electrons, they acquire a negative charge because they now possess more electrons than protons. Negatively charged ions are called anions. Most nonmetals become anions when they make ionic compounds.

A neutral chlorine atom has seven electrons in its outermost shell. Only one more electron is needed to achieve an octet in chlorine’s valence shell. (In table salt, this electron comes from the sodium atom.)

\[\ce{e^{-} +Cl -> Cl^{-}}\]

In this case, the ion has the same outermost shell as the original atom, but now that shell has eight electrons in it. Once again, the octet rule has been satisfied. The resulting anion, Cl⁻, is called the chloride ion; note the slight change in the suffix (-ide instead of -ine) to create the name of this anion. Figure \(\PageIndex{2}\) is a graphical depiction of this process.

Figure \(\PageIndex{2}\): The Formation of a Chlorine Ion. On the left, the chlorine atom has 17 electrons. On the right, the chloride ion has 18 electrons and has a 1− charge.

The names for positive and negative ions are pronounced CAT-eye-ons and ANN-eye-ons, respectively.

Ionophores

Cell membranes surround each living cell. It is a barrier that prevents unwanted substances from entering and desired substances from leaving the cell. The health of cells depends on maintaining the proper levels of ions in intracellular fluids. Any change that affects the normal flow of ions across cell membranes could well cause an organism to die. Molecules that facilitate the transport of metal ions across membranes are called ionophores (ion plus phore from the Greek phorein, meaning “to carry”). Streptomyces bacteria make valinomycin an ionophore to defend themselves against competing bacteria. Valinomycin is a potent antibiotic that can kill or inhibit the growth of bacteria. It is a cyclic molecule with a central cavity lined with oxygen atoms to bind to metal cations. Valinomycin is highly selective: its affinity for K⁺ is about 1000 times greater than that for Na⁺. By increasing the flow of K⁺ ions into cells, valinomycin disrupts the normal K⁺ levels in the cell, thereby killing the cell.

Figure \(\PageIndex{3}\): Valinomycin Is an Antibiotic That Functions Like an Ionophore

Contributions & Attributions

• Deboleena Roy (American River College)

• Marisa Alviar-Agnew (Sacramento City College)

• Henry Agnew (UC Davis)