Ions

Introduction

An atom is electrically neutral, which means that the number of protons is equal to the number of electrons. In chemical reactions, however, atoms can gain or lose electrons. This results in the formation of an ion. An ion is an atom with a positive or negative charge.

Atoms and Ions

Atoms of metallic elements tend to form positive ions by losing one or more electrons. A positive ion is called a cation (pronounced CAT-ion) and has fewer electrons than an electrically neutral atom. For example, an atom of sodium has eleven protons and eleven electrons. Sodium has one valence electron surrounding a stable core of ten electrons. In chemical reactions, a sodium atom tends to lose its one valence electron to become a sodium cation. Because this sodium ion has eleven protons and only ten electrons, it has a net charge of . An atom that loses two electrons will become an ion with a charge of , and an atom that loses three electrons will become an ion with a charge of .

Atoms of nonmetallic elements tend to form negative ions by gaining one or more electrons. A negative ion is called an anion (pronounced AN-ion). For example, an atom of fluorine has seventeen protons and seventeen electrons, of the seventeen electrons seven are valence electrons. In chemical reactions, a fluorine atom tends to gain one valence electron, becoming a fluoride anion. (Notice that the name of anions typically end in “-ide.”) Because the fluoride ion has seventeen positive protons and eighteen negative electrons, it has a net charge of . An atom that gains two electrons will become an ion with a charge of , and an atom that gains three electrons will become an ion with a charge of .

Some Common Ions

All the metals in family 1A (shown in the figure below) have one valence electron in the outer energy level. For that reason, all members of the 1A family will tend to lose only one electron when ionized. The entire family forms  ions: . Note that although hydrogen (H) is in this same column, it is not considered to be a metal. There are times when hydrogen acts like a metal and forms  ions, but most of the time it bonds with other atoms as a nonmetal. In other words, hydrogen doesn’t easily fit into any chemical family.

The metals in family 2A (shown in the figure below) all have two valence electrons. This entire family will form  ions:  and .

All members of family 2A form ions with  charge.

When family 3A members (shown in the figure below) form ions, they will almost always form  ions: , and . Notice that boron is omitted from this list. This is because boron falls on the nonmetal side of the metal/nonmetal dividing line. Boron generally doesn’t lose all of of its valence electrons during chemical reactions.

Family 4A is almost evenly divided into metals and nonmetals. The larger atoms in the family (germanium, tin, and lead) are metals. They are expected to form ions with charges of . All three of the atoms do form such ions (, and ), but tin and lead also have the ability to also form  ions. You will learn later in this chapter that some atoms have the ability to form ions of different charges, and the reasons for this will be examined later.

Like family 4A, the elements of family 5A are also divided into metals and nonmetals. The smaller atoms in this family behave as nonmetals, and the larger atoms behave as metals. Since bismuth and arsenic both have five valence electrons and must form positive ions, they form  ions. The non-metals tend to form -3 ions.

Most of the elements in family 6A (shown in figure below) are nonmetals that have 6 valence electrons they form  ions: .

Family 7A are all nonmetals with 7 valence electrons. When these atoms form ions, they form  ions: . Family 8A, of course, is made up of the noble gases, which have no tendency to either gain or lose electrons.