7.1 Ionic Bonding

Cation and anion

A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence shell, and an anion (a negative ion) forms when a neutral atom gains one or more electrons in its valence shell.

ions

atoms or molecules bearing an electrical charge.

Transition and inner transition metal elements behave differently than main group elements. Most transition metal cations have

e 2+ or 3+ charges that result from the loss of their outermost s electron(s) first, sometimes followed by the loss of one or two d electrons from the next-to-outermost shell

ionic bonds

electrostatic forces of attraction between oppositely charged cations and anions. The properties of ionic compounds shed some light on the nature of ionic bonds.

The attractive forces between ions are

isotropic—the same in all directions—meaning that any particular ion is equally attracted to all of the nearby ions of opposite charge. This results in the ions arranging themselves into a tightly bound, three-dimensional lattice structure. Sodium chloride, for example, consists of a regular arrangement of equal numbers of Na+ cations and Cl- anions

When forming a cation, an atom of a main group element tends to

lose all of its valence electrons, thus assuming the electronic structure of the noble gas that precedes it in the periodic table

Neutral atoms and their associated ions have very different

physical and chemical properties Sodium atoms form sodium metal, a soft, silvery-white metal that burns vigorously in air and reacts explosively with water. Chlorine atoms form chlorine gas, Cl2, a yellow-green gas that is extremely corrosive to most metals and very poisonous to animals and plants. The vigorous reaction between the elements sodium and chlorine forms the white, crystalline compound sodium chloride, common table salt, which contains sodium cations and chloride anions (Figure 7.2). The compound composed of these ions exhibits properties entirely different from the properties of the elements sodium and chlorine. Chlorine is poisonous, but sodium chloride is essential to life; sodium atoms react vigorously with water, but sodium chloride simply dissolves in water

Although the d orbitals of the transition elements are—according to the Aufbau principle—the last to fill when building up electron configurations, the outermost s electrons are the first

to be lost when these atoms ionize

Binary ionic compounds are composed of just

two elements: a metal (which forms the cations) and a nonmetal (which forms the anions). For example, NaCl is a binary ionic compound.

For groups 1 (the alkali metals) and 2 (the alkaline earth metals), the group numbers are equal to the numbers of

valence shell electrons and, consequently, to the charges of the cations formed from atoms of these elements when all valence shell electrons are removed.

ionic compounds (or salts),

Compounds composed of ions are called ionic compounds (or salts), and their constituent ions are held together by ionic bonds

Ionic solids

Ionic solids exhibit a crystalline structure and tend to be rigid and brittle; they also tend to have high melting and boiling points, which suggests that ionic bonds are very strong. Ionic solids are also poor conductors of electricity for the same reason—the strength of ionic bonds prevents ions from moving freely in the solid state. Most ionic solids, however, dissolve readily in water. Once dissolved or melted, ionic compounds are excellent conductors of electricity and heat because the ions can move about freely.

elements that lie to the left in a period or near the bottom of a group on the periodic table

Many metallic elements have relatively low ionization potentials and lose electrons easily

the upper-right corner of the periodic table.

Nonmetal atoms have relatively high electron affinities and thus readily gain electrons lost by metal atoms, thereby filling their valence shells. Nonmetallic elements are found in the upper-right corner of the periodic table.

Most monatomic anions form when a neutral nonmetal atom gains enough electrons to completely fill its outer s and p orbitals, thereby reaching the electron configuration of the next noble gas. Thus, it is simple to determine the charge on such a negative ion:

The charge is equal to the number of electrons that must be gained to fill the s and p orbitals of the parent atom

inert pair effect

The formation of these 1+, 2+, and 3+ cations is ascribed to the inert pair effect, which reflects the relatively low energy of the valence s-electron pair for atoms of the heavy elements of groups 13, 14, and 15

The formula of an ionic compound

The formula of an ionic compound represents the simplest ratio of the numbers of ions necessary to give identical numbers of positive and negative charges. For example, the formula for aluminum oxide, Al2O3 , indicates that this ionic compound contains two aluminum cations, Al3+, for every three oxide anions, O2− [thus, (2 × +3) + (3 × -2) = 0]. It is important to note, however, that the formula for an ionic compound does not represent the physical arrangement of its ions.

For groups 13-17, the group numbers exceed the number of valence electrons by 10 (accounting for the possibility of full d subshells in atoms of elements in the fourth and greater periods). Thus, the charge of a cation formed by

the loss of all valence electrons is equal to the group number minus 10. For example, aluminum (in group 13) forms 3+ ions (Al3+). Exceptions to the expected behavior involve elements toward the bottom of the groups