9.3: Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. 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. Compounds composed of ions are called ionic compounds (or salts), and their constituent ions are held together by ionic bonds: electrostatic forces of attraction between oppositely charged cations and anions.

Properties of Ionic Compounds

The properties of ionic compounds shed some light on the nature of ionic bonds.

• 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.

The Formation of Ionic Compounds

Many metallic elements have relatively low ionization potentials and lose electrons easily. These elements lie to the left in a period or near the bottom of a group on 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.

As all substances must be electrically neutral, the total number of positive charges on the cations of an ionic compound must equal the total number of negative charges on its anions. 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.

Ionic Compounds Form Regularly Arranged 3-Dimensional Structures

It is important to note, however, that the formula for an ionic compound does not represent the physical arrangement of its ions. It is incorrect to refer to sodium chloride (NaCl) “molecule” because there is not a single ionic bond, per se, between any specific pair of sodium and chloride ions. 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. The strong electrostatic attraction between Na⁺ and Cl^– ions holds them tightly together in solid NaCl. It requires 769 kJ of energy to dissociate one mole of solid NaCl into separate gaseous Na⁺ and Cl^– ions.

Electronic Structures of Cations

When forming a cation, an atom of the 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.

• 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.
• For example, calcium is a group 2 element whose neutral atoms have 20 electrons and a ground-state electron configuration of 1s²2s²2p⁶3s²3p⁶4s². When a Ca atom loses both of its valence electrons, the result is a cation with 18 electrons, a 2+ charge, and an electron configuration of 1s²2s²2p⁶3s²3p⁶. The Ca²⁺ ion is, therefore, isoelectronic with the noble gas Ar.
• 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

• Exceptions to the expected behavior involve elements toward the bottom of the groups.
• In addition to the expected ions Tl³⁺, Sn⁴⁺, Pb⁴⁺, and Bi⁵⁺, a partial loss of these atoms’ valence shell electrons can also lead to the formation of Tl⁺, Sn²⁺, Pb²⁺, and Bi³⁺ ions. 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.
• Mercury (group 12) also exhibits an unexpected behavior: it forms a diatomic ion, Hg₂²⁺ (an ion formed from two mercury atoms, with a Hg-Hg bond), in addition to the expected monatomic ion Hg²⁺ (formed from only one mercury atom).
• Transition and inner transition metal elements behave differently than the main group elements. Most transition metal cations have 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.
• 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. When the inner transition metals form ions, they usually have a 3+ charge, resulting from the loss of their outermost s electrons and a d or f electron.

Electronic Structures of Anions

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. Oxygen, for example, has the electron configuration 1s²2s²2p⁴, whereas the oxygen anion has the electron configuration of the noble gas neon (Ne), 1s²2s²2p⁶. The two additional electrons required to fill the valence orbitals give the oxide ion the charge of 2– (O^2–).

This text is adapted from Openstax, Chemistry 2e, Section 7.3: Ionic Bonding.

Ionic bonds are formed by the transfer of electrons from metals to nonmetals. In comparison to covalent bonds where electrons are shared, in ionic bonds metals tend to lose and nonmetals tend to accept electrons — but why? 

The most stable electron configuration of an atom is with a full octet. Achieving this state in conjunction with lowering the potential energies is a major driving force for bond formation. 

But how to predict when electrons will be transferred and not shared? For this, examine the ionization energies and electron affinities of atoms.

On the periodic table, s-block metals have low ionization energies. This facilitates easier loss of electrons to reach an octet while concurrently forming cations. 

On the contrary, nonmetals from the p-block, except for noble gases, have high electron affinities and readily accept electrons to form anions. The oppositely charged cations and anions experience strong electrostatic interactions, attracting each other to form ionic bonds.

Ionic compounds often form hard and well-defined crystal structures with high melting points, due to strong electrostatic attraction between ions.

Consider lithium fluoride, an ionic compound made of lithium and fluorine. When lithium loses an electron to become a lithium cation with a duet, the electron is being taken up by fluorine to form the anion with an octet.

According to Coulomb's law, these two charged particles are attracted to each other to form lithium fluoride, a neutral compound. 

The Lewis model can be used to describe and predict the chemical formula of ionic compounds. The Lewis symbol shows charges in the upper-right hand corner and depicts anions and cations with the symbol written within brackets. 

Consider strontium chloride, a commonly used salt to get a bright red color in fireworks. The Lewis symbol for strontium has two, and chlorine has one unpaired electron.

Strontium must lose two electrons, while chlorine must gain one electron to reach an octet. Therefore, one strontium cation binds to two chloride anions, in the ratio of 1:2.