Ionic Compounds•An ionic compound is composed of positive and negative ions that are combined so that the numbers of positive and negative charges are equal.Example: A sodium ion, Na+, has a charge of 1+. A chloride ion, Cl-, has a charge of 1-. There is an electrical force of attraction between oppositely charged ions. In sodium chloride, these ions combine in a one – to – one ratio so that each positive charge is balanced by a negative charge.

attraction

Charges balance

•Most ionic compounds exist as crystalline solids.•A crystal of any ionic compound is a 3-dimensional network of (+) and (-) ions attracted to each other.•Therefore, an ionic compound is not composed of independent neutral units that can be isolated and examined like molecular compounds.

NaCl crystals

•The chemical formula of an ionic compound represents the simplest ratio of the compound’s combined ions that gives electrical neutrality.•Example:

Electricalneutrality

Electricalneutrality

•A formula unit is the simplest collection of atoms from which an ionic compound’s formula can be established.•A formula unit represents the lowest reduced ratio of ions in the compound.•The ratio of ions in a formula unit depends on the charges of ions combined.

Formula unit

Formation of Ionic Compounds•The Octet Rule is the basis for the predictions about the charges on ions.•Ionic compounds are formed as a result of the formation of (+) and (-) ions.

•Electrons are transferred from one atom to another to form noble gas electron structure for each ion.•The atom which forms a positive ion loses electrons to the atom which gains electrons to form a negative ion.

•A compound is not stable unless the number of electrons which are lost or gained are equal.

Characteristics of Ionic Bonding•In an ionic crystal, ions minimize their energy by combining in an orderly arrangement known as a crystal lattice. (Remember nature favors minimum potential energy).

•The distance between ions and their arrangement in the crystal represents a balance between attractive and repulsive forces between ions.

Two models of the crystal structure of sodium chloride are shown.(a)To illustrate the ions’ actual arrangement, the sodium and chloride ions are shown with their electron clouds just touching.(b) In an expanded view, the distances between ions have been exaggerated in order to clarify the positioning of the ions in the structure.

(a) (b)

•Lattice energy is the energy released when one mole of an ionic crystalline compound is formed from gaseous ions.

A Comparison of Ionic and Molecular Compounds

•Forces of attraction in ionic compounds are very strong.•Molecular compounds have strong covalent bonds making up each molecule but forces between molecules are weaker than those of ionic bonding.•These differences account for different properties in the two types of compounds.

•Ionic compounds usually have higher melting and boiling points than do molecular compounds.•Many molecular compounds are completely gaseous at room temperature.

Strong attractive forces.Many are gaseous at room temperature.

NO

O

O

•A charged group of covalently bonded atoms is known as a polyatomic ion.•Polyatomic ions have both molecular and ionic characteristics.

In a polyatomic ion, two or more atoms covalently bonded together.

Together, they carry a charge,

1–

Mg

2+

NO

O

O1-

NO

O

O

1-

Mg(NO3)2

•Polyatomic ions combine with ions of opposite charge to form ionic compounds.

•The charge of a polyatomic ions results from an excess of electrons (-) or shortage of electrons (+).

1–

NO

O

O The nitrate ion, (NO3

-), has, in total, one more electron than protons.

N HH

HH

+

N OO

O

Ammonium ion Nitrate ion

S OO

O

Sulfate ion

2

O

Examples of Polyatomic Ions