Ionic Bonding and Ionic CompoundsGeneral Chemistry

Mrs. Deiseroth

Ionic Compounds Composed of positive and negative ions that are combined so that the

numbers of positive and negative charges are equal Most exist as crystalline solids

3-D network of positive and negative ions Not composed of independent, neutral units like a molecular compound

The chemical formula of an ionic compound shows the ratio of the ions present in a sample of any size

Formula unit – the simplest collection of atoms from which an ionic compound’s formula can be established Na + Cl → Na+ + Cl-

Characteristics of Ionic Bonding Remember nature favors arrangements in which potential energy is

minimized Ionic crystals – ions minimize their potential energy by combining in an

orderly arrangement known as a crystal lattice Attractive forces – forces between opposite charged ions and forces

between nuclei and electrons of adjacent ions Repulsive forces – forces between like-charged ions and forces between

electrons of adjacent ions Attraction between the adjacent oppositely charged ions is much stronger

than the repulsion by other ions of the same charge, which are farther away

Characteristics of Ionic Bonding To compare bond strengths in ionic compounds, chemists compare

the amounts of energy released when separated ions in a gas come together to form a crystalline solid. Lattice energy – is the energy released when one mole of an ionic

crystalline compound is formed from gaseous ions Table 6-3

A Comparison of Ionic and Molecular Compounds

The force that holds ions together in ionic bonding is a very strong overall attraction between positive and negative charges.

Covalent bonds of the atoms making up each molecule are also strong, but the forces of attraction between molecules are much weaker than the forces of ionic bonding.

This difference in the strength of attraction between the basic units of molecular and ionic compounds gives rise to different properties in the two types of compounds.

A Comparison of Ionic and Molecular Compounds

Melting point, boiling point, and hardness of a compound depend on how strongly its basic units are attracted to each other. Many molecular compounds melt at low temperatures Ionic compounds generally have higher melting and boiling points than molecular

compounds Many molecular compounds vaporize at room temperature Ionic compounds are hard but brittle (Figure 6-17) In the molten state, or when dissolved in water, ionic compounds are electrical

conductors because the ions can move freely to carry electrical current. Solid state – ions can’t move – poor conductors

Many ionic compounds are soluble in water

Polyatomic Ions Certain atoms bond covalently with each other to form a group of

atoms that has both molecular and ionic characteristics. Polyatomic Ion – a charged group of covalently bonded atoms They combine with ions of opposite charge to form ionic compounds Their charge results from an excess of electrons (negative charge) or

a shortage of electrons (positive charge) Let’s look at ammonium NH4+ Let’s look at Lewis structures for some common polyatomic ions