Ionic Bonding and Ionic Compounds General 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