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Chemistry – Chapter 6.3

Chemistry – Chapter 6.3. Ionic Bonding and Ionic Compounds. Objectives. Compare and contrast a chemical formula for a molecular compound with one for an ionic compound. Discuss the arrangements of ions in crystals. Define lattice energy and explain its significance.

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Chemistry – Chapter 6.3

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  1. Chemistry – Chapter 6.3 Ionic Bonding and Ionic Compounds

  2. Objectives • Compare and contrast a chemical formula for a molecular compound with one for an ionic compound. • Discuss the arrangements of ions in crystals. • Define lattice energy and explain its significance. • List and compare the distinctive properties of ionic and molecular compounds. • Write the Lewis structure for a polyatomic ion given the identity of the atoms combined and other appropriate information.

  3. Ionic Compounds • Ionic Compound: composed of positive and negative ions that are combined so that the numbers of positive and negative charges are equal. • Most ionic compounds exist as crystalline solids. • A crystal of any ionic compound is a three-dimensional network of positive ions (cations) mutually attracted to negative ions (anions).

  4. Formula Unit • The chemical formula of an ionic compound shows the ratio of the ions present ina sample of any size. • Formula Unit: The simplest collection of atoms from which an ionic compound’s formula can be established. Ex. NaCl sodium chloride. • The ratio of ions in a formula unit depends on the charges of the ions combined.

  5. Formation of Ionic Compounds • Electron-dot notation can be used to demonstrate the changes that take place in ionic bonding. • Nature favors arrangements in which potential energy is minimized. • In an ionic crystal, ions minimize their potential energy by combining in an orderly arrangement known as a crystal lattice.

  6. Lattice Energy • 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: The energy released when one mole of an ionic crystalline compound is formed from gaseous ions. • Negative energy values indicate energy is released when the crystals are formed. Lattice Energies of Some Common Ionic Compounds Compound Energy (kJ/mol) NaCl -787.5 NaBr -751.4 LiF -1032 If the lattice energy in compound A is greater than that of compound B, it will be more difficult to break the bonds in compound A.

  7. Forces of Attraction • The force that holds ions together in ionic compounds is a very strong overall attraction between positive and negative charges. • The forces of attraction between molecules in a molecular compound are weaker than the attractive forces in ionic bonding. • The melting point, boiling point, and hardness of a compound depend on how strongly its basic units are attracted to each other.

  8. Polyatomic Ions • Certain atoms bond covalently with each other to form a group of atoms that has both molecular and ionic characteristics. • Polyatomic Ions: A charged group of covalently bonded atoms. • Polyatomic ions combine with ions of opposite charge to form ionic compounds. • The charge of a polyatomic ion results from an excess of electrons or a shortage of electrons.

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