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Topic 7

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  1. Topic 7

  2. Table of Contents Topic 7 Topic 7: Types of Compounds Basic Concepts Additional Concepts

  3. Types of Compounds: Basic Concepts Topic 7 Ionic Compounds • The submicroscopic structure of ionic compounds helps explain why they share certain macroscopic properties such as high melting points, brittleness, and the ability to conduct electricity when molten or when dissolved in water. • You have learned that ionic compounds are made up of oppositely charged ions held together strongly in well-organized units.

  4. Types of Compounds: Basic Concepts Topic 7 Ionic Compounds • Because of their structure, they usually are hard solids at room temperature and are difficult to melt. Look at the structure of magnesium oxide.

  5. Types of Compounds: Basic Concepts Topic 7 Ionic Compounds • When ionic compounds melt or dissolve in water, their three-dimensional structure breaks apart, and the ions are released from the structure.

  6. Types of Compounds: Basic Concepts Topic 7 Ionic Compounds • These charged ions are now free to move and can conduct an electrical current.

  7. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • Formulas are part of the language that is used to communicate information about substances. • As a first step in studying this new language, you will learn how to name and write formulas for ionic compounds.

  8. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • Sodium chloride (NaCl) contains only sodium and chlorine, and potassium iodide (KI) contains only potassium and iodine. • Each is an example of a binary compound, which is a compound that contains only two elements.

  9. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • Binary ionic compounds can contain more than one ion of each element, as in CaF2, but they are not composed of three or more different elements, as are more complex compounds.

  10. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • To name a binary ionic compound, first write the name of the positively charged ion, usually a metal, and then add the name of the nonmetal or negatively charged ion, whose name has been modified to end in -ide. • The compound formed from potassium and chlorine is called potassium chloride. • Magnesium combines with oxygen to form a compound called magnesium oxide.

  11. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • You are already familiar with one formula for an ionic compound—NaCl. • Sodium chloride contains sodium ions that have a 1+ charge and chloride ions that have a 1– charge. • You have learned that compounds are electrically neutral.

  12. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • This means that the sum of the charges in an ionic compound must always equal zero. • Thus, one Na+ balances one Cl– in sodium chloride.

  13. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • When you write a formula, you add subscripts to the symbols for the ions until the algebraic sum of the ions’ charges is zero. • The smallest subscript to both ions that results in a total charge of zero is 1. • However, no subscript needs to be written because it is understood that only one ion or atom of an element is present if there is no subscript.

  14. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • The formula NaCl indicates that sodium chloride contains sodium and chloride ions, that there is one sodium ion present for every chloride ion in the compound, and that the compound has no overall charge.

  15. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • If more than one ion of a given element is present in a compound, the subscript indicates how many ions are present. • The mineral known as fluorite is calcium fluoride, which has the formula CaF2. • This formula indicates that there is one calcium ion for every two fluoride ions in the compound.

  16. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • In an ionic compound, a formula represents the smallest ratio of atoms or ions in the compound.

  17. Types of Compounds: Basic Concepts Topic 7 Binary Ionic Compounds • In a covalent compound, the smallest unit of the compound is a molecule, so a formula represents a single molecule of a compound. • This simplest ratio of ions in a compound is called a formula unit. • Each formula unit of calcium fluoride consists of one calcium ion and two fluoride ions. • Each of the three ions has a stable octet configuration of electrons, and the formula unit has no overall charge.

  18. Types of Compounds: Basic Concepts Topic 7 Predicting Charge on Ions • The noble gases each have eight electrons in their outer-energy levels. • Metals have few outer-level electrons so they tend to lose them and become positive ions. • Sodium must lose just one electron, becoming an Na+ ion. • Calcium must lose two electrons, becoming a Ca2+ ion.

  19. Types of Compounds: Basic Concepts Topic 7 Predicting Charge on Ions • Most nonmetals, on the other hand, have outer-energy levels that contain four to seven electrons, so they tend to gain electrons and become negative ions.

  20. Types of Compounds: Basic Concepts Topic 7 Predicting Charge on Ions • Because all elements in a given group have the same number of electrons in their outer-energy level, they must lose or gain the same number of electrons to achieve a noble-gas electron configuration. • Metals always lose electrons and nonmetals always gain electrons when they form ions. • The charge on the ion is known as the oxidation number of the atom.

  21. Types of Compounds: Basic Concepts Topic 7 Predicting Charge on Ions • The oxidation numbers for many elements in the main groups are arranged by group number.

  22. Types of Compounds: Basic Concepts Topic 7 Predicting Charge on Ions • Oxidation numbers for elements in Groups 3 through 12, the transition elements, cannot be predicted by group number.

  23. Types of Compounds: Basic Concepts Topic 7 Predicting Charge on Ions • Aluminum is in Group 13, so it loses its three outer electrons to become an Al3+ ion; oxygen is in Group 16 and has six valence electrons, so it gains two electrons to become an O2– ion.

  24. Types of Compounds: Basic Concepts Topic 7 Predicting Charge on Ions • Notice that one of aluminum’s three electrons has not been taken up by the oxygen atom. • Because all the electrons must be accounted for, more than one oxygen atom must be involved in the reaction.

  25. Types of Compounds: Basic Concepts Topic 7 Predicting Charge on Ions • But, oxygen cannot gain only one electron, so a second aluminum atom must be present to contribute a second electron to oxygen. • In all, two Al3+ ions must combine with three O2– ions to form Al2O3. • Remember that the charges in the formula for aluminum oxide must add up to zero.

  26. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions • The ions you have studied thus far have contained only one element. • However, some ions contain more than one element. • An ion that has two or more different elements is called a polyatomic ion. • Although the individual atoms have no charge, the group as a whole has an overall charge.

  27. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions

  28. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions

  29. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions • Ionic compounds may contain positive metal ions bonded to negative polyatomic ions, such as in NaOH; negative nonmetal ions bonded to positive polyatomic ions, such as in NH4I; or positive polyatomic ions bonded to negative polyatomic ions, such as in NH4NO3.

  30. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions • To write the formula for an ionic compound containing one or more polyatomic ions, simply treat the polyatomic ion as if it were a single-element ion by keeping it together as a unit. • Remember that the sum of the positive and negative charges must equal zero.

  31. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions • Multiples of a polyatomic ion in a formula can be indicated by placing the entire polyatomic ion, without the charge, in parentheses. • Write a subscript outside the parentheses to show the number of polyatomic ions in the compound. • Never change the subscripts within the polyatomic ion.

  32. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions • To do so would change the composition of the ion. • The formula for the compound that contains one magnesium ion and two nitrate ions is Mg(NO3)2.

  33. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions • To name a compound containing a polyatomic ion, follow the same rules as used in naming binary compounds. • Name the positive ion first, followed by the negative ion.

  34. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions • However, do not change the ending of the negative polyatomic ion name. • The name of the compound composed of calcium and the carbonate ion is calcium carbonate.

  35. Types of Compounds: Basic Concepts Topic 7 Compounds Containing Polyatomic Ions • Calcium is in Group 2, so its ion has a 2+ charge. The carbonate ion has a 2– charge. • To form a neutral compound, one Ca2+ ion must combine with one CO32– ion to give the formula CaCO3.

  36. Types of Compounds: Basic Concepts Topic 7 Compounds of Transition Elements • Elements known as transition elements are located in Groups 3 through 12 in the periodic table. • Transition elements form positive ions just as other metals do, but most transition elements can form more than one type of positive ion. • In other words, transition elements can have more than one oxidation number.

  37. Types of Compounds: Basic Concepts Topic 7 Compounds of Transition Elements • For example, copper can form both Cu+ and Cu2+ ions, and iron can form both Fe2+ and Fe3+ ions. • Zinc and silver are two exceptions to the variability of other transition elements; each forms one type of ion. • The zinc ion is Zn2+ and the silver ion is Ag+.

  38. Types of Compounds: Basic Concepts Topic 7 Compounds of Transition Elements • Chemists must have a way to distinguish the names of compounds formed from the different ions of a transition element. • They do this by using a Roman numeral to indicate the oxidation number of a transition element ion. Click box to view movie clip.

  39. Types of Compounds: Basic Concepts Topic 7 Compounds of Transition Elements • This Roman numeral is placed in parentheses after the name of the element. • No additional naming system is needed for zinc and silver compounds because their formulas are not ambiguous.

  40. Types of Compounds: Basic Concepts Topic 7 Compounds of Transition Elements

  41. Types of Compounds: Basic Concepts Topic 7 Compounds of Transition Elements

  42. Types of Compounds: Basic Concepts Topic 7 Compounds of Transition Elements

  43. Types of Compounds: Basic Concepts Topic 7 Hydrates • Many ionic compounds are prepared by crystallization from a water solution, and water molecules become a part of the crystal. • A compound in which there is a specific ratio of water to ionic compound is called a hydrate. • In a hydrate, the water molecules are chemically bonded to the ionic compound.

  44. Types of Compounds: Basic Concepts Topic 7 Hydrates • Some ionic compounds can easily become hydrates by absorbing water molecules from water vapor in the air. • These compounds are called hygroscopicsubstances, and one example is sodium carbonate (Na2CO3). • In a hydrate, the water molecules are chemically bonded to the ionic compound.

  45. Types of Compounds: Basic Concepts Topic 7 Hydrates • Substances that are so hygroscopic that they take up enough water from the air to dissolve completely and form a liquid solution are called deliquescent.

  46. Types of Compounds: Basic Concepts Topic 7 Hydrates • To write the formula for a hydrate, write the formula for the compound and then place a dot followed by the number of water molecules per formula unit of compound. • The dot in the formula represents a ratio of compound formula units to water molecules. • For example, CaSO4 • 2H20 is the formula for a hydrate of calcium sulfate that contains two molecules of water for each formula unit of calcium sulfate.

  47. Types of Compounds: Basic Concepts Topic 7 Hydrates • To name hydrates, follow the regular name for the compound with the word hydrate, to which a prefix has been added to indicate the number of water molecules present.

  48. Types of Compounds: Basic Concepts Topic 7 Hydrates • The name of the compound with the formula CaSO4 • 2H2O is calcium sulfate dihydrate.

  49. Types of Compounds: Basic Concepts Topic 7 Hydrates • Heating hydrates can drive off the water. • This results in the formation of an anhydrous compound—one in which all of the water has been removed. • In some cases, an anhydrous compound may have different color from that of its hydrate.

  50. Types of Compounds: Basic Concepts Topic 7 Properties of Molecular Substances • You know that ionic compounds share many properties. • The properties of a molecularsubstance—a substance that has atoms held together by covalent rather than ionic bonds—are more variable than the properties of ionic compounds.