1. Molecules and Compounds Chapter Three

2. Wolpa/Advanced Placement Chemistry Molecules Molecules are groups of atoms chemically bonded together. Molecules may be elements (the diatomic, sulfur and phosphorus) or compounds.

3. Wolpa/Advanced Placement Chemistry Allotropes/Molecular Elements Hydrogen, fluorine, chlorine, bromine, iodine and oxygen exist as diatomic molecules – HOFBrINCl’s Phosphorus exists as a tetratomic molecule Some elements exist in a variety of forms Carbon: graphite; diamond; buckminsterfullerine Phosphorus - red and white Sulfur - S 6 and S 4

4. Wolpa/Advanced Placement Chemistry Molecules and compounds Molecular compounds - molecules containing atoms from two or more different elements Covalent bonds - the force holding the atoms together in a molecular compound

5. Wolpa/Advanced Placement Chemistry Formulas A compound is represented by using the symbols for the elements of which it is composed Subscripts are used to indicate how many atoms of a particular element exist in the compound If there is only one atom of a particular element, the one is assumed

6. Wolpa/Advanced Placement Chemistry Formulas, con’t Changing the subscripts changes the compound –consider H 2 O and H 2 O 2 Two different compounds can, however, share the same chemical formula –dimethyl ether and ethyl alcohol both have the formula C 2 H 6 O

7. Wolpa/Advanced Placement Chemistry Ethyl alcohol and dimethyl ether Ethyl alcohol on the left; dimethyl ether on the right These species are termed geometric isomers Formulas that show the order and arrangement of specific atoms are known as structural formulas

8. Wolpa/Advanced Placement Chemistry Electrical nature of matter Electrostatic forces –attraction between opposite charges –repulsion between same charges Charged atoms or molecules are known as ions –cations - positively charged –anions - negatively charged

9. Wolpa/Advanced Placement Chemistry Ionic Compounds Compounds consisting of ions are known as ionic compounds. The forces holding them together are called ionic bonds

10. Wolpa/Advanced Placement Chemistry How charged species arise Neutral atoms and molecules have the same number of protons and electrons Cations have more protons than electrons Anions have more electrons than protons

11. Wolpa/Advanced Placement Chemistry Formulas of Ionic compound Formula unit - simplest whole-number ratio of ions in an ionic compound For example: Ca 2+ + Br - –you need to have the resulting formula be electrically neutral –so two Br - are needed for each Ca 2+ –the resulting formula is CaBr 2

12. Wolpa/Advanced Placement Chemistry Types of Ionic Compounds Ionic compounds will always consist of one of the following combinations: a metal and a nonmetal, a polyatomic ion and a nonmetal, a metal and a polyatomic ion or two polyatomic ions. Ionic compounds can be distinguished from molecular compounds by the kinds of elements they contain.

13. Wolpa/Advanced Placement Chemistry Polyatomic ions cations or anions consisting of groups of atoms that are covalently bonded to each other examples are NO 3 -, SO 4 2- when more than one appears in a formula unit, the polyatomic ion is put in between parentheses, and a subscript is used to indication the number of the ions that appear in the formula unit example: Ba(ClO 3 ) 2

14. Wolpa/Advanced Placement Chemistry Guidelines for determining if a compound is Ionic Metals almost always form positive ions and form ionic compounds Nonmetals form monatomic ions in ionic compounds only when combined with a metal. It is difficult to predict when the metalloids form ions The farther apart two elements are in the periodic table, the more likely they are to from an ionic compound on reaction

15. Wolpa/Advanced Placement Chemistry Ionic Crystal Lattice Ionic compounds are generally solids and have their ions arranged in extended three-dimensional networks. This regular array of positive and negative ions is called a crystal lattice.

16. Wolpa/Advanced Placement Chemistry Properties of Ionic Compounds High melting points that correlate with charges on ions Most ionic solids do not conduct electricity but molten ionic compounds do. Most ionic compounds dissolve in water

17. Wolpa/Advanced Placement Chemistry Properties cont. Solutions of ionic compounds in water conduct electricity (electrolytes) In ionic substances, each ion has its own characteristics, and these are different from the characteristics of the atom from which the ion was derived (NaCl)

18. Wolpa/Advanced Placement Chemistry Binary compound nomenclature Five types of binary compounds –Metals exhibiting only one oxidation state forming a compound with a nonmetal or polyatomic ion –Metals exhibiting two or more oxidation states forming a compound with a nonmetal or polyatomic ion –Ammonium ion with nonmetal –Two polyatomic ions –Compounds of nonmetals and nonmetals

19. Wolpa/Advanced Placement Chemistry Ionic Compound Nomenclature

20. Wolpa/Advanced Placement Chemistry Metals with only one oxidation state Groups of metals with only one common oxidation state –alkali metals - +1 –alkaline earths - +2 –Zn - +2 –Al - +3 All other metals can exhibit more that one oxidation state

21. Wolpa/Advanced Placement Chemistry Anions in negative oxidation states Nonmetallic anions usually exhibit one negative oxidation state –halogens -1 –chalcogens -2 –N, P -3

22. Wolpa/Advanced Placement Chemistry Forming compounds Metal and nonmetal combine to neutralize charge Consider - Al 3+, O 2- –cross multiply charges –2 Al 3+ + 3 O 2- = Al 2 O 3

23. Wolpa/Advanced Placement Chemistry Naming binary compounds Use name of metal with no changes Change the name of the anion by taking the “stem” and add the suffix -ide Examples –NaCl - sodium chloride –MgCl 2 - magnesium chloride

24. Wolpa/Advanced Placement Chemistry Metals with multiple oxidation states Two systems: Stock and “IUPAC” IUPAC system –metal name and the oxidation state in Roman numbers in parenthesis –Fe 2+ = iron(II) Form compound by balance charge of metal with correct number of nonmetals –CoCl 3 = cobalt(III) chloride

25. Wolpa/Advanced Placement Chemistry Stock System Nomenclature Metals in multiple oxidation states usually have one or two common oxidation states First row transition metals are +2 and +3 (except Cu 2+ and Cu + ) use -ous suffix for lower common oxidation state use -ic suffix for higher common oxidation state

26. Wolpa/Advanced Placement Chemistry Examples CoCl 3 - cobaltic chloride NiCl 2 - nickelous chloride For metals with Latin names, use them CuCl - cuprous chloride FeBr 3 - ferric bromide

27. Wolpa/Advanced Placement Chemistry Binary Molecular Nomenclature

28. Wolpa/Advanced Placement Chemistry Nonmetals + nonmetals Name nonmetal further to the left of the periodic table first with no changes Name nonmetal further to the right of the periodic table second with the -ide suffix Use Greek prefixes to indicate the number of each one

29. Wolpa/Advanced Placement Chemistry Greek prefixes NumberPrefixes 1mono 2di 3tri 4tetra 5penta 6hexa 7hepta

30. Wolpa/Advanced Placement Chemistry Examples N 2 O 3 - dinitrogen trioxide CO 2 - carbon dioxide P 2 O 5 - diphosphorus pentoxide

31. Wolpa/Advanced Placement Chemistry Oxy anions anions composed of oxygen and another elements other elements can be a metal or a nonmetals Examples –SO 4 2-, NO 2 -, MnO 4 -

32. Wolpa/Advanced Placement Chemistry Naming Need common oxidation states –most common oxidation state for nonmetals is the group number (except for the halogens) –next most common oxidation state is the group number minus one use -ate suffix for higher oxidation state and -ite suffix for next higher oxidation state

33. Wolpa/Advanced Placement Chemistry Examples SO 4 2- - sulfate SO 3 2- - sulfite NO 3 - - nitrate NO 2 - - nitrite Salts with these oxyanions –Na 2 SO 4 - sodium sulfate –KNO 3 - potassium nitrate

34. Wolpa/Advanced Placement Chemistry Acids Binary acids –name begins with hydro –then add stem of nonmetal plus -ic –end with acid Examples –HCl - hydrochloric acid –H 2 S - hydrosulfuric acid

35. Wolpa/Advanced Placement Chemistry Oxyacids Take oxyanion suffix and convert –change -ate to -ic –change -ite to -ous Do not use hydro- in the beginning Examples –H 2 SO 4 - sulfuric acid –H 2 SO 3 - sulfurous acid

36. Wolpa/Advanced Placement Chemistry Hydrates Some ionic compounds can have water molecules attached within the structure These compounds are termed hydrates and have properties distinct from the unhydrated form

37. Wolpa/Advanced Placement Chemistry Naming Hydrates Hydrates are named by naming the ionic compound and then using a Greek prefix to indicate the number of water molecules followed by the word hydrate

38. Wolpa/Advanced Placement Chemistry Chemical Formulas Molecular compounds –chemical formula represents a discrete molecular unit (e. g. CO 2 ) Ionic compounds –chemical formula represents a formula unit (the whole number ratio of cations to anions; e. g. K 2 SO 4 )

39. Wolpa/Advanced Placement Chemistry Empirical Formula Simplest whole number ratio of atoms in the compound All ionic formulas are empirical Molecular formulas are either equal to the empirical or a whole number multiple

40. Wolpa/Advanced Placement Chemistry The composition of compounds Mole composition is the number of moles of each of the elements that make up the compound –CO 2 - one mole of C and two moles of O Mass composition is the mass of each element in the compound –CO 2 - 12.0 g of C and 32.0 g of O

41. Wolpa/Advanced Placement Chemistry Percent composition mass of each element per 100 mass units of compound –in 100 g of NH 3, there is 82.0 g of N –therefore, the mass percentage of N is 82.0% N

42. Wolpa/Advanced Placement Chemistry CO 2 Calculation of % composition of carbon dioxide requires determining the number of grams of each element (C and O) in one mole

43. Wolpa/Advanced Placement Chemistry Determination of Empirical formula –Convert percent composition to an actual mass –Convert mass to moles of each element –Find the whole number ratio of the moles of different elements

44. Wolpa/Advanced Placement Chemistry Laughing gas Contains 63.6% N and 36.4% O Assume 100 g of substance, so you have 63.6 g of N and 36.4 g of O Calculation gives an empirical formula of N 2 O

45. Wolpa/Advanced Placement Chemistry

46. Empirical Formula from Experimental Analysis Combustion Method Combust compound in oxygen. Carbon is converted into carbon dioxide, hydrogen is converted into water, remaining element is found by difference.

47. Wolpa/Advanced Placement Chemistry Example An acetic acid sample with a mass of 1.000 g combusts to give 1.466 g CO 2 and 0.6001 g H 2 O. The compound is known to contain C, H, and O. 0.4001 g C from CO 2 0.0673 g H from H 2 O 0.533 g O by difference

48. Wolpa/Advanced Placement Chemistry

49. Empirical Formula From Experiment Actual measurement of masses is determined from experiment. Mass is converted to moles Simplest whole number ratio is determined

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51. Molecular formula The actual number of each atom in a formula unit Consider acetylene and benzene –both have the empirical formula CH –acetylene is actually C 2 H 2 –benzene is actually C 6 H 6

52. Wolpa/Advanced Placement Chemistry Molecular Formula from Empirical Molecular formula must be integral multiple of empirical formula therefore the mass of the molecular formula must be the same integral multiple of the mass of the empirical formula.

53. Wolpa/Advanced Placement Chemistry Example Acetic acid mass of molecular = 60 g/mol mass of empirical formula = 30 g/mol ratio = 2 empirical formula = CH 2. O molecular formula = C 2 H 4 O 2