CHEMISTRY 103 Help Sheet #2 Chapter 2 (Part I); Sections 2.1.8 Do topics appropriate for your lecture Prepared by Dr. Tony Jacob http://www.chem.wisc.edu/areas/clc (Resource page) Nuggets: Periodic Table; Isotopes; Counting neutrons, protons, electrons; Average Atomic Weight; Ionic versus molecular compounds; Naming chemicals; Writing formulas from names; Polyatomic ions; Coulomb s law EXPERIMENTS o Rutherford's Au Foil Experiment: Mass of atom is concentrated in highly dense nucleus that was positively charged; rest has little mass; later split the atom (Some instructors cover different historical experiments; be familiar with what is covered by your instructor.} ATOMIC STRUCTURE Nucleus: contains nearly all the atom s mass; neutrons and protons located in the nucleus; very small volume of atom; extremely dense; atom is mostly empty space Electron (e - ): has a charge of -1; mass 1 x 10 7 g (9.1094 x 10 8 g) Proton (p + ): has a charge of +1; mass 1 x 10 4 g (1.6726 x 10 4 g) Neutron (n): has a neutral charge; mass 1 x 10 4 g (1.6749 x 10 4 g) Relative masses: n > p + >> e - ; mass of n p + 1 with e - 1/2000 Elements: determined by the number of protons they contain Masses of all atoms are compared to mass of 12 C with 12 C defined as 12.0000 unified atomic mass units (exact value); 12 C is the standard; unified atomic mass units = u; 1 u = 1 / 6.022 x 10 23 = 1.661 x 10 4 grams ISOTOPES: elements with the same #p + (same element) but different #n (different mass) Isotope Symbol: Z A Element Symbol A = mass number = #p + + #n; Z = atomic number = #p + C isotopes: 12 6 C, 13 6 C, and 14 6C; the 3 isotopes of C each have Z = 6 (6p + ) but different #n (6, 7, and 8) H isotopes: 1 1 H = H, 2 1 H = 2 1 D = D, 3 1 H = 3 1 T = T; D = deuterium; T = tritium Abundance: percent of one isotope as compared to all atoms of that given element Average Atomic Mass (sometimes called Atomic Weight; AW): weighted average of all isotopes of one element taking into account the abundance of each isotope (also called weighted average mass); units = u; (e.g., average atomic mass Br = 79.90u) AVERAGE ATOMIC MASS (AAM) = Σ(mass of isotope) x (fractional abundance) AAM = (mass iso1 )(fractional abundance iso1 ) + (mass iso2 )(fractional abundance iso2 ) + = AAM = (M 1 )(FA 1 ) + (M 2 )(FA 2 ) + Sum of fractional abundances = 1 (i.e., 100%); for elements with only 2 isotopes: FA 1 + FA 2 = 1 Example 1: Calculate the average atomic mass of an element given following data: isotope 1: 27.977u, 92.21% abundance; isotope 2: 28.976u, 4.70% abundance; and isotope 3: 29.974u, 3.09% abundance. Answer: AAM = (M 1 )(FA 1 ) + (M 2 )(FA 2 ) + (M 3 )(FA 3 ) = (27.977)(0.9221) + (28.976)(0.0470) + (29.974)(0.0309) = 28.09u Example 2: What are the abundances of the 2 isotopes of B: 10 B (mass = 10.0129), 11 B (mass = 11.0093), and average atomic mass of B is 10.811? Answer: AAM = (M1)(FA1) + (M2)(FA2); 10.811 = 10.0129x + 11.0093y; since there are 2 unknowns 2 equations are needed; second equation: FA1 + FA2 = 1; x + y = 1 y = 1 - x; substitute this into the first equation: 10.811 = 10.0129x + 11.0093(1-x); solve for x: 10.811 = 10.0129x +11.0093 11.0093x 0.1983 = 0.9964x x = 0.1990 = 19.90%; y = 1-x = 1-0.1990 = 0.8010 = 80.10%; since x was the abundance for 10 B abundance of 10 B = 19.90% and abundance of 11 B = y = 80.10%}
Mass Spectroscopy: a technique that yields a plot (mass spectrum) of mass versus abundance (or intensity) Example: Mg has 3 isotopes: 24 Mg (79% abundance), 25 Mg (10% abundance), 26 Mg (11% abundance). Draw the mass spectrum for this element? Answer: 79 10 11 Mg 24 10 20 24 25 26 Mass 30 IONIC Compounds: contain metal + nonmetal (can substitute a polyatomic ion for either or both) Properties: extended solids; not molecules; high melting/boiling points; conducts electricity in molten state; Examples: NaCl(s), Ca(NO 3 ) 2 (s), NH 4 NO 3 (s), AgCl(s) Ions: atoms or molecules that have lost/gained e - (charges are not changed by gaining/losing protons!) Cations: positively charged atoms (electrons have been lost) Anions: negatively charged atoms (electrons have been gained) Polyatomic Ions: many-atom ions ; these compounds are not broken up and are treated as a group NH 4 +, NO 3 -, SO4, OH -, PO 4-3, CO 3, HCO 3 -, CH3 COO -, etc. (these often need to be memorized) Ionic Compound Naming Metal + Nonmetal ionic compound; example: What is the name for CaCl 2? Metal name followed by nonmetal root +"ide" Answer: calcium chloride Metal + Polyatomic ion ionic compound; example: What is the name for Ca 3 (PO 4 ) 2? Metal name followed by polyatomic ion name Answer: calcium phosphate Two Polyatomic ions ionic compound; example: What is the name for (NH 4 ) 2 SO 4? Polyatomic ion name followed by polyatomic ion name Answer: ammonium sulfate Transition Metal example: What is the name for Fe 2 (SO 4 ) 3? 1. Transition metals have charges that vary so the metal charge needs to be specified and is written in Roman numerals in parentheses after the metal name 2. Find charge on Fe. First, SO 4 charge is (memorized); 3() = -6 from 3 SO 4 ; since the compound is neutral the 2 Fe must total +6 ea Fe = +3; Fe +3 3. Metal name (charge in Roman numerals) followed by polyatomic ion name Answer: iron(iii) sulfate Ionic Compound Writing Formulas 1. Take metal name or polyatomic ion and write formula including charge If metal has a Roman numeral, then charge is determined from that numeral (e.g., copper(ii) = Cu +2 ) If it does not have a Roman numeral, it is a memorized polyatomic ion (e.g., ammonium = NH 4 + ) or determined from column Periodic Table it resides in (e.g., magnesium = Mg +2 since it is in column IIA) 2. Take nonmetal name or polyatomic ion and write formula including charge If nonmetal name ends with ide it is a single atom ion with a negative charge with the charge determined from column in Periodic Table it resides in (e.g., chloride = Cl - since column VIIA has a -1 charge); exceptions: hydroxide (OH - ) and cyanide (CN - ) are polyatomic ions even though the name ends in ide If name ends with ite or ate, or starts with hypo or per it is a memorized polyatomic ion 3. Balance charges within formula Example: Write formula for chromium(iii) carbonate Cr +3 from Roman numeral; carbonate = CO 3 (memorized) Cr x (CO 3 ) y ; choose x and y to balance charge: x = 2 [2(+3) = +6] and y = 3 [3() = -6]; +6 balances -6 Cr 2 (CO 3 ) 3 Charges on elements in Ionic Compounds based on the column of Periodic Table the element resides in: Group IA: +1 (H +, Li +,...) Group IIA: +2 (Be +2, Mg +2,...) Group IIIA: +3 (B +3, Al +3,...) Group VA: -3 (N -3, P -3,...) Group VIA: (O, S,...) Group VIIA: -1 (F -, Cl -,...)
MOLECULAR Compounds contain 2 nonmetal elements; Examples: CO, CO 2, N 2 O 4 Naming: Prefix (if greater than 1)+1st nonmetal name & Prefix(always)+2nd nonmetal root+"ide Prefixes: mono (1), di (2), tri (3), tetra (4), penta (5), hexa (6), hepta (7), octa (8), nona (9), deca (10) Example: What is the name for NO? (Has 2 nonmetals molecular compound) Has one N no prefix nitrogen; has one O prefix always used = mono; monoxide answer: nitrogen monoxide Example: What is the name for N 2 O? (Has 2 nonmetals molecular compound) Has two N use prefix = di dinitrogen; has one O prefix always used = mono; monoxide answer: dinitrogen monoxide Writing Formulas - translate name with prefixes Example: What is the formula for disulfur trioxide? disulfur 2S; trioxide 3O; answer: S 2 O 3 ACIDS Naming chemicals named as acids are neutral compounds (e.g., SO 4 and HSO 4 are not named as acids but H 2 SO 4 is named as an acid) Acids that contain anions ending in ide : change ide to ic, add hydro in front and acid at end; e.g., Cl - = chloride; HCl = hydrochloric acid ate : change ate to ic and add acid at the end; e.g., SO 4 = sulfate; H 2 SO 4 = sulfuric acid Common acids to know: Strong acids: HCl (hydrochloric acid), HBr (hydrobromic acid), HI (hydroiodic acid), HNO 3 (nitric acid), H 2 SO 4 (sulfuric acid), HClO 4 (perchloric acid); Weak acids: HF (hydrofluoric acid), HCN (hydrocyanic acid), H 3 PO 4 (phosphoric acid), HC 2 H 3 O 2 = CH 3 COOH (acetic acid) (often good to memorize acetic acid), H 2 CO 3 (carbonic acid) Organic Compounds - This material is usually not covered in Chem 103. COULOMB S LAW: Describes the attractive force between a positive (cation) and negative (anion) ion Force of Attraction = k (Q 1 )(Q 2 ) d 2 Q 1, Q 2 are charges on the ions; d is distance between the centers of the nuclei ionic charges (Q) or ionic radii = d Force mp/bp (ionic radii as you go up a column) 1. Determine the number of protons, neutrons, and electrons present in each of the following atoms. a. 53 24 Cr b. 55 25 Mn c. 9 4 Be d. 40 20 Ca e. 3 1 H 2. How many p +, n, and e - are in each of the following ions? a. 28 13 Al +3 b. 41 20 Ca +2 c. 59 28 Ni +2 d. 36 17 Cl 3. Write the complete isotopic symbol with mass and atomic number for each atom: a. a neutral atom with 28 protons and 30 neutrons b. contains 22 protons and 21 neutrons c. an oxygen atom with 10 neutrons d. chromium atom with a mass number of 54 e. a halogen with 35 protons and 44 neutrons f. an alkali metal with the fewest protons and 4 neutrons g. an iron atom with mass number of 56 and 3 more protons than electrons h. sulfide ion with 17 neutrons 4. For each, state whether the statement is true or false? a. The number of neutrons distinguishes carbon from nitrogen. b. Elements are neutral because they have equal numbers of protons and electrons. c. Elements that have different numbers of neutrons are called isotopes. 5. How many p +, n, and e - are in the isotopic sulfate (SO 3 ) ion: 33 S 16 O 18 O 18 O?
6. Which of the following statements is false? a. All atoms contain neutrons. b. All atoms contain protons. c. The mass of the neutron is greater than the mass of the proton? d. The electrons form a cloud around the nucleus. e. Atoms can easily gain or lose electrons but can not easily gain or lose neutrons. 7. Thoroughly describe the Rutherford experiment with a diagram. 8. Oxygen exists as three isotopes: 18 8 O, 17 8 O, and 16 8 O. If the average atomic weight of oxygen is 15.9994, what do you expect the abundance of 18 8 O to be approximately? <5% 25% 50% 75% 90% 9. An element has two naturally occurring isotopes with the following abundances and masses: Isotope Atomic Mass (u) Fractional Abundance 1 84.912 0.7215 2 86.909 0.2785 What is the average atomic mass of this element? What is the identity of the element? 10. Silver is found commonly as two isotopes: 107 Ag (mass = 106.91u) and 109 Ag (mass = 108.90u). The average atomic mass of silver is 107.869u. What are the percent abundances of the two isotopes, 107 Ag and 109 Ag? 11. There are 2 isotopes of Cl: 35 Cl (mass = 34.969u) and 37 Cl (mass = 36.947u). If the average atomic mass of Cl is 35.453u, what are the percent abundances of the Cl isotopes? 12. There are 3 isotopes of magnesium: 24 Mg (mass = 23.985u), 25 Mg (mass = 24.986u), and 26 Mg (mass = 25.983u). If the abundance of 25 Mg is 10.00% what are the percent abundances of the other two isotopes of magnesium? 73.7% 8.3% 7.4% 5.4% 5.2% 13. Shown is the mass spectrum of Ti. What is the average atomic mass of Ti? 46 47 48 m/e 49 50 14. Chlorine, Cl 2, exists as a diatomic gas. During a mass spectrometer experiment a sample of chlorine was analyzed. Chlorine is found as a two isotopes: 35 Cl and 37 Cl with approximate abundances: 35 Cl = 75% and 37 Cl = 25%. a. Draw a mass spectrum that would be expected for atomic chlorine (i.e., for Cl). Include on the x-axis mass values and on the y-axis the peak intensity. Assume the isotopic mass number is the mass of that isotope (i.e., 35 Cl has a mass of 35). b. How many peaks are expected in the mass spectrum for Cl 2? c. What would be the mass and intensity of each peak in the mass spectrum for Cl 2? d. Draw the expected mass spectrum for Cl 2.
15. a. Which of the following elements are metals? Ar, Mn, C, O 2, Al b. What is the name of the family of elements that contains magnesium? c. Name one halogen element. 16. Which of the following will have the highest melting point? a. BeO b. NH 3 c. MgS d. KBr 17. Which of the following compounds would have properties most similar to CaF 2? a. CO 2 b. SF 2 c. MgCl 2 d. NaCl e. CaO 18. Name the following molecules. a. NO b. PCl 3 c. KBr d. Na 2 CO 3 e. N 2 O 4 f. K 2 SO 4 g. Ca 3 (PO 4 ) 2 h. Cu 2 O i. Mn 3 (PO 4 ) 2 j. Co(CH 3 COO) 3 k. CoSO 4 l. KMnO 4 m. CH 4 19. Write the chemical formula for the following names. a. sulfur trioxide b. calcium fluoride c. chlorine monobromide d. sodium sulfate e. ammonium nitrate f. lithium carbonate g. calcium nitrate h. manganese(iii) sulfate i. chromium(vi) oxide j. titanium(iv) sulfide 20. Which of the following when placed in an ionic compound, will commonly have a charge of 2? I. Cl II. O III. SO 4 IV. Ca V. PO 4 VI. NO 3 a. II b. II and III c. II and IV d. II, III, and IV e. II and VI (see Practice Sheet #3 for polyatomic ion naming/writing formula practice; see Practice Sheet #4 for practice naming and writing formulas of compounds) ANSWERS 1. a. 24 p +, 24 e -, 29 n b. 25 p +, 25 e -, 30 n c. 4 p +, 4 e -, 5 n d. 20 p +, 20 e -, 20 n e. 1 p +, 1 e -, 2 n 2. a. 13 p +, 10 e -, 15 n b. 20 p +, 18 e -, 21 n c. 28 p +, 26 e -, 31 n d. 17 p +, 18 e -, 19 n 3. a. 58 28 Ni b. 43 22 Ti c. 18 8 O d. 54 24 Cr e. 79 35 Br f. 7 3 Li g. h. 4. a. F b. T c. T 5. protons = 40; neutrons = 45; electrons = 42 6. a (hydrogen does not) Au Foil 7. Rutherford targeted alpha particles at thin Au foil and placed a photographic detector all around the foil. α particles Nearly all the α particles went through the foil but a small number bounced back. He deduced that most of the atom is "empty" (hence most α particles pass through the foil) but there is a highly dense, positively charged, small "nucleus" (hence, a small number of α particles bounced back at sharp angles). nucleus empty space 8. <5% {since the average atomic weight of O = 15.9994 16, then the abundance of 16 O must be close to 100% since the average weight is close to 16 and therefore the abundance of 18 O must be very close to zero otherwise the average atomic mass would be higher than 16} 9. 85.47u; Rb {(84.912)(0.7215) + (86.909)(0.2785) = 85.468u} 10. 107 Ag: 51.8%, and 109 Ag: 48.2% {107.869 = 106.91(x) + 108.90(1 - x); x = abundance 107 Ag; 107.869 = 106.91x + 108.90 108.90x; -1.031 = -1.990x; x = 0.5181 = 107 Ag abundance; 109 Ag abundance = 1 x = 1 0.5181 = 0.4819}
11. 35 Cl = 75.5%; 37 Cl = 24.5% {35.453 = 34.969(x) + 36.947(1 - x); x = abundance 35 Cl; } 35.453 = 34.969x + 36.947 36.947x; -1.494 = -1.978x; x = 0.7553 = 35 Cl abundance; 37 Cl abundance = 1 x = 1 0.7553 = 0.2447} 12. 24 Mg = 78.99%; 26 Mg = 11.01% {x = abundance 24 Mg; y = abundance 26 Mg; sum of fractional abundances: 1.000 = x + 0.1000 + y; y = 1.000 0.1000 x; 24.3050 = 23.985(x) + 24.986(0.1000) + 25.983(1 0.1000 x); 24.3050 = 23.985x + 2.4986 + 23.3847 25.983x; 24.3050 = 25.8833 1.998x; 1.5783 = 1.998x; x = 0.7899 = 24 Mg abundance = 78.99%; 26 Mg abundance = 100.00 10.00 78.99 = 11.01%} 13. 47.9amu {AAM = (M 1 )(FA 1 ) + (M 2 )(FA 2 ) +; AAM = (46)(0.083) + (47)(0.074) + (48)(0.737) + (49)(0.054) + (50)(0.052) = 47.92amu} 14. a. {2 isotopes so 2 peaks are expected in the mass spectrum. The mass of each peak corresponds to the mass number (peaks at mass = 35 and mass = 37). The intensity of each peak reflects the relative abundance of each isotope so 35 Cl would be 75% and 37 Cl would be 25%. [The peaks need to be in a ratio of 3:1 for the 35 Cl and 37 Cl isotopes which are in a ratio of 3:1 from 75%/25% = 3:1.} b. 3 peaks { 35 Cl 35 Cl (mass = 70), 35 Cl 37 Cl (mass = 72) same mass as 37 Cl 35 Cl (mass = 72), 37 Cl 37 Cl (mass = 74)} c. Peak 70 : Peak 72 : Peak 74 = 56% : 38% : 6% ratio {peak 70 = (0.75)(0.75) = 0.56; peak 72 = (0.75)(0.25) + (0.25)(0.75) = 0.38; peak 74 = (0.25)(0.25) = 0.063} d. 75% 25% 35 36 37 38 Mass 56% 38% 6% 70 71 72 73 Mass 74 15. a. Mn, Al b. alkaline earth metals c. fluorine, chlorine, bromine, iodine (any one of these) 16. a {NH 3 is a molecular compound (2 nonmetals) and molecular compounds have much lower melting points than ionic compounds; KBr has +1/-1 charges while BeO and MgS have +2/ charges; higher charges will yield higher melting points so drop KBr; the other ionic compounds have the same charges: Be +2 O, Mg +2 S ; to differentiate between these the smaller the ions the higher the melting point; ions higher in a column on the Periodic Table are smaller, and therefore Be +2 is smaller than Mg +2 ; likewise, O will be smaller than S ; so BeO will have the highest melting point} 17. c 18. a. nitrogen monoxide b. phosphorus trichloride c. potassium bromide d. sodium carbonate e. dinitrogen tetroxide f. potassium sulfate g. calcium phosphate h. copper(i) oxide i. manganese(ii) phosphate j. cobalt(iii) acetate k. cobalt(ii) sulfate l. potassium permanganate m. methane (common name; memorize) 19. a. SO 3 b. CaF 2 c. ClBr d. Na 2 SO 4 e. NH 4 NO 3 f. Li 2 CO 3 g. Ca(NO 3 ) 2 h. Mn 2 (SO 4 ) 3 i. CrO 3 j. TiS 2 20. b
Polyatomic Ions, Acids, Common Names (Note: Acids and other chemicals in italics are common chemicals but may not be on the list of chemicals to be memorized!) If polyatomic ion name ends with ate then the acid (adding H + until the ion is neutral) name ends with ic acid e.g., SO 4 = sulfate H2 SO 4 = sulfuric acid If polyatomic ion name ends with ite then the acid (adding H + until the ion is neutral) name ends with ous acid e.g., NO 2 - = nitrite HNO2 = nitrous acid If ion is a single atom (e.g., Cl - ) then the ion name ends with ide (exceptions: CN -, cyanide, and OH -, hydroxide) and acid name is hydro+root+ ic acid (e.g., HCl = hydrochloric acid) NH + 4 (ammonium) H 3 O + (hydronium) OH - (hydroxide) CH 3 COO - (acetate; also written: C 2 H 3 O - 2 ) NO - 3 (nitrate) NO - 2 (nitrite) CH3 COOH (acetic acid) HNO 3 (nitric acid; strong acid) HNO 2 (nitrous acid) MnO - 4 (permanganate) CO 3 (carbonate) HCO - 3 (hydrogen carbonate) H 2 CO 3 (carbonic acid) SO 4 (sulfate) HSO - 4 (hydrogen sulfate) H 2 SO 4 (sulfuric acid; strong acid) SO 3 (sulfite) HSO - 3 (hydrogen sulfite) H 2 SO 3 (sulfurous acid) ClO - 4 (perchlorate) ClO - 3 (chlorate) ClO - 2 (chlorite) ClO - (hypochlorite) CN - (cyanide) CrO 4 (chromate) HClO 4 (perchloric acid; strong acid) HClO 3 (chloric acid) HClO 2 (chlorous acid) HClO (hypochlorous acid) HCN (hydrocyanic acid) PO 4-3 (phosphate) HPO 4 (monohydrogen phosphate) H 2 PO 4 - (dihydrogen phosphate) H 3 PO 4 (phosphoric acid) PO 3-3 (phosphite) HPO 3 (monohydrogen phosphite) H 2 PO 3 - (dihydrogen phosphite) H 3 PO 3 (phosphorous acid) F - (fluoride) Cl - (chloride) Br - (bromide) I - (iodide) NH 3 = ammonia H 2 O 2 = hydrogen peroxide CH 4 = methane C 2 H 6 = CH 3 CH 3 = ethane HF (hydrofluoric acid) HCl (hydrochloric acid; strong acid) HBr (hydrobromic acid; strong acid) HI (hydroiodic acid; strong acid) Some Common Chemicals and Common Names C 3 H 8 = CH 3 CH 2 CH 3 = propane C 4 H 10 = CH 3 CH 2 CH 2 CH 3 = butane CH 3 OH = methanol CH 3 CH 2 OH = ethanol