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Chapter 2 Atoms, Ions, and Molecules What is matter made of? Continuous or particulate? Ancient view: Four elements (fire, air, earth, water) Democritus (460–370 BC): “father of atomism”. Aristotle (384–322 BC) held that Democritus’ views were impossible. Atomic concept suppressed for ~2000 years. But by the 1700s, things had started to change. Observations Leading to the Atomic View of Matter Law of Conservation of Mass: total mass of substances does not change during a chemical reaction. [Matter can not be created or destroyed.] –Lavoisier, 1789 Law of Definite Composition: no matter its source, a substance is composed of the same elements in the same fractions (or ratio) by mass. –Proust, 1799 mass of element fraction by mass = total mass of substance percent by mass = mass of element total mass of substance 100% Example: A 15.00 gram sample of water contains 13.32 grams of oxygen. Calculate the % by mass of oxygen and hydrogen in water. (Water contains only hydrogen and oxygen.) Law of Multiple Proportions: If elements A, B form a series of substances, the different masses of B that combine with a fixed mass of A will be in a ratio of small whole numbers. –Dalton, 1803 Example: Two compounds of copper and bromine with different properties Substance mass of bromine that combines with 1g copper I 1.26 g II 2.52 g properties yellowish green solid MP = 504 °C, BP = 1345 °C light sensitive, water insoluble black solid MP = 498 °C, BP = 900 °C not light sensitive, is water soluble bromine = element B, copper = element A, ratio of B w/fixed mass of A = 2.52:1.26 or 2:1 2-1 Dalton’s Atomic Theory (1808) 1. matter is made up of microscopic, indivisible particles (atoms) that cannot be created or destroyed Our Understanding Today 2. all atoms of an element are identical in mass, and have identical physical and chemical properties 3. atoms of different elements have different masses, physical properties, and chemical properties 4. atoms of different elements combine in simple whole numbers to form compounds 5. atoms of an element cannot be converted into atoms of other elements; chemical reactions involve reorganization of the atoms But Dalton’s Atomic Theory couldn’t explain it all— Why do atoms combine the ways they do? What about the electrically charged particles that were being observed? Observations Leading to the Nuclear Model of the Atom Cathode Ray Tube Experiments (Figure 2.2): J.J. Thomson, 1897 The particles discovered were called electrons. Thomson determined the mass/charge ratio of the electron in a related experiment. Another experiment was needed to determine the mass or charge. Millikan’s Oil Drop Experiment ( Figure 2.3), 1909 Control of oil drop by electric field allowed for calculation of charge of 1 electron: e– charge = –1.602 10–19 C Since the mass/charge ratio was already known, the mass could also be calculated: e– mass = 9.109 10–28 g So, where’s the positive charge? Speculation was that it was spread throughout the atom. Rutherford’s Alpha Particle Scattering Experiment ( Figure 2.6), 1910 To test this speculation, Rutherford shot α particles (positively charged and relatively massive) at gold foil. Most of the α particles passed through the foil relatively unaffected, but a few bounced back nearly right back at the source. Conclusion: Atoms have a dense, positively charged core called the nucleus. By 1932, Chadwick discovered the neutron (neutral, not electrically charged). Figure 2.6 2-2 Describing Atoms atomic number, Z: # of protons in the nucleus of an atom mass number, A: sum of # of protons + # of neutrons in the nucleus of an atom A write as Z X or AX or “atomic name–A” (where X = atomic symbol) e.g., isotopes: atoms with same Z but different A. (atoms of same element but with different masses) e.g., Atomic Mass standard: 12C atom mass = 12 atomic mass units (amu) exactly 1 1 amu = 12 the mass of a 12C atom [= 1 dalton (Da), often used in biochemistry and biology] Using mass spectrometry, we can measure the masses of other atoms relative to 12C. mass of 20 Ne atom e.g., = 1.66604 mass of 12 C atom mass of 20Ne atom = 1.66604 12 amu = 19.9924 amu Mass spectrometry also gives information about the relative abundances (fractions) of each isotope of the element. The weighted average of the masses of the isotopes is called the atomic mass. e.g., atomic mass of Ne = 0.904838 (19.9924 amu) + 0.002696 (20.9940 amu) + 0.092465 (21.9914 amu) 20.180 amu 10 atomic symbol Ne atomic mass Neon 20.180 2-3 The atomic mass is closest to the mass of the(Z) most abundant atomic number isotope of the element. atomic name The Periodic Table Mendeleev proposed the first periodic table in 1871, similar to today’s table 1A (1) 1 H 8A (18) 2 He 3 Li 2A (2) 4 Be 3A (13) 5 B 4A (14) 6 C 5A (15) 7 N 6A (16) 8 O 7A (17) 9 F 11 Na 12 Mg 13 Al 14 Si 15 P 16 S 17 Cl 18 Ar 22.99 24.31 19 K 1.008 6.941 9.012 10.81 55.85 43 Tc 44 Ru 16.00 19.00 20.18 20 Ca 4B (4) 22 Ti 5B (5) 23 V 6B (6) 24 Cr 7B (7) 25 Mn 39.10 40.08 44.96 47.88 50.94 52.00 37 Rb 38 Sr 39 Y 40 Zr 41 Nb 42 Mo (98) 101.07 102.91 106.42 107.87 112.41 114.82 118.71 121.76 127.60 126.90 131.29 73 Ta 74 W 75 Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn (10) 28 Ni 1B (11) 29 Cu 2B (12) 30 Zn 26.98 28.09 30.97 32.07 35.45 39.95 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 58.93 58.69 63.55 65.39 69.72 72.61 74.92 78.96 79.90 83.80 45 Rh 46 Pd 47 Ag 48 Cd 49 In 50 Sn 51 Sb 52 Te 53 I 54 Xe 85.47 87.62 88.91 91.22 55 Cs 56 Ba 71 Lu 72 Hf 132.91 137.33 174.97 178.49 180.95 183.84 186.21 190.23 192.22 195.08 197.00 200.59 204.38 207.2 208.98 (209) (210) (222) 87 Fr 88 Ra 103 Lr 104 Rf 105 Db 106 Sg 107 Bh 108 Hs 109 Mt 110 Ds 111 Rg 112 Cn 113 Nh 114 Fl 115 Mc 116 Lv 117 Ts 118 Og (267) (268) (271) (270) (269) (278) (281) (281) (285) (286) (289) (289) (293) (294) (294) 57 La 58 Ce 59 Pr 60 Nd 61 Pm 62 Sm 63 Eu 64 Gd 65 Tb 66 Dy 67 Ho 68 Er 69 Tm 70 Yb 138.91 140.12 140.91 144.24 (145) 150.36 151.96 157.25 158.93 162.50 164.93 167.26 168.93 173.04 89 Ac 90 Th 91 Pa 92 U 93 Np 94 Pu 95 Am 96 Cm 97 Bk 98 Cf 99 Es 100 Fm 101 Md 102 No [227] 232.04 231.04 238.03 [237] (244) (243) (247) (247) (251) (252) (257) (258) (259) (223) [226] (262) 95.94 54.94 14.01 10 Ne 3B (3) 21 Sc 92.91 (8) 26 Fe 8B (9) 27 Co 12.01 4.003 METALS, METALLOIDS, NONMETALS Columns: “Groups”; Rows: “Periods” Group Names to LEARN ELEMENTS IN THE O 1A(1): Alkali Metals SAME GROUP O 2A(2): Alkaline Earth Metals ARE USUALLY O 6A(16): Chalcogens CHEMICALLY SIMILAR O 7A(17): Halogens O 8A(18): Noble Gases (somewhat old-fashioned, but still in use) Know/be aware of the A and B group designations as shown above. While now not official, they are too popular and enduring to not be aware of. (They also have uses that we will take advantage of.) “A” group elements are “main group” or “representative” elements. “B” group elements are “transition metals.” The 14-column-wide section at the bottom would fit in between Groups 2A and 3B if there were space. Two numbering conventions are in use. American convention: “A” and “B” groups, as shown above [Europeans reverse the “A” and “B” designations for 3 to 8!] New International convention: Consecutive numbers 1 to 18 (future: 1 to 32?) 2-4 LEARN the names/symbols of the following elements now. (You’ll pick up others as we go along.) Al aluminum Au gold P phosphorus Ar argon He helium Pt platinum As arsenic H hydrogen Pu plutonium Ba barium I iodine K potassium Bi bismuth Kr krypton Rn radon B boron Fe iron Si silicon Br bromine Pb lead Ag silver Cd cadmium Li lithium Na sodium Ca calcium Mg magnesium Sr strontium C carbon Mn manganese S sulfur Cl chlorine Hg mercury Sn tin Cr chromium Ne neon Ti titanium Co cobalt N nitrogen U uranium Cu copper Ni nickel Xe xenon F fluorine O oxygen Zn zinc Bonding in Compounds ionic compound o formed from transfer of electron(s) from one atom (usually a metal) to another (usually a nonmetal) Figure 2.10 o composed of ions (+ ion = cation; – ion = anion) o possesses no net charge (electrically neutral) o contains ionic bonds: interaction between + and – charges In formation of ions, A-group elements gain or lose electrons so as to have the same number of electrons as the nearest noble gas. covalent compound o formed from sharing of electrons between atoms (both usually nonmetals) o most consist of molecules o contains covalent bonds: sharing of electrons between nuclei of bonded atoms Also, some elements exist naturally as covalently bonded molecules (Fig. 2.14) H2 N2 O2 P4 S8 Se8 F2 Cl2 Br2 I2 Plus carbon—diamond, graphite, C60, C70, etc. 2-5 Naming Compounds and Writing Formulas (Chemical Nomenclature) You need to learn the language of chemistry, otherwise you would sound like this…. Backwards words say to used I. Again go I there. [Shucks] oh! --paraphrased from George Carlin LEARN the following: o FIGURE 2.17, Common Monatomic Ions o TABLE 2.3, Common Polyatomic Ions o TABLE 2.2, Numerical Prefixes for Hydrates and Binary Covalent Compounds o SECTION 2.6: Rules for naming and writing formulas for Ionic Compounds and Hydrates Rules for naming and writing formulas for Inorganic Acids Rules for naming and writing formulas for Binary Covalent Compounds 2-6