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Chapter 2 — Atoms, Molecules, Ions ATOMIC COMPOSITION 1 1 • Protons – – – The atom is mostly empty space + electrical charge mass = 1.672623 x 10-24 g relative mass = 1.007 atomic mass units (u or amu) • protons and neutrons in the nucleus. • Electrons – – • electrons in space around the nucleus. no electrical charge mass = 1.009 u • extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water. • 1 u = ? g © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage Mass Number, A • C atom with 6 protons and 6 neutrons is the mass standard • = 12 atomic mass units (u) • Mass Number (A) = # protons + # neutrons • A boron atom can have A = 5 p + 5 n = 10 u © 2009 Brooks/Cole - Cengage PLAY MOVIE • the number of electrons is equal to the number of protons. negative electrical charge relative mass = 0.0005 u • Neutrons – – 2 ATOM COMPOSITION A 10 Z 5 Isotopes 3 • Atoms of the same element (same Z) but different mass number (A). • Boron-10 has 5 p and 5 n: 105B • Boron-11 has 5 p and 6 n: 115B 11B 10B B © 2009 Brooks/Cole - Cengage 4 Chapter 2 — Atoms, Molecules, Ions Hydrogen Isotopes 2 5 6 Mass Spectrometer separates isotopes Hydrogen has 3 isotopes 1 1 H 1 proton + 0 neutrons, protium 2 1 H 1 proton + 1 neutron, deuterium 3 1 H 1 proton + 2 neutrons, tritium © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage Mass Spectrum • a mass spectrum is a graph that gives the relative mass and relative abundance of each particle • relative mass of the particle is plotted in the x-axis • relative abundance of the particle is plotted in the y-axis © 2009 Brooks/Cole - Cengage 7 % Abundance vs % by Weight A charter school class has 6 girls, who each weight 100 lbs, and 4 boys, who each weigh 150 lbs. • What is the % abundance of girls? • What is the average weight of the students? • What is the % by weight of girls? • Isotopic abundance tells % by count © 2009 Brooks/Cole - Cengage 8 Chapter 2 — Atoms, Molecules, Ions 3 9 Isotopes 11B Isotopes & Atomic Weight • Except for carbon-12 with mass = 12 u, isotopic masses are not integer values. However they are very close to integers 10B • Because of the existence of isotopes, the mass of a collection of atoms has an average value. – 10B = 10.0129 u and 11B = 11.0093 u • Actual masses of atoms are always less than the sum of the masses of subatomic particles because of the mass defect • If isotope masses and abundances are known, calculate atomic mass: • Average mass = ATOMIC WEIGHT or Atomic Mass • Boron is 19.9% 10B and 80.1% 11B. That is, 11B is 80.1 percent abundant on earth. Atomic Mass = ∑ (fractional abundance of isotope)n × (mass of isotope)n • For boron atomic mass = 0.199(10.0 u) + 0.801 (11.0 u) = 10.8 u isotope 1 (mass of isotope 1) ( % abundance ) 100 isotope 2 ) (mass of isotope 2) + . . . ( % abundance 100 Atomic Mass = + © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage Elements, Molecules, Ions, Compounds 11 ELEMENTS THAT EXIST AS DIATOMIC MOLECULES PLAY MOVIE NaCl, salt Ethanol, C2H6O Buckyball, C60 © 2009 Brooks/Cole - Cengage 10 © 2009 Brooks/Cole - Cengage 12 Chapter 2 — Atoms, Molecules, Ions ELEMENTS THAT EXIST AS POLYATOMIC MOLECULES White P4 and polymeric red phosphorus S8 sulfur molecules 4 13 O2 and O3 oxygen molecules © 2009 Brooks/Cole - Cengage 15 Charges on Common Ions -3 -2 -1 +1 +2 +3 By losing or gaining e-, atom has same number of e-’s as nearest Group 8A atom. © 2009 Brooks/Cole - Cengage 14 Carbon Allotropes (different structural forms with different chemical & physical properties) © 2009 Brooks/Cole - Cengage IONS AND IONIC COMPOUNDS ELEMENTS THAT EXIST AS MOLECULES © 2009 Brooks/Cole - Cengage 16 Chapter 2 — Atoms, Molecules, Ions 5 Predicting Charges on Monatomic Ions 17 18 METALS M ➔ Mn+ + n ewhere n = periodic group Na+ sodium ion Mg2+ magnesium ion Al3+ aluminum ion Transition metals ➔ M2+ or M3+ are common Fe2+ iron(II) ion Fe3+ iron(III) ion Always the same charge: 1A, 2A, 3A, 5A, 6A, 7A, Ag+, Zn2+ © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage 19 NONMETALS NONMETAL + n e– ➔ Xnwhere n = 8 – Group # Group 4A Group 5A Group 6A Group 7A C4-,carbide N3-, nitride O2-, oxide F-, fluoride Name derived by adding -ide to stem Ion Formation Reaction of aluminum and bromine S2-, sulfide Cl-, chloride PLAY MOVIE Br-, bromide I-, iodide © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage 20 Chapter 2 — Atoms, Molecules, Ions POLYATOMIC IONS 6 21 22 Groups of atoms with a single charge. MEMORIZE the names and formulas in Table 2.4, page 74. Celestite, SrSO4 Note: many O containing anions have names ending in –ate (or -ite). © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage Polyatomic Ions 23 Polyatomic Ions NH4+ ammonium ion One of the few common polyatomic cations © 2009 Brooks/Cole - Cengage HNO3 nitric acid © 2009 Brooks/Cole - Cengage NO3nitrate ion 24 Chapter 2 — Atoms, Molecules, Ions Polyatomic Ions 7 25 CO32carbonate ion Polyatomic Ions 26 CH3CO2acetate ion HCO3bicarbonate ion hydrogen carbonate © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage Naming Oxyanions 27 Polyatomic Ions increasing oxygen content (they contain oxygen) ClO4- perchlorate ClO3- chlorate (most common oxyanion) ClO2 chlorite ClO- - SO42sulfate ion hypochlorite SO32sulfite ion All have the same overall charge and different number of O atoms, so oxidation number of Cl is different in each © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage 28 Chapter 2 — Atoms, Molecules, Ions Polyatomic Ions 8 29 IONIC COMPOUNDS NH4+ NO3nitrate ion Cl- NO2nitrite ion ammonium chloride, NH4Cl © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage Electrostatic Forces 31 Electrostatic Forces COULOMB’S LAW PLAY MOVIE The oppositely charged ions in ionic compounds are attracted to one another by ELECTROSTATIC FORCES. These forces are governed by COULOMB’S LAW. © 2009 Brooks/Cole - Cengage 30 As ion charge increases, the attractive force _______________. As the distance between ions increases, the attractive force ________________. © 2009 Brooks/Cole - Cengage 32 Chapter 2 — Atoms, Molecules, Ions Electrostatic Forces 9 33 Importance of Coulomb’s Law COULOMB’S LAW NaCl, m.p. 804oC Na2O, Na+ and O2-, m.p. 1132oC © 2009 Brooks/Cole - Cengage NaF, m.p. 993oC CaO, Ca2+ and O2m.p. 2572oC (Na+ and Ca2+ are similar size) © 2009 Brooks/Cole - Cengage Naming Molecular Compounds Counting Atoms 35 Chemistry is a quantitative science—we need a “counting unit.” MOLE CO2 Carbon dioxide All are formed from two or more nonmetals. 1 mole is the amount of substance that contains as many particles (atoms, molecules) as there are in 12.0 g of 12C. 518 g of Pb, 2.50 mol CH4 methane © 2009 Brooks/Cole - Cengage 34 BCl3 boron trichloride © 2009 Brooks/Cole - Cengage 36 Chapter 2 — Atoms, Molecules, Ions Molar Mass 10 37 Note that the mass of water is included in the molar mass of a compound. Empirical & Molecular Formulas 38 A pure compound always consists of the same elements combined in the same proportions by weight. Therefore, we can express molecular composition as PERCENT BY WEIGHT Ethanol, C2H6O 52.13% C 13.15% H 34.72% O © 2009 Brooks/Cole - Cengage Determining Formulas In chemical analysis we determine the % by weight of each element in a given amount of pure compound and derive the EMPIRICAL or SIMPLEST formula. © 2009 Brooks/Cole - Cengage 39 A compound of B and H is 81.10% B. What is its empirical formula? • 81.1% B => 81.1 g B, 18.9% H => 18.9 g H • 81.1 g B x 1 mol/10.81 g = 7.50 mol B • 18.9 g H x 1 mol/1.00 g = 18.9 mol H • 18.9 mol/7.50 mol = 2.52 • Ratio (2.5 H : 1 B) x 2 • B2H5 © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage 40 Chapter 2 — Atoms, Molecules, Ions A compound of B and H is 81.10% B. Its empirical 11 41 formula is B2H5. What is its molecular formula? How to Determine a Formula? We need to do an EXPERIMENT to find the MOLAR MASS. Experiment gives molar mass = 53.3 g/mol Compare with the mass of B2H5 = 26.66 g/unit Find the ratio of these masses. 2 units of B2H5 53.3 g/mol = 26.66 g/unit of B2H5 1 mol Molecular formula = B4H10 © 2009 Brooks/Cole - Cengage © 2009 Brooks/Cole - Cengage Mass Spectrum of C2H6O: is it CH3CH2OH or CH3OCH3? CH3O+ C2H5O+ C2H5+ CH3+ © 2009 Brooks/Cole - Cengage C2H6O+ 43 42