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John E. McMurry • Robert C. Fay C H E M I S T R Y Sixth Edition Chapter 2 Atoms, Molecules, and Ions Conservation of Mass and the Law of Definite Proportions Law of Conservation of Mass: Mass is neither created nor destroyed in chemical reactions. Conservation of Mass and the Law of Definite Proportions 3.25 g + 3.32 g = 6.57 g Hg(NO3)2(aq) + 2KI(aq) HgI2(s) + 2KNO3(aq) 4.55 g + 2.02 g = 6.57 g Conservation of Mass and the Law of Definite Proportions Law of Definite Proportions: Different samples of a pure chemical substance always contain the same proportion of elements by mass. By mass, water is: 88.8 % oxygen 11.2 % hydrogen The Law of Multiple Proportions and Dalton’s Atomic Theory Law of Multiple Proportions: Elements can combine in different ways to form different substances, whose mass ratios are small whole-number multiples of each other. nitrogen monoxide (NO): nitrogen dioxide (NO2): 7 grams nitrogen per 8 grams oxygen 7 grams nitrogen per 16 grams oxygen Insert Figure 2.2 p37 The Law of Multiple Proportions and Dalton’s Atomic Theory • Elements are made up of tiny particles called atoms. • Each element is characterized by the mass of its atoms. Atoms of the same element have the same mass, but atoms of different elements have different masses. • The chemical combination of elements to make different chemical compounds occurs when atoms join in small whole-number ratios. • Chemical reactions only rearrange how atoms are combined in chemical compounds; the atoms themselves don’t change. Atomic Structure: Electrons Cathode-Ray Tubes: J. J. Thomson (1856-1940) proposed that cathode rays must consist of tiny negatively charged particles. We now call them electrons. Atomic structure: Electrons The strength of deflecting magnetic electric field The size of the negative charge of electron The mass of the electron Atomic Structure: Electrons Millikan’s oil drop experiment Atomic Structure: Protons and Neutrons Rutherford’s Gold Foil Experiment In Rutherford’s gold foil experiment (1871-1937),positively charged particles • were aimed at atoms of gold. • mostly went straight through the atoms. • were deflected only occasionally. Conclusion: There must be a small, dense, positively charged nucleus in the atom that deflects positive particles that come close. Rutherford proposed that the atom must consist mainly of empty space with the mass concentrated in a tiny central core—the nucleus Atomic Structure: Protons and Neutrons The mass of the atom is primarily in the nucleus. Atomic Structure: Protons and Neutrons The charge of the proton is opposite in sign but equal to that of the electron. Atomic Numbers Atomic Number (Z): Number of protons in an atom’s nucleus. Equivalent to the number of electrons around an atom’s nucleus Mass Number (A): The sum of the number of protons and the number of neutrons in an atom’s nucleus Isotope: Atoms with identical atomic numbers but different mass numbers Atomic Numbers Atomic Numbers carbon-12 mass number 12 6 C 6 protons 6 electrons 6 neutrons C 6 protons 6 electrons 8 neutrons atomic number carbon-14 mass number 14 6 atomic number Atomic Masses and the Mole Atomic Mass: The weighted average of the isotopic masses of the element’s naturally occurring isotopes The mass of 1 atom of carbon-12 is defined to be 12 amu. Examples An atom of zinc has a mass number of 65. A. How many protons are in this zinc atom? B. How many neutrons are in the zinc atom? C. What is the mass number of a zinc atom that has Examples An atom has 14 protons and 20 neutrons. A. Its atomic number is B. Its mass number is C. The element is Example Write the atomic symbols for atoms with the following subatomic particles: A. 8 p+, 8 n, 8 e- ___________ B. 47p+, 60 n, 47 e- ___________ Atomic Masses and the Mole Why is the atomic mass of the element carbon 12.01 amu? carbon-12: 98.89 % natural abundance 12 amu carbon-13: 1.11 % natural abundance 13.0034 amu mass of carbon = (12 amu)(0.9889) + (13.0034 amu)(0.0111) = 11.87 amu + 0.144 amu = 12.01 amu Atomic Masses and the Mole Atomic mass unit (amu) is the mass in grams of a single atom Isotopic mass 1 amu = 1.6605 x 10-24g Atomic Mass of Magnesium The atomic mass of Mg • is due to all the Mg isotopes. • is a weighted average. • is not a whole number. 23 Calculating Atomic Mass The calculation for atomic mass requires the • percent(%) abundance of each isotope. • atomic mass of each isotope of that element. • sum of the weighted averages. mass of isotope(1)x (%) + mass of isotope(2) x (%) + 100 100 Calculating Atomic Mass for Cl 35Cl has atomic mass 34.97 amu (75.76%) and 37C has atomic mass 36.97 amu (24.24%). • Use atomic mass and percent of each isotope to calculate the contribution of each isotope to the weighted average. Atomic mass 35Cl x % abundance = Atomic mass 37Cl x % abundance = • Sum is atomic mass of Cl is 25 Learning Check Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69Ga (atomic mass 68.926) atoms and 39.90% of 71Ga (atomic mass 70.925) atoms. What is the atomic mass of gallium? Which isotope has more atoms? 26 Atomic Masses and the Mole Avogadro’s Number (NA): One mole of any substance contains 6.022 x 1023 formula units. Molar Mass: The mass in grams of one mole of any element. It is numerically equivalent to its atomic mass. Molar Mass from Periodic Table Molar mass is the atomic mass expressed in grams. 1 mole of Ag = 107.9 g 1 mole of C = 12.01 g 1 mole of S = 32.07 g Copyright © 2009 by Pearson Education, Inc. 28 Learning Check Give the molar mass for each A. 1 mole of K atoms = ________ B. 1 mole of Sn atoms = ________ 29 Collection Terms A collection term states a specific number of items. • 1 dozen donuts = 12 donuts • 1 ream of paper = 500 sheets • 1 case = 24 cans Copyright © 2009 by Pearson Education, Inc. 2 A Mole of Atoms A mole is a collection that contains • the same number of particles as there are carbon atoms in 12.0 g of carbon. • 6.02 x 1023 atoms of an element (Avogadro’s number). 1 mole element Number of Atoms 1 mole C = 6.02 x 1023 C atoms 1 mole Na = 6.02 x 1023 Na atoms 1 mole Au = 6.02 x 1023 Au atoms 3 Samples of 1 Mole Quantities 1 mole of C atoms = 6.02 x 1023 C atoms 1 mole of Al atoms = 6.02 x 1023 Al atoms 1 mole of S atoms = 6.02 x 1023 S atoms 5 Avogadro’s Number and the Mole Avogadro’s Number (NA): One mole of any substance contains 6.022 x 1023 formula units. One mole of any substance is equivalent to its molar mass. H 1 mole = 1.08 g 6.022 x 1023 molecules = 1.08 g C: 1 mole = 12.01 g 6.022 x 1023 molecules = 12.01 g Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 3/33 Avogadro’s Number Avogadro’s number, 6.02 x 1023, can be written as an equality and two conversion factors. Equality: 1 mole = 6.02 x 1023 particles Conversion Factors: 6.02 x 1023 particles and 1 mole 1 mole 6.02 x 1023 particles 6 Learning Check 1. The number of atoms in 2.0 g of Al atoms is 2. The number of moles of S in 1.8 x 1024 atoms of S 10 Compounds and Mixtures Variable composition Same composition Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 2/36 Compounds and Mixtures Variable properties Similar properties The same number Cannot be of protons separated physically Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 2/37 Molecules and Covalent Bonds Covalent Bond: Results when two atoms share several (usually two) electrons. Typically a nonmetal bonded to a nonmetal Molecules and Covalent Bonds Covalent Bond: Results when two atoms share several (usually two) electrons. Typically a nonmetal bonded to a nonmetal Molecule: The unit of matter that results when two or more atoms are joined by covalent bonds. Molecules and Covalent Bonds Insert Figure 2.12 p56 Ions and Ionic Bonds Ionic Bond: A transfer of one or more electrons from one atom to another. A strong electrical attraction between charged particles. Typically a metal bonded to a nonmetal. Ion: A charged particle Cation: A positively charged particle. Metals tend to form cations. Anion: A negatively charged particle. Nonmetals tend to form anions. Molecules, Ions, and Chemical Bonds In the formation of sodium chloride, one electron is transferred from the sodium atom to the chlorine atom. 11 protons 11 electrons 17 protons 17 electrons Na + Cl Na1+ 11 protons 10 electrons + Cl117 protons 18 electrons Naming Chemical Compounds Cation Charges for Typical Main-Group Ions 1+ 2+ 3+ Naming Chemical Compounds Anion Charges for Typical Main-Group Ions 3- © 2012 Pearson Education, Inc. 2- 1- Chapter 2/44 Naming Chemical Compounds Ionic Compound: A neutral compound in which the total number of positive charges must equal the total number of negative charges. Binary Ionic Compounds Na+ Cl- NaCl magnesium oxide: Mg2+ O2- MgO aluminum sulfide: Al3+ S2- Al2S3 sodium chloride: Naming Chemical Compounds Some transition metals form more than one cation Naming Chemical Compounds Use Roman numerals in parentheses to indicate the charge on metals that form more than one kind of cation. Binary Ionic Compounds iron(III) oxide: Fe3+ O2- Fe2O3 tin(II) chloride: Sn2+ Cl- SnCl2 lead(II) fluoride: Pb2+ F- PbF2 Naming Ionic Compounds To name a compound that contains two elements, • identify the cation and anion. • name the cation first, followed by the name of the anion with an – ide ending. 48 Example Write the formula and names of the following compounds: 1) K+ and Br2) Ca2+ and O23) Pb4+ and S24) Cu2+ and F- 49 Naming Chemical Compounds Binary Molecular Compounds Because nonmetals often combine with one another in different proportions to form different compounds, numerical prefixes are usually included in the names of binary molecular compounds. Naming Chemical Compounds N2F4 The first element listed is more cationlike and takes the name of the element. The second element listed is more anionlike and takes the name of the element with an “ide” modification to the ending. The prefix is added to the front of each to indicate the number of each atom. dinitrogen tetrafluoride Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 2/51 Naming Chemical Compounds Binary Molecular Compounds Whenever the prefix ends in “a” or “o” and the element name begins with a vowel, drop the “a” or “o” in the prefix. N2O4 dinitrogen tetroxide Whenever the prefix for the first element is “mono,” drop it. CO2 carbon dioxide CO carbon monoxide Learning Check Write the name of each covalent compound. P2O5 _____________________ SiCl4 _____________________ CCl4 _____________________ N2O _____________________ 53 Naming Chemical Compounds Naming Chemical Compounds Polyatomic Ionic Compounds Na+ OH- NaOH Mg2+ CO32- MgCO3 sodium carbonate: Na+ CO32- Na2CO3 iron(II) hydroxide: Fe2+ OH- Fe(OH)2 sodium hydroxide: magnesium carbonate: Learning Check Write formula and name of the following compounds: A. Mg2+ and NO3- B. Al3+ and BrO3- C. Fe2+ and SO42- D. Ba2+ and PO32- 56 Examples Write the correct formula for each. A. aluminum nitrate B. copper(II) nitrate C. iron(III) hydrogen sulfite D. tin(IV) hydroxide 57