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Chapter Two: ATOMS, MOLECULES, AND IONS Early History of Chemistry • Greeks were the first to attempt to explain why chemical changes occur. • Alchemy dominated for 2000 years: – Several elements discovered – Mineral acids prepared • Robert Boyle was the first “chemist”: – Performed quantitative experiments 2.1 Chapter 2 | Slide 2 Copyright © Houghton Mifflin Company. All rights reserved. Three Important Laws • Law of conservation of mass (Lavoisier): – Mass is neither created nor destroyed • Law of definite proportion (Proust): – A given compound always contains exactly the same proportion of elements by mass 2.2 Chapter 2 | Slide 3 Copyright © Houghton Mifflin Company. All rights reserved. Three Important Laws (continued) • Law of multiple proportions (Dalton): – When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers 2.2 Chapter 2 | Slide 4 Copyright © Houghton Mifflin Company. All rights reserved. Dalton’s Atomic Theory (1808) • Each element is made up of tiny particles called atoms. 2.3 Chapter 2 | Slide 5 Copyright © Houghton Mifflin Company. All rights reserved. Dalton’s Atomic Theory (continued) • The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways. 2.3 Chapter 2 | Slide 6 Copyright © Houghton Mifflin Company. All rights reserved. Dalton’s Atomic Theory (continued) • Chemical compounds are formed when atoms of different elements combine with each other. A given compound always has the same relative numbers and types of atoms. 2.3 Chapter 2 | Slide 7 Copyright © Houghton Mifflin Company. All rights reserved. Dalton’s Atomic Theory (continued) • Chemical reactions involve reorganization of the atoms—changes in the way they are bound together. • The atoms themselves are not changed in a chemical reaction. 2.3 Chapter 2 | Slide 8 Copyright © Houghton Mifflin Company. All rights reserved. Concept Check Which of the following statements regarding Dalton’s atomic theory are still believed to be true? I. Elements are made of tiny particles called atoms. II. All atoms of a given element are identical. III. A given compound always has the same relative numbers and types of atoms. IV. Atoms are indestructible. Chapter 2 | Slide 9 Copyright © Houghton Mifflin Company. All rights reserved. Avogadro’s Hypothesis (1811) • At the same temperature and pressure, equal volumes of different gases contain the same number of particles: – 5 liters of oxygen – 5 liters of nitrogen – Same number of particles! 2.3 Chapter 2 | Slide 10 Copyright © Houghton Mifflin Company. All rights reserved. Gay-Lussac and Avogadro • Gay-Lussac measured (under same conditions of T and P) the volumes of gases that reacted with each other. • Avogadro’s Hypothesis: – At the same T and P, equal volumes of different gases contain the same number of particles 2.3 Chapter 2 | Slide 11 Copyright © Houghton Mifflin Company. All rights reserved. Representing Gay-Lussac’s Results 2.3 Chapter 2 | Slide 12 Copyright © Houghton Mifflin Company. All rights reserved. Representing Gay-Lussac’s Results 2.3 Chapter 2 | Slide 13 Copyright © Houghton Mifflin Company. All rights reserved. Representing Gay-Lussac’s Results 2.3 Chapter 2 | Slide 14 Copyright © Houghton Mifflin Company. All rights reserved. Representing Gay-Lussac’s Results 2.3 Chapter 2 | Slide 15 Copyright © Houghton Mifflin Company. All rights reserved. Early Experiments to Characterize the Atom • J. J. Thomson (1898-1903): – Postulated the existence of electrons using cathode-ray tubes – Determined the charge-to-mass ratio of an electron – The atom must also contain positive particles that balance exactly the negative charge carried by the electrons 2.4 Chapter 2 | Slide 16 Copyright © Houghton Mifflin Company. All rights reserved. Cathode Ray Tube 2.4 Chapter 2 | Slide 17 Copyright © Houghton Mifflin Company. All rights reserved. Early Experiments to Characterize the Atom • Robert Millikan (1909): – Performed experiments involving charged oil drops – Determined the magnitude of the electron charge – Calculated the mass of the electron 2.4 Chapter 2 | Slide 18 Copyright © Houghton Mifflin Company. All rights reserved. Millikan Oil Drop Experiment 2.4 Chapter 2 | Slide 19 Copyright © Houghton Mifflin Company. All rights reserved. Early Experiments to Characterize the Atom • Ernest Rutherford (1911): – Explained the nuclear atom – Atom has a dense center of positive charge called the nucleus – Electrons travel around the nucleus at a relatively large distance 2.4 Chapter 2 | Slide 20 Copyright © Houghton Mifflin Company. All rights reserved. Rutherford’s Gold Foil Experiment 2.4 Chapter 2 | Slide 21 Copyright © Houghton Mifflin Company. All rights reserved. The Modern View of Atomic Structure • The atom contains: Electrons Protons – found in the nucleus; positive charge equal in magnitude to the electron’s negative charge Neutrons – found in the nucleus; no charge; virtually same mass as a proton 2.5 Chapter 2 | Slide 22 Copyright © Houghton Mifflin Company. All rights reserved. The Modern View of Atomic Structure • The nucleus is: – Small compared with the overall size of the atom – Extremely dense; accounts for almost all of the atom’s mass 2.5 Chapter 2 | Slide 23 Copyright © Houghton Mifflin Company. All rights reserved. Nuclear Atom Viewed in Cross Section Chapter 2 | Slide 24 Copyright © Houghton Mifflin Company. All rights reserved. The Modern View of Atomic Structure • Isotopes: – Atoms with the same number of protons but different numbers of neutrons – Show almost identical chemical properties; chemistry of atom is due to its electrons – In nature most elements contain mixtures of isotopes 2.5 Chapter 2 | Slide 25 Copyright © Houghton Mifflin Company. All rights reserved. Two Isotopes of Sodium 2.5 Chapter 2 | Slide 26 Copyright © Houghton Mifflin Company. All rights reserved. Exercise A certain isotope X+ contains 54 electrons and 78 neutrons. What is the mass number of this isotope? 2.5/2.6 Chapter 2 | Slide 27 Copyright © Houghton Mifflin Company. All rights reserved. Chemical Bonds • Covalent Bonds: – Bonds form between atoms by sharing electrons – Resulting collection of atoms is called a molecule 2.6 Chapter 2 | Slide 28 Copyright © Houghton Mifflin Company. All rights reserved. Covalent Bonding 2.6 Chapter 2 | Slide 29 Copyright © Houghton Mifflin Company. All rights reserved. Chemical Bonds • Ionic Bonds: Bonds form due to force of attraction between oppositely charged ions Ion – atom or group of atoms that has a net positive or negative charge Cation – positive ion; lost electron(s) Anion – negative ion; gained electron(s) 2.6 Chapter 2 | Slide 30 Copyright © Houghton Mifflin Company. All rights reserved. Molecular vs. Ionic Compounds 2.6 Chapter 2 | Slide 31 Copyright © Houghton Mifflin Company. All rights reserved. The Periodic Table • Metals vs. Nonmetals • Groups or Families – elements in the same vertical columns – Alkali metals, alkaline earth metals, halogens, and noble gases • Periods – horizontal rows of elements 2.7 Chapter 2 | Slide 32 Copyright © Houghton Mifflin Company. All rights reserved. The Periodic Table 2.7 Chapter 2 | Slide 33 Copyright © Houghton Mifflin Company. All rights reserved. Naming Compounds • Binary Compounds: – Composed of two elements – Ionic and covalent compounds included • Binary Ionic Compounds: – Metal-nonmetal • Binary Covalent Compounds: – Nonmetal to nonmetal 2.8 Chapter 2 | Slide 34 Copyright © Houghton Mifflin Company. All rights reserved. Binary Ionic Compounds (Type I) • Cation is always named first and the anion second. • Monatomic cation has the same name as its parent element. • Monatomic anion is named by taking the root of the element name and adding –ide. 2.8 Chapter 2 | Slide 35 Copyright © Houghton Mifflin Company. All rights reserved. Binary Ionic Compounds (Type I) • Examples: KCl Potassium chloride MgBr2 Magnesium bromide CaO Calcium oxide 2.8 Chapter 2 | Slide 36 Copyright © Houghton Mifflin Company. All rights reserved. Binary Ionic Compounds (Type II) • Metals in these compounds form more than one type of positive charge. • Charge on the metal ion must be specified. • Roman numeral indicates the charge of the metal cation. • Transition metal cations usually require a Roman numeral. 2.8 Chapter 2 | Slide 37 Copyright © Houghton Mifflin Company. All rights reserved. Binary Ionic Compounds (Type II) • Examples: CuBr Copper(I) bromide FeS Iron(II) sulfide PbO2 Lead(IV) oxide 2.8 Chapter 2 | Slide 38 Copyright © Houghton Mifflin Company. All rights reserved. Polyatomic Ions • Must be memorized (see Table 2.5 on pg. 62 in text). • Examples of compounds containing polyatomic ions: NaOH Mg(NO3)2 (NH4)2SO4 Sodium hydroxide Magnesium nitrate Ammonium sulfate 2.8 Chapter 2 | Slide 39 Copyright © Houghton Mifflin Company. All rights reserved. Formation of Ionic Compounds 2.8 Chapter 2 | Slide 40 Copyright © Houghton Mifflin Company. All rights reserved. Binary Covalent Compounds (Type III) • Formed between two nonmetals. • First element is named first, using the full element name. • Second element is named as if it were an anion. • Prefixes are used to denote the numbers of atoms present. • Prefix mono- is never used for naming the first element. 2.8 Chapter 2 | Slide 41 Copyright © Houghton Mifflin Company. All rights reserved. Binary Covalent Compounds (Type III) • Examples: CO2 Carbon dioxide SF6 Sulfur hexafluoride N 2O 4 Dinitrogen tetroxide 2.8 Chapter 2 | Slide 42 Copyright © Houghton Mifflin Company. All rights reserved. Overall Strategy for Naming Chemical Compounds Chapter 2 | Slide 43 Copyright © Houghton Mifflin Company. All rights reserved. Flowchart for Naming Binary Compounds Chapter 2 | Slide 44 Copyright © Houghton Mifflin Company. All rights reserved. Acids • Acids can be recognized by the hydrogen that appears first in the formula—HCl. • Molecule with one or more H+ ions attached to an anion. 2.8 Chapter 2 | Slide 45 Copyright © Houghton Mifflin Company. All rights reserved. Acids • If the anion does not contain oxygen, the acid is named with the prefix hydro- and the suffix -ic. • Examples: HCl HCN H2S Hydrochloric acid Hydrocyanic acid Hydrosulfuric acid 2.8 Chapter 2 | Slide 46 Copyright © Houghton Mifflin Company. All rights reserved. Acids • If the anion does contain oxygen: – The suffix -ic is added to the root name if the anion name ends in -ate. • Examples: HNO3 H2SO4 HC2H3O2 Nitric acid Sulfuric acid Acetic acid 2.8 Chapter 2 | Slide 47 Copyright © Houghton Mifflin Company. All rights reserved. Acids • If the anion does contain oxygen: – The suffix -ous is added to the root name if the anion name ends in -ite. • Examples: HNO2 H2SO3 HClO2 Nitrous acid Sulfurous acid Chlorous acid 2.8 Chapter 2 | Slide 48 Copyright © Houghton Mifflin Company. All rights reserved. Flowchart for Naming Acids Chapter 2 | Slide 49 Copyright © Houghton Mifflin Company. All rights reserved. Exercise Which of the following compounds is named incorrectly? a) KNO3 b) TiO2 c) Sn(OH)4 d) PBr5 e) CaCrO4 potassium nitrate titanium(II) peroxide tin(IV) hydroxide phosphorus pentabromide calcium chromate 2.8 Chapter 2 | Slide 50 Copyright © Houghton Mifflin Company. All rights reserved.