Chapter 2. Atoms, Molecules, and Ions

Conservation of Mass Chapter 2 z z Atoms, Molecules, and Ions An STM image of tiny wires, just 10 atoms thick and laid down on a surface of silicon. z Nickel atoms – STM http://www.almaden.ibm.com/vis/stm/atomo.html Law of Definite Proportions z Lavoisier and Proust discovered that all samples of a single substance exist in the same ratio a o by mass. ass 1600’s: Boyle studies chemistry 1774: Priestley heated HgO (mercury(II) oxide) in a closed container the mass didn’t container, didn t change. Lavoisier determined O2 is in combustion. Law of Conservation of Mass: mass is neither created nor destroyed in chemical reactions. Dalton’s Atomic Theory z Dalton's Theory: n Elements are made up of tiny particles called atoms. o Each element is characterized by the mass of its atoms atoms. Atoms of the same element have the same mass, but atoms of different elements have different masses. p Chemical combination of elements to make different substances occurs when atoms join together in small, whole-number ratios. q Chemical reactions only rearrange the way that atoms are combined; the atoms themselves don’t change.  H2O: 1 part H, 8 parts O, by mass  CO2: 1 part C, 2.7 parts O, by z Conservation of Mass mass Different samples of a pure chemical substance always contain the same proportion of elements by mass. 4 Law of Multiple Proportions z Elements can combine in different ways to form different substances, whose mass ratios are small, whole-number multiples of each o e other. z Worked Ex. 2.1; Problem 2.1 The Structure of the Atom z Late-1880’s, J.J. Thomson experimented with cathode ray tubes, electricity, and magnetism to learn more about atoms’ structures. He discovered the presence of negatively charged particles now called electrons particles, electrons. 6 1 Cathode Ray Tubes z Why does the beam bend? Oil Drop Experiment z TV CRT www.shiningltd.com/tv-crt.htm Cathode Ray In early 1900’s, Millikan used an oil drop experiment to determine the charge of an electron (1.6 x 10-19 C). Oil Drop 7 Nuclear Atom z z History Overview In 1911, Rutherford, tried to determine the arrangement of negative electrons and positive nucleus in an atom. Gold foil experiment: p Law of Electrostatic Attraction; positive alpha particles Rutherford’s Experiment 8 z z z z Boyle: first to study chemistry Priestley & Laviosier: Conservation of Mass Proust: Law of Definite Proportions D l Dalton: 4 postulates l – Theory Th off matter  Law of Multiple Proportions Gold Foil z z z Thomson: presence of electrons; plum pudding Millikan: charge of an electron Rutherford: presence of small, positive nucleus 9 Relative Sizes z Atomic Structure Nucleus is to an atom what a pea is to a stadium Electron, proton, Electron proton & neutron z Worked Ex. 2.2; Problems 2.2, 2.3 2 Atomic Numbers z Atomic #, Mass #, and Isotopes: Carbon proton What makes one atom different from another?  Their number of protons, aka their atomic number (Z) neutron electron 12C 13C 13C– 6 protons 6 neutrons 6 protons 7 neutrons 6 protons 7 neutrons (subscript) z Where can we find this information on the periodic table?  H = 1 proton, t H He = 2 protons, t Li = 3 protons t z Number of neutrons in nucleus also influences an atom’s mass and, therefore, its behavior. How many protons in each box? How many neutrons in each box? How many electrons in each box? What distinguishes a neutral atom from an ion? How is the charge on an ion determined? What do all carbon atoms have in common? What feature is different in isotopes of an element? How is the mass number (A) determined?  Mass number (A) is sum of protons & neutrons (superscript).  Most hydrogen: Z = 1, A = 1. How many neutrons?  Most helium: Z = 2, A = 4. How many neutrons? z z How many protons and neutrons in 54Cr (chromium-54)? Worked Ex. 2.4; Problems 2.4 – 2.6 Atomic #, Mass #, and Isotopes z z Atomic Mass How many of each particle in these atoms (protons, electrons, neutrons)? 1H+ 3H z z z z 12C 14C4+ z Nuclear Particles z 15O 16O2- z 14N 15N3- z 238U 234U z z Atomic mass unit: exactly one-twelfth the mass of a carbon-12 atom. Mass of 1 12C atom = 12 amu (exactly) 6.022 x 1023 amu = 1 g Atomic mass on periodic table is weighted average of abundance of an atom’s isotopes.  12C  13C = 12 amu; 98.89% = 13.0034 amu; 1.11% What is the mass of Cu if 69.09% has a mass of 62.93 amu and 30.91% has a mass of 64.94 amu? Worked Ex. 2.5; Problems 2.7, 2.8 15 Terminology z z z z z Atom: individual atom (one sphere in an image) Molecule: substance made up of two or more atoms Compound: Co pou d subs substance a ce made ade up o of two oo or more oe different atoms or ions Pure substance: all atoms, compounds, or molecules are the same in a sample Mixture: blends of two or more different substances Classifications of Matter z Describe each picture below using the terms  mixture or pure substance  atom, element, molecule, and/or compound  Homogeneous: uniform throughout (seawater)  Heterogeneous: not uniform (water and oil) 18 3 Classify z z Answers Mixture or Pure Substance Atom, element, molecule, and/or compound z z z z z z z z How many boxes are mixtures? How many boxes contain only compounds? How many boxes contain elements? z z z z z z z A1: pure substance; molecules of a compound A2: pure; molecules of a compound A3: pure; molecules of a compound B1: pure; molecules of a compound B2: pure; molecules of a compound B3: mixture; molecules of elements C1: pure; atoms of an element C2: mixture; molecules of compounds C3: mixture; atoms and molecules of elements D1: pure; atoms of an element D2: pure; molecules of a compound D3: mixture; molecules of compound, atoms of element 19 Classifications of Matter z Covalent Chemical Bonds How would you classify the following? z Diatomic molecules: z Covalent bonds: form when atoms share electrons; typically between nonmetals  Air  Distilled water  Ice  H2, N2, F2, O2, I2, Cl2, Br2 Cereal & iron  Ice Cream  Rocks  Gold Ink Chromatography 21 Covalent Compounds z z Write formulas for the following structures: Ethanol Glucose Ionic Chemical Bonds z z Ionic bonds form when atoms transfer electrons; typically between metal and Form NaCl nonmetal Ions are atoms or groups of atoms that have a positive or negative charge (have gained or lost electrons)  Anions have gained one or more electrons; since electrons are negative, anions have negative charges  Cations have lost one or more electrons; cations have positive charges z z NaCl (salt); NaOH (in bleach) Worked Ex. 2.6; Key Concept 2.7, 2.11, 2.12; Problem 2.9, 2.10 4 Molecules, Ions, and Chemical Bonds z What do Ionic & Covalent Compounds Look Like? Which side of the periodic table gains electrons? Loses electrons?  Metals tend to lose electrons, have a positive charge (cation)  Nonmetals N t l ttend d tto gain i electrons, l t h have a negative ti charge (anion) z z z z Na Æ Cl Æ Mg Æ S Æ Na1+ Cl1Mg2+ S2NaCl CH4 26 Covalent vs Ionic Bonding Formulas of Ionic Compounds Ca in CaF2 z Classify the following types of bonds as covalent or ionic: NO2 CaO KBr CO2 KrF2 CuCl2 z Worked Ex. 2.8; Problems 2.13, Key Concept 2.14 z z z z z z z z z z Fluorite: CaF2 How are these ions bonded? Which picture below could be CaF2? Wh iisn’t Why ’ the h structure C Ca – F – F ? F in CaF2 Green = metal, blue = nonmetal 5