The Early History Of Chemistry Atoms, Molecules, Alchemy .

Atoms,Molecules,and IonsThe Early History of ChemistryBefore 16th Century–Alchemy: Attempts (scientific or otherwise) tochange cheap metals into gold17th CenturyChapter 2–Robert Boyle: First “chemist” to performquantitative experiments18th Century–George Stahl: Phlogiston flows out of a burningmaterial.–Joseph Priestley: Discovers oxygen gas,“dephlogisticated air.”Law of Conservation of MassOther Fundamental Chemical LawsLaw of Definite ProportionDiscovered by Antoine LavoisierMass is neither created nor destroyedA given compound always contains exactlythe same proportion of elements by mass.Combustion involves oxygen, notphlogistonCarbon tetrachloride is always 1 atomcarbon per 4 atoms chlorine.35.45 g Cl x 412.01g COther Fundamental Chemical Laws 11.83 g Cl1.000 g CCCl4Dalton’s Atomic Theory (1808)Law of Multiple ProportionsWhen two elements form a series ofcompounds, the ratios of the masses of thesecond element that combine with 1 gram ofthe first element can always be reduced tosmall whole numbers.The ratio of the masses of oxygen in H2Oand H2O2 will be a small whole number(“2”).H 2O8.0 g O , H2O21.0 g HEach element is made up of tiny particlescalled atoms.The atoms of a given element areidentical; the atoms of different elementsare different in some fundamental way orways.16.0 g O, so 16.0 g O 2.01.0 g H8.0 g O1

Dalton’s Atomic Theory(continued)Chemical compounds are formed whenatoms combine with each other. A givencompound always has the same relativenumbers and types of atoms.Chemical reactions involve reorganizationof the atoms - changes in the way they arebound together. The atoms themselves arenot changed in a chemical reaction.Figure 2.4: A representation ofsome of Gay-Lussac's experimentalresults on combining gas volumes.Avogadro’s Hypothesis (1811)At the same temperature and pressure, equalvolumes of different gases contain the samenumber of particles. 5 liters of oxygen 5 liters of nitrogen Same number of particles!Figure 2.5: A representation ofcombining gases at the molecular level.The spheres represent atoms in themolecules.2 H2 (g) O2 (g) º 2 H2O (g)H2 (g) Cl2 (g) º 2 HCl (g)Early Experiments toCharacterize the AtomJ. J. Thomson - postulated the existence ofelectrons using cathode ray tubes.Figure 2.7: A cathode-ray tube. Thefast-moving electrons excite the gas inthe tube, causing a glow between theelectrodes.Ernest Rutherford - explained the nuclearatom, containing a dense nucleus withelectrons traveling around the nucleus at alarge distance.2

Figure 2.8: Deflection of cathoderays by an applied electric field.Figure 2.5: Thecathode-raybeam travelsright to left.Photo courtesyof JamesScherer.Figure 2.5: Thebeam of negativeparticles bendsdownward.Photo courtesyof JamesScherer.Figure 2.5: Themagnet is turnedaround and thebeam bends inopposite direction.Photo courtesy ofJames Scherer.Atomic Theory of Matter The Structure of the Atom– In 1909, U.S. physicist, Robert Millikan hadobtained the charge on the electron. (see Figure2.6)– These two discoveries combined provided uswith the electron’s mass of 9.109 x 10-31 kg,which is more than 1800 times smaller than themass of the lightest atom (hydrogen).– These experiments showed that the electronwas indeed a subatomic particle.Figure 2.10: A schematicrepresentation of the apparatusMillikan used to determine the chargeon the electron.3

Figure 2.9: The plum puddingmodel of the atom.Figure 2.13: (a) The expectedresults of the metal foil experiment ifThomson's model were correct.(b)Actual results.Figure 2.12: Rutherford'sexperiment onα-particle bombardment of metalfoil.The Modern View of AtomicStructureThe atom contains:electronsprotons: found in the nucleus, they have apositive charge equal in magnitude to theelectron’s negative charge.neutrons: found in the nucleus, virtuallysame mass as a proton but no charge.Figure 2.14: Anuclear atomviewed in crosssection. Notethat thisdrawing isnot to scale.1 atomic mass unit (or Dalton) 1/12 of a 12C atom or1.66054 x 10-27 kg. You cannot calculate the absolutemass of a 12C nucleus by adding the masses of 6 protonsand 6 neutrons - the mass that you calculate will be greaterthan (12)(1.66054 x 10-27 kg). While mass is conserved inchemical reactions it is not conserved in nuclear reactionsand formation of the carbon nucleus from protons &neutrons results in a small mass loss (see Chapter 18,your text.)4

Figure 2.15: Two isotopes of sodium. Bothhave eleven protons and eleven electrons,but they differ in the number of neutrons intheir nuclei.The Chemists’ Shorthand: AtomicSymbolsMass number Atomic number 3919 K Element SymbolFor a neutral K atomFigure 2.14: A modern form of the periodic table.Return to Slide 20Periodic TableElements classified by:propertiesatomic numberGroups (vertical)1A2A7A8A alkali metalsalkaline earth metalshalogensnoble gases28IonsCation: A positive ionMg2 , NH4 Anion: A negative ionCl , SO42 Ionic Bonding: Force of attraction betweenoppositely charged ions.Periods (horizontal)5

Monatomic IonsElements which have an excess or deficit of electronsExamples: Sodium ion has 11 protons and on 10 electrons.The net charge on the atom is 1 (obtained byadding the charge of the protons and electrons)We would write the symbol for sodium ion orNa 23 11NaWhat would be the symbol of an element withatomic number 17 and 18 electrons?35Cl17Chemical Substances;Formulas and Names Rules for predicting charges onmonatomic ions– Most of the main group metals form cations withthe charge equal to their group number.– The charge on a monatomic anion for a nonmetalequals the group number minus 8.– Most transition elements form more than one ion,each with a different charge. (see Table 2.5)The Chemists’ Shorthand:Formulas Chemical Formula: Symbols types of atoms Subscripts relative numbers of each type ofatomCO2 Structural Formula: Individual bonds are shown by lines.O C OChemical Formulas; Molecularand Ionic SubstancesChemical Formulas; Molecularand Ionic Substances Ionic substances Ionic substances– The formula of an ionic compound is written bygiving the smallest possible whole-number ratio ofdifferent ions in the substance.– The formula unit of the substance is the group ofatoms or ions explicitly symbolized by its formula.NaClCaCl2– When an atom picks up extra electrons, it becomesa negatively charged ion, called an anion.– An atom that loses electrons becomes a positivelycharged ion, called a cation.– An ionic compound is a compound composed ofcations and anions held together by ionic bonds.6

Figure 2.19: Sodium metal reacts withchlorine gas to form solid sodiumchloride.Chemical Bonds The forces that hold atoms together incompounds. Covalent bonds result from atomssharing electrons. Molecule: a collection of covalently-bondedatoms.2Na (s) Cl2 (g)º2 NaCl (s)Figure 2.17: Molecular models of some elementarysubstances.Figure 2.25: Representation of the reaction of methane withoxygen.An elemental substance is composed of molecules containingatoms from only one element.CH4 (g) 2 O2 (g) º CO2 (g) 2 H2O (g)Figure 2.16: Molecular and structural formulas and molecular models.Chemical Formulas; Molecularand Ionic Substances Molecular substances (see Figure 2.16)– A molecule is a definite group of atoms that arechemically bonded together – that is, tightlyconnected by attractive forces.– A molecular substance is a substance that iscomposed of molecules, all of which are alike.– A molecular formula gives the exact number ofatoms of elements in a molecule.– Structural formulas show how the atoms arebonded to one another in a molecule.Return to Slide 267

Figure 2.16:Thestructuralformula formethane.Figure 2.17: Space-filling model of methane.This type of model shows both the relativesizes of the atoms in the molecule and theirspatial relationships.Figure 2.18: Ball-and-stickmodel of methane.Chemical Formulas; Molecularand Ionic Substances The chemical formula of a substance is a notationusing atomic symbols with subscripts to conveythe relative proportions of atoms of the differentelements in a substance.– Consider the formula of aluminum oxide,Al2O3. This formula implies that the compoundis composed of aluminum atoms and oxygenatoms in the ratio 2:3.Chemical Substances;Formulas and Names Ionic compounds (Type I binarycompounds– Most ionic compounds contain metal andnonmetal atoms; for example, NaCl.– You name an ionic compound by giving the nameof the cation followed by the name of the anion.– A monatomic ion is an ion formed from a singleatom.Naming CompoundsBinary Ionic Compounds:1. Cation first, then anion2. Monatomic cation name of the elementCa2 calcium ion3. Monatomic anion root -ideCl chlorideCaCl2 calcium chloride8

Figure 2.22: The commoncations and anions49Naming Compounds(continued)Binary Ionic Compounds (Type II):metal forms more than one cation(transition metals)use Roman numeral in namePbCl2Pb2 is the cationPbCl2 lead (II) chloride52Crystals of copper(II) sulfate.Various chromium compounds dissolved in water. From leftto right; CrCl2, K2Cr2O7, Cr(NO3)3, CrCl3, K2CrO4.9

Chemical Substances;Formulas and Names Polyatomic ionsFigure 2.20: Ball-and-stick modelsof the ammonium ion and thenitrate ion.– A polyatomic ion is an ion consisting of two ormore atoms chemically bonded together andcarrying a net electric charge.– Table 2.6 lists some common polyatomic ions. Herea few examples.2 SO 4 sulfate SO 3 sulfiteNO 3 nitrateNO 2 nitrite2 Polyatomic Ions You ShouldKnow NH4 - AmmoniumOH- - HydroxideCN- - CyanideSO42- - SulfateHSO4- - bisulfateClO4- - PerchlorateNO3- - NitrateNO2- - Nitrite O22- - Peroxide PO43- - Phosphate HPO42- - monohydrogen phosphate H2PO4- - dihydrogenphosphate CO32- - Carbonate HCO3- - BicarbonateNaming CompoundsMore Practice Na2SO4Sodium Sulfate AgCNSilver Cyanide Ca(OCl)2Calcium HypochloriteNa2SO3Sodium SulfiteCd(OH)2Cadmium HydroxideKClO4Potassium Perchlorate(continued)Binary molecular compounds (Type III):Compounds between two nonmetalsFirst element in the formula is named first.Second element is named as if it were ananion.Use prefixesNever use mono- for the first element informulaP2O5 diphosphorus pentoxide10

For binary molecular compounds, usethe following to determine the orderof the nonmetal or metalloid elementsin the formula:Element: B Si,C Sb,As,P,N H Te,Se,S I,Br,Cl O FGroupIIIA IVAVAVIAVIIAH-P-H is written PH3H61PracticePracticeNitrogen trifluorideNF3Seleniumdisulfur dichloridetrioxide, SeO3S2Cl2carbon monoxideCOFigure 2.23: A flowchart for naming n difluorideOF2Figure 2.24: Overall strategy fornaming chemical compounds.11

Chemical Substances;Formulas and Names Acids– Acids are traditionally defined as compounds with apotential H as the cation.– Binary acids consist of a hydrogen ion and anysingle anion. For example, HCl is hydrochloricacid.– An oxoacid is an acid containing hydrogen, oxygen,and another element. An example is HNO3, nitricacid. (see Figure 2.23)Figure 2.25: A flowchart for naming acids. Anacid is best considered as one or more H ionsattached to an anion.Figure 2.23:Molecularmodel ofnitric acid.HNO312

The forces that hold atoms together in compounds. Covalent bonds result from atoms sharing electrons. Molecule: a collection of covalently-bonded atoms. Figure 2.17: Molecular models of some elementary substances. An elemental substance is composed of molecules containing atoms from only one element.