Chapter 2 Atoms, Molecules, And Ions

Chapter 2Atoms, Molecules,and IonsJim GeigerCem 151Atoms,Molecules,and Ions

Atomic Theory of MatterThe theory of atoms:Original to the GreeksLeuccipus, Democritus and Lucretius(Aristotle thought they were nuts)He believed that one could divide up a piece of matter an infinite numberof times, that is, one never came up with a piece of matter thatcould not be further divided. He suggested that everything in theworld was made up of some combination of four elements: earth,fire, water, and air. The elements were acted upon by the twoforces of gravity and levity. Gravity was the tendency for earthand water to sink, and levity the tendency for air and fire to rise.John Dalton (1805-1808)Revived the idea and made itscience by measuring theatomic weights of 21 elements.That’s the key thingbecause then you can seehow elements combine.Atoms,Molecules,and Ions

Dalton’s PostulatesEach element is composed ofextremely small particles called atoms.Tiny balls make up the worldAtoms,Molecules,and Ions

Dalton’s PostulatesAll atoms of a given element are identicalto one another in mass and otherproperties, but the atoms of one elementare different from the atoms of all otherelements.ONAtoms,Molecules,and Ions

Dalton’s PostulatesAtoms of an element are not changed intoatoms of a different element by chemicalreactions; atoms are neither created nordestroyed in chemical reactions. (As far asDalton knew, they couldn’t be changed at all).ONONRed O’s stay Os and aqua N’sstay N’s.Atoms,Molecules,and Ions

Dalton’s PostulatesCompounds are formed when atoms ofmore than one element combine; agiven compound always has the samerelative number and kind of atoms.HNNH3ammoniaChemistry happens when the ballsrearrangeAtoms,Molecules,and Ions

Law of Constant CompositionJoseph Proust (1754–1826) Also known as the law of definite proportions. The elemental composition of a puresubstance never varies. The relative amounts of each element in acompound doesn’t vary.HNNH3ammoniaammonia always has 3 H and 1 N.Atoms,Molecules,and Ions

Law of Conservation of MassThe total mass of substances present atthe end of a chemical process is thesame as the mass of substancespresent before the process took place.3H2 N22NH3ammoniaThe atoms on the left all appear on the rightAtoms,Molecules,and Ions

The Electron Streams of negatively charged particles were foundto emanate from cathode tubes. J. J. Thompson (1897). Maybe atoms weren’t completely indivisible after all.Atoms,Molecules,and Ions

The ElectronThompson measured the charge/mass ratio of theelectron to be 1.76 108 coulombs/g.How? by manipulating the magnetic and electric fieldsand observing the change in the beam position on a Atoms,Molecules,fluorescent screen.and Ions

Millikan Oil Drop Experimentmeasured charge of electronUniv. Chicago (1909).How?Vary the electric field (E) until thedrops stop.Vary the charge (q) on the dropwith more X-rays. Get amultiple of 1.6x10-19 Coulombs.The charge of 1 electron.Eq mgYou set E, measure mass ofdrop (m) & know g. Findq.Major result: q couldn’tbe any number. It wasa multiple of 1.6x10-19Atoms,Molecules,Coulombsand Ions

Radioactivity: The spontaneous emission of radiationby an atom. First observed by Henri Becquerel. (1903 Nobel Prize with Pierre and Marie Curie) Also studied by Marie and Pierre Curie.“rays” not particlesparticles of some sort.Stuff comes out of atoms, “subatomic particles”Atoms,Molecules,and Ions

Radioactivity Three types of radiation were discovered by ErnestRutherford: (memorize the 3 types of particle)Ø α particles, attracted to negative electrode, so they have apositive charge, much more mass than negative stuff (turnout to be He nuclei)Ø β particles, attracted to positive electrode, sothey have a negative charge, 1000s of times less massive (turnout to be electrons coming from nucleus).Ø γ rays, no charge, no mass, like light.Atoms,Molecules,and Ions

The Atom, circa 1900: “Plum pudding” model, putforward by Thompson. Positive sphere of matterwith negative electronsimbedded in it. most of the volume positivestuff because most of themass is positive Expectation: density more orless uniform throughout.Atoms,Molecules,and Ions

Discovery of the NucleusThe Gold Foil ExperimentRutherford andMarsden shot αparticles at a thin sheetof gold foil andobserved the pattern ofscatter of the particles.Atoms,Molecules,and Ions

The Nuclear AtomVirtually all theparticles went straightthroughMost of the atomessentially emptyA few particlesdeflected,some straightback.A very small part ofthe atom is verydense, impenetrable.The mass must beconcentrated.The size of nucleus will beproportional to the # ofhighly scattered particles.Atoms,Molecules,and Ions

The Nuclear Atom Rutherford and Marsden postulated a verysmall, dense nucleus with the negativeelectrons around the outside of the atom. Most of the volume of the atom is emptyspace.Atoms,Molecules,and Ions

Other Subatomic Particles Protons were discovered by Rutherfordin 1919.Ø Have the positive charge in the atom. Neutrons were discovered by JamesChadwick in 1932.Ø Have mass like proton, but no charge.Why was it harder to discover them?Atoms,Molecules,and Ions

Subatomic Particles Protons and electrons are the only particles thathave a charge. Protons and neutrons have similar mass. The mass of an electron is so small we will oftenignore it.Atoms,Molecules,and Ions

Symbols of Elements:depicting the subatomic particlesElements are symbolized by one or twoletters.Atoms,Molecules,and Ions

Atomic NumberAll atoms of the same element have the samenumber of protons:The atomic number (Z)Atoms,Molecules,and Ions

Atomic MassThe mass of an atom in atomic mass units(amu) is approximately the total number ofprotons and neutrons in the atom.Atoms,Molecules,and Ions

Isotopes: Elements are defined by the number of protons. Isotopes are atoms of the same element with differentmasses. Isotopes have different numbers of les,and Ions

Atomic MassAtomic and molecular masses can be measured with greataccuracy with a mass spectrometer. Heavier ion turns less in themagnetic field (more momentum, because of higher mass (mv))Atoms,(magnetic moments of ions similar). Aston, 1919.Molecules,and Ions

Average Mass Because in the real world all theelements exist as mixtures of isotopes. And we measure many many atoms ata time“Natural abundance”Ø Average mass is calculated from theisotopes of an element weighted by theirrelative abundances.Atoms,Molecules,and Ions

Average mass, exampleIsotope abundance Atomic mass24Mg78.99%23.98504 amu25Mg10.00%24.98584 amu26Mg11.01%25.98259 amuGiven the above data, what is the average molecularmass of magnesium (Mg)?.7899(23.98504) 0.1000(24.98584) 0.1101(25.98259) 18.95 2.499 2.861 24.31Atoms,Molecules,and Ions

Periodic Table: A systematiccatalog ofelements. Elements arearranged in orderof atomic number. But why like this?Atoms,Molecules,and Ions

PeriodicityWhen one looks at the chemical properties ofelements, one notices a repeating pattern ofreactivities.Atoms,Molecules,and Ions

Periodic Table The rows on the periodic chart are periods. Columns are groups. Elements in the same group have similar chemicalproperties.Atoms,Molecules, Derived empirically, no theory to explain it.and Ions

GroupsThese five groups are known by their names.You gotta know these very well.Atoms,Molecules,and Ions

Periodic TableNonmetals are on the upper right-hand corner ofthe periodic table (with the exception of H).Atoms,Molecules,and Ions

Periodic TableMetalloids border the stair-step line (with theexception of Al and Po, which are both metals).Atoms,Molecules,and Ions

Periodic TableMetals are on the left side of the chart.Atoms,Molecules,and Ions

Elements of life Elements required for living organisms. Red, most abundantblue, next most abundantGreen, trace amounts.Atoms,Molecules,and Ions

Chemical FormulasThe subscript to the rightof the elementtells the number ofatoms of that elementin the compound.Atoms,Molecules,and Ions

Molecular CompoundsMolecular compoundsare composed ofmolecules and almostalways contain onlynonmetals.Atoms,Molecules,and Ions

Diatomic MoleculesThese seven elements occur naturally asmolecules containing two atoms.You should know these guys.Atoms,Molecules,and Ions

Types of Formulas Empirical formulas give the lowestwhole-number ratio of atoms of eachelement in a compound. Molecular formulas give the exactnumber of atoms of each element in acompound.Example: ethane:Empirical formula: CH3Molecular formula: C2H6Atoms,Molecules,and Ions

Types of Formulas Structural formulas show theorder in which atoms arebonded. Perspective drawings alsoshow the three-dimensionalarray of atoms in acompound.Atoms,Molecules,and Ions

Ions When atoms lose or gain electrons, they becomeions. Often they lose or gain electrons to have thesame number of electrons as the nearest noble gas.Ø Cations are positive and are formed by elements on the leftside of the periodic chart (metals).Ø Anions are negative and are formed by elements on the rightAtoms,Molecules,side of the periodic chart (nonmetals).and Ions

Mono-atomic ionsmetalsnonmetals Metals usually become cations ( ) Nonmetals usually become anions (-)Atoms,Molecules,and Ions

Ionic compounds A metal will give up electrons to a nonmetalforming a cation ( ) (the metal), and an anion(-) (the nonmetal).Na ClNa Cl-NaClMg 2ClMg2 2Cl-MgCl2Note, everybody gains or loses electrons tobe like the nearest noble gas.Compounds are always electrically neutral!!Atoms,Molecules,and Ions

Writing Formulas Because compounds are electrically neutral,one can determine the formula of acompound this way:Ø The charge on the cation becomes the subscripton the anion.Ø The charge on the anion becomes the subscripton the cation.Ø If these subscripts are not in the lowest wholenumber ratio, divide them by the greatest commonfactor.Mg2 O2-MgO Not Mg2O2Atoms,Molecules,and Ions

Common Cations**********************You should know these.Atoms,Molecules,and Ions

Common AnionsAtoms,Molecules,and Ions

Polyatomic O3ClO4MnO4CrO4-2Cr2O7-2hydrogen phosphatedihydrogen hloratePermanganateChromateDichromateYou must memorizeALL of these!Atoms,Molecules,and Ions

Patterns in Oxyanion Nomenclature When there are only two oxyanionsinvolving the same element:Ø The one with fewer oxygens ends in -ite NO2 : nitrite; SO32 : sulfiteØ The one with more oxygens ends in -ate NO3 : nitrate; SO42 : sulfateAtoms,Molecules,and Ions

Patterns in OxyanionNomenclatureWhen there are more than two: The one with the fewest oxygens has the prefix hypo- and ends in -iteØ ClO : hypochloriteThe one with the second fewest oxygens ends in -iteØ ClO2 : chloriteThe one with the second most oxygens ends in -ateØ ClO3 : chlorateThe one with the most oxygens has the prefix per- and ends in -ateØ ClO4 : perchlorateAtoms,Molecules,and Ions

Inorganic Nomenclature name of cation goes first. If anion is element, change ending to -ide; If anion is polyatomic ion, simply write thename of the polyatomic ion. If the cation can have more than onepossible charge, write the charge as aRoman numeral in parentheses. (Fe(II),Fe(III))Atoms,Molecules,and Ions

Examplesnaming inorganic compounds Write the name of the cation.If the anion is an element, change its ending to -ide; if the anion is apolyatomic ion, simply write the name of the polyatomic ion.If the cation can have more than one possible charge, write the chargeas a Roman numeral in NaClO4K2CrO4NaHsodium chlorideammonium nitrateIron(II) sulfatepotassium cyanideRubidium hydroxidelithium acetatesodium chloratesodium perchloratepotassium chromateSodium hydrideAtoms,Molecules,and Ions