Chapter 3Chapter 3

Atoms: The Building Atoms: The Building Blocks of MatterBlocks of Matter

Sect. 3-1: The Atom: From Sect. 3-1: The Atom: From Philosophical Idea to Philosophical Idea to Scientific TheoryScientific Theory

Democritus vs. AristotleDemocritus vs. Aristotle Atom vs. infinitely divisibleAtom vs. infinitely divisible Aristotle’s idea won out for ~ 2000 yr.Aristotle’s idea won out for ~ 2000 yr. Problem with both ideas…Problem with both ideas…

Foundations of Atomic Foundations of Atomic TheoryTheory

Law of conservation of massLaw of conservation of mass Mass not created or destroyedMass not created or destroyed

Law of definite proportionsLaw of definite proportions Same compound always same ratio Same compound always same ratio

of elementsof elements Water is always 2 H and 1 OWater is always 2 H and 1 O

Law of multiple proportionsLaw of multiple proportions Compounds of same elements Compounds of same elements

combine in different ratioscombine in different ratios Carbon dioxide (1 C : 2 O) and carbon Carbon dioxide (1 C : 2 O) and carbon

monoxide (1 C : 1 O)monoxide (1 C : 1 O)

Dalton’s Atomic TheoryDalton’s Atomic Theory1)1) AtomsAtoms

2)2) Atoms of an element are the same (mass, Atoms of an element are the same (mass, size, etc), atoms of different atoms are size, etc), atoms of different atoms are differentdifferent

3)3) Atoms can’t be dividedAtoms can’t be divided

4)4) Atoms combine in ratios to form compoundsAtoms combine in ratios to form compounds

5)5) Atoms combine, separate, or rearrange Atoms combine, separate, or rearrange during a chemical reactionduring a chemical reaction

What’s the difference between What’s the difference between Democritus and Dalton???Democritus and Dalton???

Experimental Data!!!Experimental Data!!!

Modern Atomic TheoryModern Atomic Theory What’s no longer held as true from What’s no longer held as true from

Dalton’s theory?Dalton’s theory? Atoms being divisibleAtoms being divisible Atoms of same element having same Atoms of same element having same

massmass

Sect. 3-2: The Structure of Sect. 3-2: The Structure of the Atomthe Atom

Atoms made up of:Atoms made up of: ProtonsProtons NeutronsNeutrons ElectronsElectrons

Discovery of the ElectronDiscovery of the Electron Cathode Ray Tube experiments Cathode Ray Tube experiments

concluded that the “cathode rays” had concluded that the “cathode rays” had mass and a negative chargemass and a negative charge

http://images.google.com/imgres?imgurl=http://www.hcc.mnscu.edu/programs/dept/chem/abomb/Thomson_ExpBoth.jpg&imgrefurl=http://www.hcc.mnscu.edu/programs/dept/chem/abomb/page_id_15138.html&h=342&w=512&sz=35&hl=en&start=44&tbnid=niL-YcGGzm9sqM:&tbnh=88&tbnw=131&prev=/images%3Fq%3Dcathode%2Bray%2Btube%26start%3D36%26gbv%3D2%26ndsp%3D18%26svnum%3D10%26hl%3Den%26sa%3DN

J.J. Thomson measured ratio of charge J.J. Thomson measured ratio of charge to mass for these particles to mass for these particles

concluded particles were the same for all concluded particles were the same for all elementselements

Cathode-ray particles later named Cathode-ray particles later named electronselectrons

Millikan determined exact mass of Millikan determined exact mass of electron (about 1/1837 mass of hydrogen electron (about 1/1837 mass of hydrogen atom)atom)

Confirms negative charge of electronConfirms negative charge of electron Inferences made:Inferences made:

must be something positive to balance must be something positive to balance negativenegative

Atoms must have more particles due to tiny Atoms must have more particles due to tiny mass of electron and much larger mass of mass of electron and much larger mass of atomatom

Discovery of Atomic NucleusDiscovery of Atomic Nucleus Rutherford’s gold foil experimentRutherford’s gold foil experiment

http://www.dlt.ncssm.edu/TIGER/diagrams/structure/GoldFoilExperiment.jpg

Significance of Rutherford’s experiment:Significance of Rutherford’s experiment: Nucleus of the atomNucleus of the atom

Very denseVery dense Positive chargePositive charge

Nucleus = marble, whole atom = football Nucleus = marble, whole atom = football fieldfield

Composition of the Atomic Composition of the Atomic NucleusNucleus

Protons – positive chargeProtons – positive charge Very massive compared to electronsVery massive compared to electrons Determine what type of element the atom isDetermine what type of element the atom is

Neutrons – neutral (no charge)Neutrons – neutral (no charge) Slightly more massive than protonsSlightly more massive than protons

Nuclear forcesNuclear forces – short range forces that – short range forces that hold protons and neutrons together in the hold protons and neutrons together in the nucleusnucleus

Sect. 3-3: Counting Sect. 3-3: Counting AtomsAtoms Atomic number = # protonsAtomic number = # protons IsotopesIsotopes

Same element, different massesSame element, different masses same # protons, different # of neutronssame # protons, different # of neutrons

Mass # = protons + neutronsMass # = protons + neutrons Hyphen notation Hyphen notation

Hydrogen - 3Hydrogen - 3

Nuclear symbolNuclear symbol 33

11HH

# neutrons = mass # - atomic ## neutrons = mass # - atomic #

How many of each of the following are in How many of each of the following are in the carbon – 14 isotope?the carbon – 14 isotope? ProtonsProtons NeutronsNeutrons ElectronsElectrons

6, 8, and 6 respectively6, 8, and 6 respectively

Relative Atomic MassesRelative Atomic Masses 1 atomic mass unit (amu) = 1/12 the 1 atomic mass unit (amu) = 1/12 the

mass of a carbon-12 atommass of a carbon-12 atom Isotopes differ in mass, but not Isotopes differ in mass, but not

significantly different in chemical significantly different in chemical behaviorbehavior

Average Atomic Masses Average Atomic Masses of Elementsof Elements

Weighted average of atomic masses of Weighted average of atomic masses of all naturally occurring isotopes of an all naturally occurring isotopes of an elementelement

(%of isotope) (mass isotope) + (%of isotope) (mass isotope) +

(%of isotope) (mass isotope) = (%of isotope) (mass isotope) =

weighted averageweighted average

Relating Mass to Relating Mass to Numbers of AtomsNumbers of Atoms

MoleMole – SI unit for amount of substance – SI unit for amount of substance Based on Carbon-12Based on Carbon-12 Abbreviated molAbbreviated mol Counting unit (like dozen)Counting unit (like dozen)

Avogadro’s NumberAvogadro’s Number - # of particles in one - # of particles in one mol of a substancemol of a substance 6.022 x 106.022 x 102323 particles = 1 mol particles = 1 mol

Ex: How many moles of silver are in Ex: How many moles of silver are in

3.01 x 103.01 x 102323 atoms of silver? atoms of silver? 0.500 mol0.500 mol

Molar MassMolar Mass – mass of one mole of a pure – mass of one mole of a pure substancesubstance Units are typically reported in g/molUnits are typically reported in g/mol

What is the molar mass of Cu?What is the molar mass of Cu? 63.55 g/mol63.55 g/mol

Gram to mole conversionGram to mole conversion Use molar mass as conversion factorUse molar mass as conversion factor

What is the mass in grams of 3.50 mol of What is the mass in grams of 3.50 mol of Cu?Cu? 222g 222g