Chapter 3Chapter 3

Atoms: The Building Blocks of Atoms: The Building Blocks of MatterMatter

3-1 An Ancient Idea3-1 An Ancient Idea

Water: atoms Water: atoms smooth and round smooth and round so it flowed with no so it flowed with no permanent shapepermanent shape

Fire: atoms Fire: atoms thorny,making thorny,making burns painfulburns painful

Earth: atoms rough Earth: atoms rough and jagged so they and jagged so they held together to held together to make hard stable make hard stable substancessubstances

3-1 Opposing View3-1 Opposing View

Did not believe in Did not believe in atomsatoms

Believed matter Believed matter was continuouswas continuous

Very influentialVery influential Neither Democritus Neither Democritus

nor Aristotle nor Aristotle supported their supported their ideas with ideas with experimentation. experimentation.

3-1 Law of 3-1 Law of Conservation of MassConservation of Mass Antoine Laurent Antoine Laurent

Lavoisier (1743-1794) – Lavoisier (1743-1794) – the father of modern the father of modern chemistrychemistry

Mass is neither created Mass is neither created nor destroyed during nor destroyed during ordinary chemical ordinary chemical reactions or physical reactions or physical changeschanges

Arrested by French Arrested by French Revolutionary Tribunal Revolutionary Tribunal for his membership in for his membership in the Ferme Generale and the Ferme Generale and executedexecuted

3-1 Law of Definite 3-1 Law of Definite Proportions (Constant Proportions (Constant Composition)Composition)

A chemical A chemical compound contains compound contains the same the same proportions by mass proportions by mass regardless of the regardless of the size of the sample size of the sample or source of the or source of the compoundcompound

Sodium chloride Sodium chloride (NaCl) is always (NaCl) is always 39.34% sodium and 39.34% sodium and 60.66% chlorine60.66% chlorine

3-1 John Dalton3-1 John Dalton

English English schoolteacher – schoolteacher – 18081808

Proposed Proposed explanation for explanation for these lawsthese laws

3-1 Dalton’s Atomic 3-1 Dalton’s Atomic TheoryTheory All matter is composed of extremely small All matter is composed of extremely small

particles called atoms.particles called atoms. Atoms of a given element are identical in size, Atoms of a given element are identical in size,

mass and other properties. Atoms of different mass and other properties. Atoms of different elements differ in size, mass and other properties. elements differ in size, mass and other properties.

Atoms cannot be subdivided, created or Atoms cannot be subdivided, created or destroyed.destroyed.

Atoms of different elements combine in simple Atoms of different elements combine in simple whole-number ratios to form chemical whole-number ratios to form chemical compounds. compounds.

In chemical reactions, atoms are combined, In chemical reactions, atoms are combined, separated or rearranged. separated or rearranged.

3-13-1 Dalton and Conservation Dalton and Conservation of Massof Mass

In chemical reactions, atoms are rearranged – bonds are In chemical reactions, atoms are rearranged – bonds are broken and new bonds are formed.broken and new bonds are formed.

Same number of atoms of each type exist before and after Same number of atoms of each type exist before and after the reaction. the reaction.

No atoms are created or destroyed.No atoms are created or destroyed.

3-1 Dalton and Definite 3-1 Dalton and Definite ProportionsProportions Each kind of atom has its own mass Each kind of atom has its own mass

and atoms in a compound always and atoms in a compound always exist in fixed whole number ratios. exist in fixed whole number ratios.

Sodium chloride always 1:1 Sodium chloride always 1:1 sodium:chlorinesodium:chlorine

Na: 23, Cl: 35.5 Na: 23, Cl: 35.5 Na: 23/58.5 *100 = 39.34%Na: 23/58.5 *100 = 39.34% Cl: 35.5/58.5 *100 = 60.66%Cl: 35.5/58.5 *100 = 60.66%

3-1 Law of Multiple 3-1 Law of Multiple ProportionsProportions

If two or more different compounds are composed of the If two or more different compounds are composed of the same elements, then the ratio of the masses of the second same elements, then the ratio of the masses of the second element combined with a certain mass of the first element element combined with a certain mass of the first element is always a whole number ratio.is always a whole number ratio.

3-1 Dalton and 3-1 Dalton and Multiple ProportionsMultiple Proportions Each kind of atom has a unique mass Each kind of atom has a unique mass

and atoms in compounds are combined and atoms in compounds are combined in fixed whole number ratios. in fixed whole number ratios.

3-1 Impact of Dalton’s 3-1 Impact of Dalton’s Atomic TheoryAtomic Theory Related Democritus’s idea of atoms to Related Democritus’s idea of atoms to

the MEASURABLE property of mass – the MEASURABLE property of mass – atomic theory could then be TESTED atomic theory could then be TESTED by EXPERIMENTby EXPERIMENT

Dalton’s theory has been modified Dalton’s theory has been modified over the years as new information has over the years as new information has become available, but the become available, but the fundamental principles hold true fundamental principles hold true todaytoday

3-1 Dalton’s Atomic 3-1 Dalton’s Atomic ModelModel

3-2 Structure of the 3-2 Structure of the AtomAtom

ATOMATOM - The - The smallest smallest particle of an particle of an element that element that retains the retains the chemical chemical properties of properties of that element.that element.

3-2 Discovery of the 3-2 Discovery of the ElectronElectron

18971897 J. J. Thomson, J. J. Thomson,

English physicistEnglish physicist Did experiments Did experiments

with with cathode cathode ray tubesray tubes – – glass tubes glass tubes containing gases containing gases at low pressureat low pressure

3-2 Cathode Ray Tubes3-2 Cathode Ray Tubes

Cathode – negative electrode, anode – Cathode – negative electrode, anode – positive electrodepositive electrode

When a current is passed through the When a current is passed through the tube, the end opposite the cathode tube, the end opposite the cathode glowsglows

Glow caused by stream of particles Glow caused by stream of particles called called cathode raycathode ray because is because is originated at cathode – ray traveled originated at cathode – ray traveled from cathode to anodefrom cathode to anode

3-2 CRT Experiments3-2 CRT Experiments

Observation – a paddle wheel placed on rails Observation – a paddle wheel placed on rails between electrodes rolled along rails from between electrodes rolled along rails from cathode to anodecathode to anode

Conclusion – existence of cathode ray Conclusion – existence of cathode ray supported, supported, cathode ray has MASScathode ray has MASS

3-2 CRT Experiments3-2 CRT Experiments

Observations – cathode rays deflected by a Observations – cathode rays deflected by a magnetic field in the same way as a wire carrying magnetic field in the same way as a wire carrying an electric current (cathode ray acts NEGATIVE); an electric current (cathode ray acts NEGATIVE); deflected away from negatively charged objectsdeflected away from negatively charged objects

Conclusion – Conclusion – cathode ray is made of cathode ray is made of negatively charged particlesnegatively charged particles

3-2 Charge and Mass 3-2 Charge and Mass of the Electronof the Electron J. J. Thomson called these tiny J. J. Thomson called these tiny

negatively charged particles negatively charged particles “electrons”“electrons”

He calculated the He calculated the mass to mass to charge ratiocharge ratio of the electron – it of the electron – it is always the same no matter is always the same no matter what metal is used for the what metal is used for the electrodeselectrodes

3-2 Charge and Mass 3-2 Charge and Mass of the Electronof the Electron Robert Millikan, Robert Millikan,

American physicistAmerican physicist 19091909 Showed that mass Showed that mass

of electron is of electron is 1/2000 the mass 1/2000 the mass of the simplest of the simplest known atom known atom (hydrogen)(hydrogen)

9.109 x 109.109 x 10-31-31 kg kg

3-2 Important Points3-2 Important Points

Atoms contain tiny negatively charged Atoms contain tiny negatively charged particles called electrons. particles called electrons.

Electrons are present in atoms of all Electrons are present in atoms of all elements.elements.

Atoms are divisible and one of parts is Atoms are divisible and one of parts is negatively charged.negatively charged.

Because atoms are neutral, there must Because atoms are neutral, there must also be a positive component. also be a positive component.

Because electrons have such small Because electrons have such small mass, atoms must contain other parts mass, atoms must contain other parts that make up most of their mass.that make up most of their mass.

3-2 The Plum Pudding 3-2 The Plum Pudding Model of the AtomModel of the Atom

3-2 Discovery of the 3-2 Discovery of the Atomic NucleusAtomic Nucleus

19111911 Ernest Rutherford, Ernest Rutherford,

New Zealand (with New Zealand (with Hans Geiger and Hans Geiger and Ernest Marsden)Ernest Marsden)

Important Important experiment experiment providing more providing more detail into atom’s detail into atom’s structure.structure.

3-2 The Gold Foil 3-2 The Gold Foil ExperimentExperiment Thin, gold foil Thin, gold foil

bombarded bombarded with alpha with alpha particles particles (positively (positively charged charged particles with particles with 4x mass of 4x mass of hydrogen hydrogen atom)atom)

3-2 The Gold Foil 3-2 The Gold Foil Experiment - ResultsExperiment - Results Most particles went right through Most particles went right through

gold foilgold foil A few were slightly deflectedA few were slightly deflected A few bounced off the gold A few bounced off the gold

foil!foil!

3-2 The Gold Foil 3-2 The Gold Foil Experiment - Experiment - ConclusionsConclusions Particles that pass through foil – hit nothingParticles that pass through foil – hit nothing Particles slightly deflected – come close to a Particles slightly deflected – come close to a

positive chargepositive charge Particles that bounce back – hitting a positive Particles that bounce back – hitting a positive

chargecharge

3-2 Rutherford’s 3-2 Rutherford’s ExplanationExplanation The atom has a densely packed The atom has a densely packed

bundle of matter with a positive bundle of matter with a positive charge (he called it the charge (he called it the nucleusnucleus).).

The nucleus contains all of the The nucleus contains all of the positive charge and most of the positive charge and most of the mass. mass.

The nucleus has very little volume – The nucleus has very little volume – the atom is mostly empty spacethe atom is mostly empty space. .

3-2 Rutherford’s 3-2 Rutherford’s Atomic ModelAtomic Model Small nucleus in Small nucleus in

the centerthe center Electrons orbit Electrons orbit

nucleus like nucleus like planets around planets around the Sun.the Sun.

Sometimes called Sometimes called planetary atomic planetary atomic model.model.

3-2 Composition of the 3-2 Composition of the Atomic NucleusAtomic Nucleus

Nuclei contain two kinds of particles – protons and Nuclei contain two kinds of particles – protons and neutronsneutrons

Electrons are outside the nucleus, in the electron cloudElectrons are outside the nucleus, in the electron cloud Table 3-1 on p. 74Table 3-1 on p. 74ParticlParticlee

SymbolSymbol RelativRelative e ChargeCharge

Mass Mass NumbeNumberr

RelativRelative Mass e Mass (amu)(amu)

Actual Actual Mass Mass (kg)(kg)

electronelectron ee-- -1-1 00 0.000540.000548686

9.109x109.109x10--

3131

protonproton pp++ +1+1 11 1.007271.0072766

1.673x101.673x10--

2727

neutronneutron nn00 00 11 1.008661.0086655

1.675x101.675x10--

2727

3-2 Forces in the 3-2 Forces in the NucleusNucleus Like charges generally repel each Like charges generally repel each

other BUT at very close range other BUT at very close range there is an attraction between there is an attraction between themthem

Nuclear Forces – short range Nuclear Forces – short range proton-neutron, proton-proton, proton-neutron, proton-proton, and neutron-neutron forces that and neutron-neutron forces that hold the nuclear particle togetherhold the nuclear particle together

3-2 The Sizes of Atoms3-2 The Sizes of Atoms

Expressed as atomic radius – Expressed as atomic radius – distance from center of nucleus to distance from center of nucleus to outer edge of electron cloudouter edge of electron cloud

Measured in picometers (10Measured in picometers (10-12-12 m) m) Generally range from 40 pm to Generally range from 40 pm to

270 pm270 pm Nuclear radius is ~0.001 pm (like Nuclear radius is ~0.001 pm (like

a dime in a football field)a dime in a football field)

3-3 Counting Atoms3-3 Counting Atoms

Atoms of the same Atoms of the same element all have the element all have the same number of same number of protons – this is the protons – this is the atomic numberatomic number

Atomic number is Atomic number is found on the found on the periodic table – periodic table – elements arranged elements arranged in order of in order of increasing atomic increasing atomic numbernumber

3-3 Isotopes3-3 Isotopes

Atoms of the same element that have Atoms of the same element that have different masses (different numbers of different masses (different numbers of neutrons) – same chemical behaviorneutrons) – same chemical behavior

3-3 Isotopes3-3 Isotopes

Most of the elements Most of the elements consist of mixtures of consist of mixtures of isotopes.isotopes.

To identify an isotope, To identify an isotope, must know atomic must know atomic number AND mass number AND mass number (protons + number (protons + neutrons)neutrons)

Isotopes usually Isotopes usually identified by identified by specifying mass specifying mass numbernumber

3-3 Isotopes3-3 Isotopes

Nuclide – general term for any isotope of any elementNuclide – general term for any isotope of any element

Hyphen NotationHyphen Notation

neon-20, uranium-235neon-20, uranium-235

Nuclear SymbolNuclear Symbol

3-3 Relative Atomic 3-3 Relative Atomic MassMass One atom has been chosen as a One atom has been chosen as a

standard, masses of all other standard, masses of all other atoms expressed in relation to atoms expressed in relation to this standard this standard

Atomic mass unitAtomic mass unit – exactly 1/12 – exactly 1/12 the mass of a carbon-12 atomthe mass of a carbon-12 atom

Atomic mass of any other atom is Atomic mass of any other atom is determined by comparing it with determined by comparing it with the mass of carbon-12the mass of carbon-12

3-3 Average Atomic 3-3 Average Atomic MassMass Most elements occur naturally as Most elements occur naturally as

mixtures of isotopesmixtures of isotopes The percentage of each isotope in an The percentage of each isotope in an

element is constant – same no matter element is constant – same no matter where sample comes fromwhere sample comes from

Average atomic mass Average atomic mass is the is the weighted average of the atomic weighted average of the atomic masses of the naturally occurring masses of the naturally occurring isotopes of an elementisotopes of an element

3-3 Calculating 3-3 Calculating Average Atomic MassAverage Atomic Mass

Copper has 2 isotopes – copper-63 Copper has 2 isotopes – copper-63 and copper-65and copper-65

Copper-63: 69.17%, 62.929599 amuCopper-63: 69.17%, 62.929599 amu

Copper-65: 30.83%, 64.927793 amuCopper-65: 30.83%, 64.927793 amu

(.6917)(62.929599 amu) + (.3083)(64.927793 amu) = 63.55 amu(.6917)(62.929599 amu) + (.3083)(64.927793 amu) = 63.55 amu

3-3 Average Atomic 3-3 Average Atomic MassMass Atomic masses that appear on the periodic table are AVERAGE atomic Atomic masses that appear on the periodic table are AVERAGE atomic

massesmasses

3-3 Relating Mass to 3-3 Relating Mass to Number of AtomsNumber of Atoms THE MOLE!THE MOLE! The amount of a The amount of a

substance that substance that contains as many contains as many particles as there particles as there are atoms in are atoms in exactly 12 g of exactly 12 g of carbon-12carbon-12

The mole is a The mole is a counting unit, like a counting unit, like a dozen or a reamdozen or a ream

1 mole of 1 mole of something is 6.022 something is 6.022 x 10x 102323

3-3 Avogadro’s 3-3 Avogadro’s NumberNumber 12 g of carbon-12 12 g of carbon-12

contains exactly contains exactly 6.0221367 x 106.0221367 x 102323 atoms. atoms.

Named Named Avogadro’s Avogadro’s number after number after Amedeo Amedeo Avogadro, Italian Avogadro, Italian scientistscientist

3-3 Molar Mass3-3 Molar Mass

A mole is the amount of a substance that A mole is the amount of a substance that contains Avogadro’s number of particlescontains Avogadro’s number of particles

The mass of one mole of a pure substance is The mass of one mole of a pure substance is called the molar mass of that substance called the molar mass of that substance (g/mol)(g/mol)

3-3 Molar Mass3-3 Molar Mass

To find the molar To find the molar mass of an atom, mass of an atom, find the atomic find the atomic mass in amu on mass in amu on the periodic table the periodic table and change the and change the unit to g/molunit to g/mol

The molar mass is The molar mass is numerically equal numerically equal to the atomic to the atomic massmass

elementelement Atomic Atomic mass mass (amu)(amu)

Molar Molar mass mass (g/mol)(g/mol)

carbon carbon 12.01112.011 12.01112.011

coppercopper 63.5563.55 63.5563.55

ironiron 55.8555.85 55.8555.85

neonneon 20.1820.18 20.1820.18

3-3 Gram/Mole 3-3 Gram/Mole ConversionsConversions Molar mass can be used as a Molar mass can be used as a

conversion factorconversion factor Can convert between mass and Can convert between mass and

moles for any substancemoles for any substance MMoles to oles to GGrams rams MMultiply (by ultiply (by

molar mass)molar mass) GGrams to rams to MMoles oles DDivide (by molar ivide (by molar

mass)mass)

3-3 Conversions with 3-3 Conversions with Avogadro’s NumberAvogadro’s Number If amount in moles is known, can If amount in moles is known, can

calculate number of particles (and calculate number of particles (and vice versa)vice versa)

Moles to Particles Multiply (by Moles to Particles Multiply (by Avogadro’s number)Avogadro’s number)

Particles to Moles Divide (by Particles to Moles Divide (by Avogadro’s number)Avogadro’s number)

Mole ConversionsMole Conversions