Chapter 4Chapter 4“Atomic Structure”“Atomic Structure”

Section 4.1 Defining the AtomSection 4.1 Defining the Atom Greek philosopher Greek philosopher DemocritusDemocritus

(460 (460 B.C.B.C. – 370 – 370 B.CB.C.) suggested.) suggested

existence of atoms (Greek wordexistence of atoms (Greek word

“ “atomos”)atomos”) Believed atoms were Believed atoms were indivisibleindivisible and and

indestructibleindestructible His ideas agreed with later scientific theory, His ideas agreed with later scientific theory,

but didn’t explain chemical behavior - was but didn’t explain chemical behavior - was not based on scientific methodsnot based on scientific methods – only – only philosophyphilosophy

Dalton’s Atomic TheoryDalton’s Atomic Theory (experiment based!) (experiment based!)

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

4) In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.

1) All elements composed of tiny indivisible particles called atoms

2) Atoms of same element identical. Atoms of any one element are different from all other elements.

John Dalton(1766 – 1844)

Sizing up the AtomSizing up the Atom Elements subdivided into smaller particles – called atoms, and they still have properties of that element

1.0 x 108 copper atoms in a single file, they would be approximately 1 cm longindividual atoms are observable with instruments such as scanning tunneling (electron) microscopes

Section 4.2Section 4.2Structure of the Nuclear AtomStructure of the Nuclear Atom

One change to Dalton’s atomic One change to Dalton’s atomic theory - theory - atoms are divisibleatoms are divisible into into subatomic particles:subatomic particles:

Electrons, protons, and Electrons, protons, and neutronsneutrons

Discovery of ElectronDiscovery of Electron1897 - J.J. Thomson used cathode ray tube to deduce presence of negatively charged

particle…….the electronelectron

Modern Modern CCathode athode RRay ay TTubesubes

CRT’s pass electricity through gas contained - very low pressure.

Television Computer Monitor

Mass of the ElectronMass of the Electron

1916 – Robert Millikan determines mass of electron: 1/1840 the mass of hydrogen atom; has one unit of negative charge

The oil drop apparatus

Mass of the electron is 9.11 x 10-28 g

Conclusions from the Study Conclusions from the Study of the Electron:of the Electron:

a) Cathode rays have identical properties regardless of element used to produce them. All elements must contain identically charged electrons.

b) Atoms are neutral, so there must be positive particles in atom to balance negative charge of electrons

c) Electrons have so little mass that atoms must contain other particles that account for most of mass

Conclusions from the Study Conclusions from the Study of the Electron:of the Electron:

Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)

1932 – James Chadwick confirmed the existence of “neutron” – particle with no charge, but mass nearly equal to proton

Subatomic ParticlesSubatomic Particles

ParticleParticle ChargeCharge Mass (g)Mass (g) LocationLocation

ElectronElectron

(e(e--)) -1-1 9.11 x 109.11 x 10-28-28 Electron Electron cloudcloud

ProtonProton (p(p++)) +1+1 1.67 x 101.67 x 10-24-24 NucleusNucleus

NeutronNeutron

(n(noo)) 00 1.67 x 101.67 x 10-24-24 NucleusNucleus

Thomson’s Atomic ModelThomson’s Atomic Model

Believed electrons were like plums embedded in + charged “pudding,” called “plum pudding” model.

J. J. Thomson

Ernest Rutherford’sErnest Rutherford’sGold Foil Experiment - 1911Gold Foil Experiment - 1911

Alpha particles - helium nuclei w/ + charge - The alpha particles were fired at thin sheet of gold foil Particles that hit on the detecting screen (film) were recorded

Rutherford’s problem:Rutherford’s problem:In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

Target #1

Target #2

The Answers:The Answers:

Target #1 Target #2

Rutherford’s FindingsRutherford’s Findings

a) Small nucleusb) Dense nucleus c) + charge nucleus

Most particles passed through Few deflected VERY FEW greatly deflected

Conclusions:

The Atom SongAtoms song - Mark Rosen

garten

“Like howitzer shells bouncing off of tissue paper!”

The Rutherford Atomic ModelThe Rutherford Atomic Model His experimental evidence:His experimental evidence:

atom mostly empty spaceatom mostly empty space All positive charge, almost all mass in All positive charge, almost all mass in

small center. small center. “Nucleus“Nucleus”” protons and neutrons mprotons and neutrons makeake nucleus! nucleus! electrons distributed around nucleus…electrons distributed around nucleus…

occupy most volumeoccupy most volume His model called “His model called “nuclear modelnuclear model””

Rutherford’s Atom 3:08

Section 4.3Section 4.3Atomic NumberAtomic Number

All atoms composed of All atoms composed of identicalidentical protons, neutrons, and electronsprotons, neutrons, and electrons How then are atoms of one element How then are atoms of one element

different from another element?different from another element? Elements different b/c they contain Elements different b/c they contain

different # of different # of PROTONSPROTONS ““atomic numberatomic number” of element is ” of element is number number

of protonsof protons in nucleus in nucleus # protons in atom = # electrons# protons in atom = # electrons

Atomic NumberAtomic NumberAtomic number (Z) of element is # of protons in nucleus of each atom of that element.

ElementElement # of protons# of protons Atomic # (Z)Atomic # (Z)

CarbonCarbon 66 66

PhosphorusPhosphorus 1515 1515

GoldGold 7979 7979

Mass NumberMass Number

Mass number is # of protons and neutrons in nucleus of an isotope:

Mass # = p+ + n0

NuclideNuclide pp++ nn00 ee-- Mass #Mass #

Oxygen Oxygen - - 1010

- - 3333 4242

- - 3131 1515

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

Complete SymbolsComplete Symbols

Contain symbol of element, mass Contain symbol of element, mass number & atomic number.number & atomic number.

X Massnumber

Atomicnumber

Subscript →

Superscript →

SymbolsSymbols Find each of these: Find each of these:

a)a) number of protonsnumber of protons

b)b) number of number of neutronsneutrons

c)c) number of number of electronselectrons

d)d) Atomic numberAtomic number

e)e) Mass NumberMass Number

Br80 35

SymbolsSymbols If an element has an atomic If an element has an atomic

number of 34 and a mass number of 34 and a mass number of 78, what is the: number of 78, what is the:

a)a) number of protonsnumber of protons

b)b) number of neutronsnumber of neutrons

c)c) number of electronsnumber of electrons

d)d) complete symbolcomplete symbol

SymbolsSymbols If an element has 91 If an element has 91

protons and 140 neutrons protons and 140 neutrons what is the what is the

a)a) Atomic numberAtomic number

b)b) Mass numberMass number

c)c) number of electronsnumber of electrons

d)d) complete symbolcomplete symbol

SymbolsSymbols If an element has 78 If an element has 78

electrons and 117 neutrons electrons and 117 neutrons what is the what is the

a)a) Atomic numberAtomic number

b)b) Mass numberMass number

c)c) number of protonsnumber of protons

d)d) complete symbolcomplete symbol

IsotopesIsotopes

Dalton was wrong about Dalton was wrong about elements of same type being elements of same type being identical…identical…

Atoms of same element Atoms of same element cancan have different numbers of have different numbers of neutronsneutrons..

different mass numbersdifferent mass numbers isotopesisotopes

IsotopesIsotopes

Frederick SoddyFrederick Soddy (1877-1956) (1877-1956) proposed idea of isotopes in 1912proposed idea of isotopes in 1912

Isotopes - atoms of same element with different masses, b/c varying #s of neutrons

Won 1921 Nobel Prize in Chemistry Won 1921 Nobel Prize in Chemistry has a small crater named for him on the has a small crater named for him on the

far side of the Moon.far side of the Moon.

Naming IsotopesNaming Isotopes

We can also put mass We can also put mass number number afterafter name of the name of the element:element:carbon-12carbon-12carbon-14carbon-14uranium-235uranium-235

IsotopeIsotope ProtonsProtons ElectronsElectrons NeutronsNeutrons NucleusNucleus

Hydrogen–1Hydrogen–1

(protium)(protium) 11 11 00

Hydrogen-2Hydrogen-2

(deuterium)(deuterium) 11 11 11

Hydrogen-3Hydrogen-3

(tritium)(tritium)

11 11 22

IsotopesIsotopesElements occur in nature as mixtures of isotopes.

Atomic MassAtomic Mass How heavy is an oxygen atom?How heavy is an oxygen atom?

Depends, b/c different Depends, b/c different kindskinds of oxygen of oxygen atoms exist.atoms exist.

We’re more concerned with We’re more concerned with average average atomic mass.atomic mass.

Based on abundance (%) of each variety Based on abundance (%) of each variety of that element in nature.of that element in nature. Don’t use grams - numbers tooooo small.Don’t use grams - numbers tooooo small.

Measuring Atomic MassMeasuring Atomic Mass

Atomic Mass UnitAtomic Mass Unit (amu)(amu) one-twelfth mass of a carbon-12 one-twelfth mass of a carbon-12

atom.atom. Carbon-12 chosen b/c of its Carbon-12 chosen b/c of its isotope purityisotope purity..

Each isotope has own atomic mass Each isotope has own atomic mass we determine average from % we determine average from %

abundance.abundance.

To calculate the average:To calculate the average:

Multiply atomic mass of each Multiply atomic mass of each isotope by abundance isotope by abundance (decimal), then add results.(decimal), then add results.

If not told otherwise, mass of isotope If not told otherwise, mass of isotope expressed in expressed in atomic mass unitsatomic mass units (amu)(amu)

Atomic MassesAtomic Masses

IsotopeIsotope SymbolSymbol Composition Composition of the nucleusof the nucleus

% in nature% in nature

Carbon-Carbon-1212

1212CC 6 protons6 protons

6 neutrons6 neutrons

98.89%98.89%

Carbon-Carbon-1313

1313CC 6 protons6 protons

7 neutrons7 neutrons

1.11%1.11%

Carbon-Carbon-1414

1414CC 6 protons6 protons

8 neutrons8 neutrons

<0.01%<0.01%

Atomic mass is average of all naturally occurring isotopes of that element.

12.01What is the average atomic mass of Carbon?

Atomic Atomic mass mass (amu)(amu)

12

13.00

14.00

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Question

Solution

Answer

Knowns and Unknown