Atomic TheoryAtomic Theory

Defining the AtomDefining the Atom The Greek philosopher The Greek philosopher DemocritusDemocritus (460 B.C. – 370 (460 B.C. – 370

B.C.) was among the first to suggest the existence B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)of atoms (from the Greek word “atomos”) He believed that atoms were He believed that atoms were indivisibleindivisible and and

indestructibleindestructible His ideas did agree with later scientific theory, but His ideas did agree with later scientific theory, but

did not explain chemical behavior, and was did not explain chemical behavior, and was not not based on the scientific methodbased on the scientific method – but just – but just 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 are composed of tiny indivisible particles called atoms

2) Atoms of the same element are identical. Atoms of any one element are different from those of any other element.

John Dalton(1766 – 1844)

Sizing up the AtomSizing up the Atom Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element

If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm longDespite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes

Structure of the Nuclear AtomStructure of the Nuclear Atom

One change to Dalton’s atomic theory is that One change to Dalton’s atomic theory is that atoms are divisibleatoms are divisible into subatomic particles: into subatomic particles: Electrons, protons, and neutronsElectrons, protons, and neutrons are are

examples of these fundamental particlesexamples of these fundamental particles There are many other types of particles, There are many other types of particles,

but we will study these threebut we will study these three

Discovery of the ElectronDiscovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron

Modern Modern CCathode athode RRay ay TTubesubes

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Television Computer Monitor

Mass of the ElectronMass of the Electron

1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a 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 the element used to produce them. All elements must contain identically charged electrons.

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

c) Electrons have so little mass that atoms must contain other particles that account for most of the 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 the “neutron” – a particle with no charge, but a mass nearly equal to a 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

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

J. J. Thomson

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

Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are 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) The nucleus is smallb) The nucleus is densec) The nucleus is positively

charged

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:

The Rutherford Atomic ModelThe Rutherford Atomic Model Based on his experimental evidence:Based on his experimental evidence:

The atom is mostly empty spaceThe atom is mostly empty space All the positive charge, and almost all the mass is All the positive charge, and almost all the mass is

concentrated in a small area in the center. He concentrated in a small area in the center. He called this a “called this a “nucleusnucleus””

The nucleus is composed of protons and neutrons The nucleus is composed of protons and neutrons (they (they makemake the nucleus!) the nucleus!)

The electrons distributed around the nucleus, and The electrons distributed around the nucleus, and occupy most of the volumeoccupy most of the volume

His model was called a “His model was called a “nuclear modelnuclear model””

Bohr ModelBohr Model In 1913, the Danish In 1913, the Danish

scientist Niels Bohr scientist Niels Bohr proposed an proposed an improvement. In his improvement. In his model, he placed model, he placed each electron in a each electron in a specificspecific energy level. energy level.

Bohr ModelBohr Model According to Bohr’s According to Bohr’s

atomic model, atomic model, electrons move in electrons move in definite definite orbitsorbits around around the nucleus, much the nucleus, much like planets circle the like planets circle the sun. These orbits, or sun. These orbits, or energy energy levelslevels, are , are located at certain located at certain distances from the distances from the nucleus.nucleus.

Wave Model

The Wave ModelThe Wave Model Today’s atomic model Today’s atomic model

is based on the is based on the principles of principles of wavewave mechanicsmechanics..

According to the According to the theory of wave theory of wave mechanics, electrons mechanics, electrons do not movedo not move about an about an atom in a atom in a definite definite path,path, like the planets like the planets around the sun.around the sun.

The Wave ModelThe Wave Model

In fact, it is In fact, it is impossibleimpossible to determine the exact to determine the exact location of an electron. The location of an electron. The probableprobable location of location of an electron is based on how much an electron is based on how much energyenergy the the electron has.electron has.

According to the modern atomic model, at atom According to the modern atomic model, at atom has a has a small positively charged nucleussmall positively charged nucleus surrounded by a large region in which there are surrounded by a large region in which there are enough electrons to make an atom neutral.enough electrons to make an atom neutral.

Electron CloudElectron Cloud A space in which A space in which

electrons are likely to electrons are likely to be found.be found.

Electrons Electrons whirlwhirl about about the nucleus billions of the nucleus billions of times in one secondtimes in one second

They are not moving They are not moving around in around in randomrandom patterns.patterns.

Location of electrons Location of electrons depends upon how depends upon how much much energyenergy the the electron has.electron has.

Electron CloudElectron Cloud

Depending on their energy they are locked Depending on their energy they are locked into a certain area in the cloud.into a certain area in the cloud.

Electrons with the Electrons with the lowestlowest energy are found energy are found in the energy level in the energy level closestclosest to the nucleus to the nucleus

Electrons with the Electrons with the highesthighest energy are energy are found in the found in the outermostoutermost energy levels, energy levels, farther from the nucleus.farther from the nucleus.

IndivisibleIndivisible ElectronElectron NucleusNucleus OrbitOrbit Electron Electron CloudCloud

GreekGreek XX

DaltonDalton XX

ThomsonThomson XX

RutherfordRutherford XX XX

BohrBohr XX XX XX

WaveWave XX XX XX

Atomic StructureAtomic Structure

An atom consists of:An atom consists of: nucleusnucleus – contains protons and neutrons – contains protons and neutrons electronselectrons in space around the nucleus. in space around the nucleus. The atom is mostly empty spaceThe atom is mostly empty space

The The AtomAtom

NucleusNucleus

Electron cloudElectron cloud

ATOMIC COMPOSITIONATOMIC COMPOSITION Protons (pProtons (p++))

+ electrical charge+ electrical charge mass = 1.672623 x 10mass = 1.672623 x 10-24-24 g g relative mass = 1.007 atomic mass units relative mass = 1.007 atomic mass units

(amu) (amu) but we can round to 1but we can round to 1 Electrons (eElectrons (e--))

negative electrical chargenegative electrical charge relative mass = 0.0005 amu relative mass = 0.0005 amu

but we can round to 0but we can round to 0 Neutrons (nNeutrons (noo))

no electrical chargeno electrical charge mass = 1.009 amu mass = 1.009 amu but we can round to 1but we can round to 1

Atomic Number, ZAtomic Number, ZAll atoms of the same element have All atoms of the same element have

the same number of protons in the the same number of protons in the nucleus, nucleus, ZZ

1313

AlAl

26.98126.981

Atomic numberAtomic number

Atom symbolAtom symbol

AVERAGE Atomic MassAVERAGE Atomic Mass

Atomic NumberAtomic Number

Atoms are composed of Atoms are composed of identicalidentical protons, protons, neutrons, and electronsneutrons, and electrons How then are atoms of one element different How then are atoms of one element different

from another element?from another element? Elements are different because they contain Elements are different because they contain

different numbers of different numbers of PROTONSPROTONS The “The “atomic numberatomic number” of an element is the ” of an element is the number number

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

Atomic NumberAtomic Number

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

CarbonCarbon 66 66

PhosphorusPhosphorus 1515 1515

GoldGold 7979 7979

Mass Number, AMass Number, A A carbon atom with 6 protons and 6 A carbon atom with 6 protons and 6

neutrons is the mass standard = 12 neutrons is the mass standard = 12 atomic mass unitsatomic mass units

Mass NumberMass Number (A) (A)= # protons + # neutrons= # protons + # neutrons

NOT on the periodic table…(Round the NOT on the periodic table…(Round the AVERAGE atomic mass on the table)AVERAGE atomic mass on the table)

A boron atom can have A boron atom can have A = 5 p + 5 n = 10 amuA = 5 p + 5 n = 10 amu

A

Z

10

5B

A

Z

10

5B

Mass NumberMass Number

Mass number is the number of protons and neutrons in the 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 the symbol of the element, Contain the symbol of the element, the mass number and the atomic the mass number and the atomic number.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

IsotopesIsotopes

Dalton was wrong about all Dalton was wrong about all elements of the same type elements of the same type being identicalbeing identical

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

Thus, different mass numbers.Thus, different mass numbers.These are called These are called isotopesisotopes..

IsotopesIsotopes

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

Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

Soddy won the Nobel Prize in Chemistry in Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and 1921 for his work with isotopes and radioactive materials.radioactive materials.

Naming IsotopesNaming Isotopes

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

Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

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.

Isotopes are atoms of the same element that differ in the number of neutrons.

Isotopes Isotopes & Their & Their

UsesUsesBone scans with Bone scans with radioactive radioactive technetium-99. technetium-99.

Isotopes & Their Isotopes & Their UsesUses

The The tritiumtritium content of ground water is content of ground water is used to discover the source of the used to discover the source of the water, for example, in municipal water water, for example, in municipal water or the source of the steam from a or the source of the steam from a volcano. volcano.

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

It depends, because there are different It depends, because there are different kindskinds of oxygen atoms. of oxygen atoms.

We are more concerned with the We are more concerned with the average average atomic mass.atomic mass.

This is based on the abundance This is based on the abundance (percentage) of each variety of that (percentage) of each variety of that element in nature.element in nature. We don’t use grams for this mass because We don’t use grams for this mass because

the numbers would be too small.the numbers would be too small.

Measuring Atomic MassMeasuring Atomic Mass Instead of grams, the unit we use is the Instead of grams, the unit we use is the Atomic Atomic

Mass UnitMass Unit (amu)(amu) It is defined as one-twelfth the mass of a carbon-12 It is defined as one-twelfth the mass of a carbon-12

atom.atom. Carbon-12 chosen because of its Carbon-12 chosen because of its isotope purityisotope purity. .

Each isotope has its own atomic mass, thus we Each isotope has its own atomic mass, thus we determine the average from percent abundance.determine the average from percent abundance.

To calculate the average:To calculate the average: Multiply the atomic mass of each isotope Multiply the atomic mass of each isotope

by it’s abundance (expressed as a by it’s abundance (expressed as a decimal), then add the results.decimal), then add the results.

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

Atomic MassesAtomic Masses

IsotopeIsotope SymbolSymbol Composition of Composition of the nucleusthe nucleus

% in nature% in nature

Carbon-12Carbon-12 1212CC 6 protons6 protons

6 neutrons6 neutrons

98.89%98.89%

Carbon-13Carbon-13 1313CC 6 protons6 protons

7 neutrons7 neutrons

1.11%1.11%

Carbon-14Carbon-14 1414CC 6 protons6 protons

8 neutrons8 neutrons

<0.01%<0.01%

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

Carbon = 12.011

Average Atomic MassAverage Atomic Mass

weighted average of all isotopesweighted average of all isotopes on the Periodic Tableon the Periodic Table round to 2 decimal placesround to 2 decimal places

100

(%)(mass(mass)(%) )

Avg.AtomicMass

Avg.AtomicMass

D. Average Atomic MassD. Average Atomic Mass

EXEX: Calculate the avg. atomic mass of oxygen if : Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% its abundance in nature is 99.76% 1616O, 0.04% O, 0.04% 1717O, and 0.20% O, and 0.20% 1818O.O.

100

(18)(0.20)(17)(0.04))(16)(99.76 16.00amu

Avg.AtomicMass

D. Average Atomic MassD. Average Atomic Mass

EXEX: Find chlorine’s average atomic mass if : Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.35 and 2 are chlorine-37.

10

(37)(2)(35)(8)35.40 amu