Ch. 3

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

AnAn STM image of nickel STM image of nickel atoms placed on a copper atoms placed on a copper

surface.surface.

Source: IBM Research

Red ridge is a series of Cesium atoms

Image of a ring of cobalt Image of a ring of cobalt atoms placed on a copper atoms placed on a copper

surface.surface.

Source: IBM Research

Atom - smallest particle making up elements

One teaspoon of water has 3 times One teaspoon of water has 3 times as many atoms as the Atlantic as many atoms as the Atlantic Ocean has teaspoons of water!Ocean has teaspoons of water!

Think about the technological advances of the past 100 years! They have been nothingshort of miraculous!

Radios CalculatorsTelevisions ComputersAutomobiles Cell phonesJet airplanes IpodsPlastic VelcroRefrigerators Internet (thanks, Al Gore)

Penicillin CD’s & DVD’sInsulin and, of course -Electric guitars Sliced Bread!

Where did it all Where did it all begin?begin?

The word “The word “atomatom” ” comes from the comes from the

Greek word Greek word ““atomos”atomos” which means which means

indivisibleindivisible..

The idea that all The idea that all matter is made up of matter is made up of

atoms wasatoms was first first proposed by theproposed by the

Greek philosopher Greek philosopher DemocritusDemocritus in the 5th in the 5th

century B.Ccentury B.C..

Then came the idea of “The 4 Basic Elements”Then came the idea of “The 4 Basic Elements”Earth, Air, Fire, & WaterEarth, Air, Fire, & Water

After that came After that came Alchemy.Alchemy.

The change to “real” The change to “real” Chemistry didn’t occur Chemistry didn’t occur until the first true until the first true element was element was discovered! (1774)discovered! (1774)The first element The first element discovered wasdiscovered was

The discovery of oxygen is attributed to 3 scientistsThe discovery of oxygen is attributed to 3 scientists (working independently)(working independently)

Karl ScheeleKarl Scheele (1771) (German) (1771) (German)first to prepare and describe oxygenfirst to prepare and describe oxygen

Joseph PriestleyJoseph Priestley (1774) (British) (1774) (British)isolated oxygen gas from mercuric oxide.isolated oxygen gas from mercuric oxide.observed accelerated burningobserved accelerated burning

Antoine LavoisierAntoine Lavoisier (1784) (French) (1784) (French)made accurate measurements and interpreted made accurate measurements and interpreted

Priestley’s resultsPriestley’s results

Carl Wilhelm Scheele beat Priestley to the Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards.discovery but published afterwards.

Too bad! – So sad!Too bad! – So sad!

Priestley MedalPriestley Medal

Source: Roald Hoffman, Cornell University

Priestley gets the main credit for discovering oxygen!

Priestley produced a gas (oxygen) by using sunlight to Priestley produced a gas (oxygen) by using sunlight to heat mercuric oxide kept in a closed container. The heat mercuric oxide kept in a closed container. The oxygen forced some of the mercury out of the jar as it oxygen forced some of the mercury out of the jar as it was produced, increasing the volume about five times. was produced, increasing the volume about five times.

2HgO(s) → 2Hg(l) + O2 (g)

Priestley: Scientific Priestley: Scientific ContributionsContributions

DISCOVERY OF 8 GASESDISCOVERY OF 8 GASES OxygenOxygen NitrogenNitrogen Carbon DioxideCarbon Dioxide Carbon MonoxideCarbon Monoxide Sulfur DioxideSulfur Dioxide Nitrous OxideNitrous Oxide Nitric OxideNitric Oxide Hydrogen ChlorideHydrogen Chloride

Priestley: Additional Scientific Priestley: Additional Scientific ContributionsContributions

Discovered the interconnection Discovered the interconnection between photosynthesis and between photosynthesis and respirationrespiration

Discovered carbonated waterDiscovered carbonated waterDiscovered that India rubber Discovered that India rubber

removed graphite pencil marks - the removed graphite pencil marks - the first rubber eraserfirst rubber eraserNow we can make mistakes!!

Antoine-Laurent Lavoisier

Lavoisier: the Founder of Modern Chemistry

•Lavoisier continued the investigations of Priestly

•Quantitative experiments led to:

Law of Conservation of Matter.

•He systematized the language of chemistry, its nomenclature and rhetoric.

•He was beheaded during the Reign of Terror for his role as a tax “farmer” prior to the Revolution (Priestley escaped to America!)

Lavoisier heated a measured amount of Lavoisier heated a measured amount of mercury to form the red mercuric oxide. He mercury to form the red mercuric oxide. He measured the amount of oxygen removed from measured the amount of oxygen removed from the jar and the amount of red oxide formed. the jar and the amount of red oxide formed. When the reaction was reversed, he found the When the reaction was reversed, he found the original amounts of mercury and oxygen. original amounts of mercury and oxygen.

2Hg(l) + O2 (g) → 2HgO(s)

Properties of OxygenProperties of OxygenColorlessColorlessOdorlessOdorlessTastelessTastelessGas at room temperatureGas at room temperatureSlightly soluble in waterSlightly soluble in water Inflammable (does NOT burn)Inflammable (does NOT burn)Only part of air that supports Only part of air that supports

combustioncombustion

Physical Property or Chemical Property?

PPPPPCC

These properties of oxygen were later used to determine the properties of other substances.

By the late 18th century, scientists finally came to the conclusion that Oxygen was truly an element (can’t be broken down into simpler forms without losing its properties)

Scientists began to search for & test other new elements.

Sometimes, when they tried to react substances together, nothing happened!

Substances that DO NOT react are Inert

They found that most materials will react to form new substances. These elements are said to be chemically active (reactive)

Oxygen is very reactive, so is hydrogen which we will look at next!

Increasing chemical reactivityinertinert OxygenOxygenhydrogenhydrogen

Discovery of

Henry Cavendish (1766) Reacted various metals with acids  

producing a salt and hydrogen gas Acid + metal → hydrogen gas + salt Zinc + sulfuric acid → Hydrogen + zinc sulfate Zn(s) + H2SO4(aq) → H2 (g) + ZnSO4 (aq)

While testing the properties of Hydrogen While testing the properties of Hydrogen he he

found that water is a compoundfound that water is a compound

(1731 – 1810)

WordEquation

Chemicalequation

Hydrogen + Oxygen WaterHydrogen + Oxygen Water

2H2 + O2 2H2O

Antoine LavoisierAntoine Lavoisier

Named Priestly’s newly discovered gas - Named Priestly’s newly discovered gas - “oxygen” - meaning “acid former”“oxygen” - meaning “acid former”

Named Cavendish’s new gas “hydrogen” -Named Cavendish’s new gas “hydrogen” -meaning “water former”meaning “water former”

Dalton’s Atomic TheoryDalton’s Atomic Theory

John Dalton (1766-1844)

While his theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today (instead of alchemy)

So, it’s an important landmark in the history of science.

Dalton’s Modern Atomic TheoryDalton’s Modern 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.

Law of Definite ProportionsLaw of Definite Proportions

Each compound has a specific ratio Each compound has a specific ratio of elements by mass.of elements by mass.

Ex: Water is always 8 grams of oxygen Ex: Water is always 8 grams of oxygen for each gram of hydrogen. for each gram of hydrogen.

Discovery of the ElectronDiscovery of the Electron

Began with the invention of the Crooke’s Tube (cathode ray tube) c. 1875

Cathode RayCathode Ray TubeTube

Electric current sent through gases sealed in tube at low pressure

Anode- positive electrodeCathode- negative electrode

Voltage source

Metal Disks - electrodes

-+ gasgas

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

In 1897, J.J. Thomson used a cathode ray tube to study gases.

Passing an electric current makes Passing an electric current makes a beam appear to move from the a beam appear to move from the negative to the positive end – so negative to the positive end – so the ‘beam’ was called a “Cathode the ‘beam’ was called a “Cathode Ray”Ray”

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Voltage source

Thomson’s ExperimentThomson’s Experiment

Thomson found that cathode rays Thomson found that cathode rays were deflected from a negatively-were deflected from a negatively-charged plate.charged plate.

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

and that cathode rays were attracted to and that cathode rays were attracted to plates with a positive chargeplates with a positive charge

Does light bend like this?Does light bend like this?

+

Light doesn’t ‘bend’ so the cathode ray must be made of particles rather than Light!Since they are attracted to a positive plate & repelled by a negative one the particles aren’t neutral –What charge must they have? That’s right! NEGATIVE!!Thomson called these negative particles –

ELECTRONS

Mass of the ElectronMass of the Electron

1916 – Robert Millikan determined the mass of the electron: 1/1840 the mass of a hydrogen atom; and, 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 of Conclusions from the Study of the Electron:the Electron:

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

b) Atoms are neutral, so there must be a positive substance 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 their mass

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

In 1903, An In 1903, An important discovery important discovery leading to further leading to further understandings of understandings of atomic structure atomic structure happened by happened by accident. accident.

Henri Becquerel Henri Becquerel discovered discovered radioactivityradioactivity

Radioactivity is the spontaneous emission of energy from an object

1903: Shared a Nobel Prize with Pierre and Marie Curie for discovering radioactivity.

Ernest Rutherford (1871-1937)The Nobel Prize in Chemistry 1908

Studied under J. J. Thomson

3 Types of Radiation 3 Types of Radiation discovered discovered by Ernest Rutherfordby Ernest Rutherford

e01

He42

• Alpha (Alpha (άά) – a positively ) – a positively charged charged helium helium nucleusnucleus 44

2 2 HeHe+2+2

•Beta (Beta (ββ) – fast-moving ) – fast-moving electrons electrons

--ee

•Gamma (Gamma (γγ) – like high-) – like high-energy energy

x-raysx-rays

00

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

Shot alpha particles at a thin sheet of gold foil

Particles that hit on a detecting screen (film) were recorded

Lead block

Polonium

Gold Foil

Flourescent Screen

He Expected:He Expected:

The alpha particles to pass through The alpha particles to pass through the foil without changing direction the foil without changing direction very much.very much.

Because…Because…The positive charges were spread The positive charges were spread

out evenly (according to Thomson’s out evenly (according to Thomson’s atomic theory). Alone they were not atomic theory). Alone they were not enough to stop the alpha particles.enough to stop the alpha particles.

What he expected

Again, because he thought the mass was evenly distributed in the atom

What he got

Rutherford’s ObservationsRutherford’s Observations

Most of the particles went straight Most of the particles went straight through the foilthrough the foil (what he expected) (what he expected)

A few particles were slightly deflectedA few particles were slightly deflectedStill fewer actually bounced back Still fewer actually bounced back

towards the sourcetowards the source!!Astonishing!!!Astonishing!!!Rutherford said it was like firing a Rutherford said it was like firing a

Howitzer shell at a piece of tissue Howitzer shell at a piece of tissue paper & having it bounce back & hit paper & having it bounce back & hit you!you!

“Like howitzer shells bouncing off of tissue paper!”

+

Rutherford’s ConclusionsRutherford’s Conclusions

Since most of the particles went through Since most of the particles went through the foil - the foil - atoms are mostly empty space.atoms are mostly empty space.

Because a few Because a few + particles were deflected particles were deflected they must have come close to a they must have come close to a positively positively charged core.charged core.

Since a very few particles were deflected Since a very few particles were deflected straight back, the positively-charged core straight back, the positively-charged core must be must be very densevery dense..

This small dense positive area is the This small dense positive area is the nucleusnucleus..

+

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

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

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

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

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

Discovery of ProtonsDiscovery of Protons Eugen Goldstein in 1886

observed particles with a positive charge passing through a perforated cathode.

In 1920, Rutherford studied these particles & called them protons.

They have a charge of positive 1and a mass of 1.7 x 10-24 grams.

This is not a ‘handy’ number to work with so we use a mass of 1 amu.Amu stands for “atomic mass unit”

1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton(1 amu).

Discovery of the NeutronRutherford predicted the existence of the neutron in 1920. Twelve years later, his assistant found it!

So now we have a more complete picture of an atom!

Subatomic Subatomic ParticlesParticles

ParticleParticle ChargeCharge Mass (g)Mass (g) LocationLocation

ElectronElectron

(e(e--)) -1-1

9.11 x 109.11 x 10-28-28gg

(virtually 0)(virtually 0) outside outside nucleusnucleus

ProtonProton (p (p++))

(H(H++))+1+1 1 amu 1 amu

(1.7 x 10(1.7 x 10-24-24g)g)

in nucleusin nucleus

NeutronNeutron

(n(noo)) 00

1 amu1 amu

(1.67 x 10(1.67 x 10-24-24g)g) in nucleusin nucleus

Elements are the new building blocksElements are the new building blocks

Hydrogen

Nitrogen-7

Oxygen-8Carbon-6

Between 1912 and 1914, the physicist H.G.J. Moseley conducted a series of experiments where he bombarded targets made out of different kinds of metals with cathode rays. Each metal he studied emitted X-rays of a characteristic frequency, almost like a set of "fingerprints".

Henry Moseley (1887 – 1915)

The pattern that emerged when the observed X-rays were organized in order of increasing frequency suggested to Moseley a regular increase in the positive charge on the nuclei of the atoms.

He called this positive nuclear charge-the Atomic Number of the element

Atomic NumberAtomic Number

Elements are different because they contain different numbers of PROTONS

The “atomic number” of an element is the number of protons in the nucleus

Since all atoms are neutral - the

# protons in an atom = # electrons

Henry Moseley – used x-ray spectra& came up with the idea of the Atomic Number

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

Mass NumberMass Number

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

Mass # = # protons + # neutrons

Subatomic ParticlesSubatomic Particles

POSIT IVECHARG E

PROT ONS

NEUT RALCHARG E

NEUT RONS

NUCLEUS

NEG AT IVE CHARG E

ELECT RONS

AT OM

Most of the atom’s mass.

NUCLEUS ELECTRONS

PROTONS NEUTRONS NEGATIVE CHARGE

POSITIVE CHARGE

NEUTRAL CHARGE

ATOM

Atomic Numberequals the # of...

equal in a neutral atom

IsotopesIsotopesFrederick SoddyFrederick Soddy (1877-1956) (1877-1956)

proposed the idea of isotopes in 1912 in 1912 (worked with Rutherford)(worked with Rutherford)

IsotopesIsotopes are atoms of theare atoms of the same elementsame element having having different mass numbersdifferent mass numbers, due to , due to varying numbers of neutrons.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 radioactive 1921 for his work with isotopes and radioactive materialsmaterials..

IsotopesIsotopesAtoms of the same element (same Z) Atoms of the same element (same Z)

but different mass number (A).but different mass number (A).Boron-10 (B-10) has 5 p and 5 n Boron-10 (B-10) has 5 p and 5 n

Boron-11 (B-11) has 5 p and 6 nBoron-11 (B-11) has 5 p and 6 n

10B

11B

IsotopesIsotopes Radioisotopes (radioactive isotopes) - Radioisotopes (radioactive isotopes) - unstable

isotopes that spontaneously decay emitting radiation

They play an important part in the technologies They play an important part in the technologies that provide us with food, water and good health. that provide us with food, water and good health. Radio-carbon dating of fossilsRadio-carbon dating of fossils In medicine, diagnosis, treatment, and In medicine, diagnosis, treatment, and

research research Sterilization of meat

Disinfestation of grain and spicesIncreasing shelf life (eg, fruits)

Nuclear SymbolsNuclear SymbolsContain the symbol of the element, Contain the symbol of the element,

the mass number and the atomic the mass number and the atomic number (represent isotopes of number (represent isotopes of elements)elements)

X Massnumber

Atomicnumber

Subscript →

Superscript →

Element symbol

REMEMBER! number of electrons = number of protonsSo all atoms are neutral!

RheniumRhenium

Re186

75

Protons: 75Neutrons: 111Electrons: 75

Nuclear SymbolsNuclear Symbols

Find each of these: Find each of these:

a)a) number of protonsnumber of protons

b)b) number of neutronsnumber of neutrons

c)c) number of electronsnumber of electrons

d)d) Atomic numberAtomic number

e)e) Mass NumberMass Number

Br80

35

Nuclear SymbolsNuclear Symbols 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) Write the complete symbolWrite the complete symbol

Se78

34

34

44

34

Naming IsotopesNaming Isotopes

We can name isotopes by We can name isotopes by placing the mass number placing the mass number afterafter the name of the the name of the element:element:carbon-12carbon-12carbon-14carbon-14uranium-235uranium-235

Mass numbers

ISOTOPESISOTOPES

IsotopeIsotope pp++ nn00 ee-- Mass #Mass #

Oxygen Oxygen - - 1010

- - 3333 4242

- - 3131 1515

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

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

The element hydrogen has 3 isotopes

Examples of IsotopesExamples of Isotopes

Learning Check – CountingLearning Check – Counting Naturally occurring carbon consists of three isotopes, Naturally occurring carbon consists of three isotopes,

1212C, C, 1313C, and C, and 1414C. State the number of protons, C. State the number of protons, neutrons, and electrons in each of these carbon atoms.neutrons, and electrons in each of these carbon atoms.

1212CC 1313CC 1414CC 66 6 6 6 6

#p#p++ _______ _______ _______ _______ _______ _______

#n#noo _______ _______ _______ _______ _______ _______

#e#e-- _______ _______ _______ _______ _______ _______

AnswersAnswers

1212CC 1313CC 1414CC 66 66 66

#p#p++ 66 66 66

#n#noo 66 77 88

#e#e-- 66 66 66

Learning CheckLearning Check

An atom has 14 protons and 20 neutrons.An atom has 14 protons and 20 neutrons.A.A. Its atomic number isIts atomic number is

1) 141) 14 2) 162) 16 3) 343) 34

B. Its mass number isB. Its mass number is1) 141) 14 2) 162) 16 3) 343) 34

C. The element isC. The element is1) Si1) Si 2) Ca2) Ca 3) Se3) Se

D.D. Another isotope of this element isAnother isotope of this element is

1) 1) 3434X X 2) 2) 3434X X 3) 3) 3636XX 16 16 14 1414 14

Atomic MassAtomic Mass How heavy is an How heavy is an atomatom of oxygen? 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 atomic mass.average atomic mass.

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

because the numbers would be too because the numbers would be too small – small –

Measuring Atomic MassMeasuring Atomic MassInstead of grams, the unit we use Instead of grams, the unit we use

is the is the Atomic Mass UnitAtomic Mass Unit (amu) (amu)It is defined as It is defined as one-twelfth the one-twelfth the

mass of a carbon-12 atom.mass of a carbon-12 atom. Carbon-12 chosen because of its isotope purity.Carbon-12 chosen because of its isotope purity.

Each isotope has its own mass Each isotope has its own mass number, sonumber, so we determine the average we determine the average atomic mass from the element’s atomic mass from the element’s percent abundancepercent abundance..

To calculate the average atomic mass:To calculate the average atomic mass:

Multiply the mass of each Multiply the mass of each isotope by it’s abundance, isotope by it’s abundance, then add the results. then add the results.

Abundance may be expressed Abundance may be expressed as a decimal or a %, (Divide as a decimal or a %, (Divide by 100 if using %’s)by 100 if using %’s)

100

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

Avg.AtomicMass

IsotopeIsotope SymbolSymbol Composition of Composition of the nucleusthe nucleus

% in nature% in nature

Carbon-12Carbon-12 C-12C-12 6 protons6 protons

6 neutrons6 neutrons

98.89%98.89%

Carbon-13Carbon-13 C-13C-13 6 protons6 protons

7 neutrons7 neutrons

1.11%1.11%

Carbon-14Carbon-14 C-14C-14 6 protons6 protons

8 neutrons8 neutrons

<0.01%<0.01%

Atomic Mass is the weighted average of all the naturally occurring isotopes of an element. on the Periodic Tableon the Periodic Table

= 12.011(98.89 x 12) + (1.11 x 13) + (0.01 x 14)

100

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

Sub-atomic Particles - Summary

Protons p+ - positive charge, in nucleus , mass of 1 amu

Electrons - e- negative

charge, orbiting nucleus, “no mass”

Neutrons n0 – no charge, in nucleus, mass of 1 amu