Chapter 4 (Partial)Structure of the Atom

4.1 Early Theories of Matter (& Early Chemistry – Alchemy and Related)4.2 Subatomic Particles & Nuclear Atom4.2.5 Ultimate Structure of Matter – The Standard Model (Not in Book)

Section 4.1 Early Ideas About Matter

• Compare and contrast the atomic models of Democritus, Aristotle, and Dalton.

• Describe the activities related to the chemical sciences that occurred between the time of Aristotle and the early 19th century when Dalton’s theory was published.

• List the components of Dalton’s atomic theory.

• Explain how Dalton's theory explains the conservation of mass.

The ancient Greeks tried to explain matter, but the scientific study of the atom began with John Dalton in the early 1800's.

Section 4.1 Early Ideas About Matter

• Identify the components of Dalton’s theory that are not strictly correct and provide examples of why they aren’t.

• Name the two instruments that are routinely used to obtain images of atoms.

• Describe the basic operational principles of the Scanning Tunneling Microscope (STM).

(Cont.)

Key Concepts• Democritus was the first person to propose the

existence of atoms.

• According to Democritus, atoms are solid, homogeneous, and indivisible.

• Aristotle did not believe in the existence of atoms.

• John Dalton’s atomic theory is based on numerous scientific experiments.

• The scanning tunneling microscope (STM) and the modified scanning transmission electron microscope (modified STEM) are instruments capable of atomic scale imaging.

Section 4.1 Early Ideas About Matter

Early Philosophers Thought matter formed of:• Earth• Air• Fire• Water

History: Development of Atomic Model

EmpedoclesAristotle

-500 -250 0 250 500 750 1000 1250 1500 1750 2000-500 -250 0 250 500 750 1000 1250 1500 1750 2000

Atomic Theory Timeline

DemocritusLeucippus

Zeno

J. Dalton

J. Proust

A. Lavoisier

R. Boyle

R. Bacon

Democritus Aristotle Boyle Lavoisier Dalton

Democritus Greek, 460-370 BC

First to propose matter was not infinitely divisible = concept of atom

Democritus, Greek Philosopher (460-370 BC)

His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained

This piece would be indivisible

Named the smallest piece of matter “atomos,” meaning “not to be cut”

Democritus – Atomic TheoryTo him, atoms were small, hard particles that were all made of same material but were different shapes and sizes

Atoms were infinite in number, surrounded by empty space, and always moving and capable of joining together

Democritus’ Concept of Matter• Matter is empty space through which

atoms move• Atoms are solid, homogeneous,

indestructible, indivisible• Different kinds of atoms have different

sizes and shapes• Differing properties of matter are due to

atoms size, shape, & movement• Changes in matter result from changes

in groupings of atoms and not changes in atoms themselves

Aristotle Greek Philosopher (384-322 BC)

14 years old when Democritus diedBelieved matter made of 4 basic elements (earth, air, fire and water)Disagreed with Democritus - believed matter was continuous (did not accept idea of the “void”)His ideas endured for 2000 yrs

Alchemy for 2000 years

Aristotle believed any substance could be transmuted (transformed) into any other substance simply by changing relative proportions of the 4 basic qualities

This mindset dominated quest for new substances done by the alchemists

Alchemy for 2000 yearsIdea of transmutation laid foundation for alchemy Alchemists were searching for evolution from ignorance to enlightenment by searching for: • elixir of life (source of eternal life/youth)• philosopher’s stone (substance to turn base

metals into gold; also el. of life)• aqua vitae (“water of life” – concentrated

ethanol solution – whiskey etc)• panacea (substance meant to cure all

diseases)

During the search for ability to transmute matter (e.g., change lead into gold), they did a lot of good experimentation that laid foundation for modern science

Alchemy"The hopeless pursuit of the practical transmutation of metals was responsible for almost the whole of the development of chemical technique before the seventeenth century, and further led to the discovery of many important materials.”

http://www.levity.com/alchemy/

Alchemy

Popular belief is that Alchemists made contributions to the "chemical" industries of the day—ore testing and refining, metalworking, production of gunpowder, ink, dyes, paints, cosmetics, leather tanning, ceramics, glass manufacture, preparation of extracts, liquors, and so on

Alchemists contributed distillation to Western Europe

http://en.wikipedia.org/wiki/Alchemy

Science During 1600’s to1800’sScientists were discovering concepts and relationships by doing large, basic experiments with stoves, pots, ovens, and basic glassware, much of which had been developed by alchemists

With observable properties came explanations!

Robert Boyle 1627-1691

Sometimes referred to as Father of Modern Chemistry

One of first to publish all experimental details of his work, including experiments that did not work

Boyle revived Democritus’ ideas by proposing that a substance was not element if it were made of two or more components

Robert Boyle ~ 1660

Best known for his quantitative work with gases (Boyle’s Law)

Still believed in alchemy – that metals could be converted into gold

Was first to propose existence of elements in the modern sense

Boyle considered a substance to be an element unless it can be broken down into simpler substances

Marie-Anne and Antoine Lavoisier 1743-1794

Mother and father of modern chemistry?Studied various types of reactions involving oxygen: respiration, burning, rusting

Antoine Lavoisier (France) ~1760

Studied chemical reactions quantitatively

Credited with being first to propose law of conservation of matter

LavoisierWas sure that air contained > one elementWas able to determine amount of “reacting component” in air - named this component oxygen

Lavoisier

Pictured experiment demonstrates Law of Conservation of Mass

Lavoisier Law of Conservation of Mass

There is no detectable change in total mass of materials when they react chemically to form new materials

Mass of products will equal mass of reactants in a chemical reaction

During chemical reaction, matter neither created nor destroyed

Joseph Proust (France, 1754-1826)~1794 Studied chemical composition of compound copper carbonate (CuCO3)

Found all samples of CuCO3 had same relative composition of elements by mass: 5.3 parts Copper: 4 parts Oxygen: 1 part Carbon

This finding led to law of definite proportion

John Dalton (1766-1844)

A schoolteacher!

Devised Law of Multiple Proportions “when two elements form more than one compound, they come together in whole number ratios”

John Dalton (1766-1844)Used work of Lavoisier, Proust, and Gay-Lussac to revive Democritus’ idea that matter was made of atomsBased much of his theory on• Law of Conservation

of Mass• Law of Constant

Composition

John Dalton’s Atomic Theory

Matter made up of atoms. Atoms of given element identical.

Atoms can’t be created, destroyed or divided.

Atoms may combine in the ratio of small, whole numbers to form compounds. In chemical reactions, atoms are separated, combined, or rearranged.

All atoms of one element have the same mass. Atoms of two different elements have different masses.

John Dalton’s Atomic Theory1. Matter composed of extremely small

atoms

2. Atoms of given element are identical

3. Atoms of different elements are different

4. Can’t be created, divided, or destroyed

5. Different atoms combine in whole number ratios to form compounds

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

Dalton’s Atomic TheoryExperimental evidence

• looked at mass ratios of compounds

Theory explained conservation of mass

Element A Element B Compound AB2

mass = mA

mass = mA + mB

mass = mB

Dalton’s Atomic TheorySlightly wrong about• Indivisibility of atoms (subatomic particles)• All atoms of same element having identical

properties (isotopes)• Although atoms themselves not created or

destroyed, slight changes in mass occur as energy absorbed/released (thanks to James Kong & A Einstein)

• “Exotic” matter (neutron stars, plasmas, dark matter, etc) not composed of atoms as such (thanks to Adam Sorrentino)

Atom DefinitionSmallest particle of an element that retains the property of the element

This simple definition does not deal with the reality uncovered by modern nanotechnology research – individual atoms or small clusters of atoms of an element do not always behave in the same way as a bulk sample of the element

Imaging Atoms

Atom diameters ~ 0.1 to 0.5 nm (water molecule diameter ~0.3 nm)

Techniques exist to “image” atoms (not really “seeing” them in the conventional sense of the word)

Not readily available until STM commercialized (see following)

http://en.wikipedia.org/wiki/Scanning_tunneling_microscope

Schematic of STMhttp://www.iap.tuwien.ac.at/www/surface/STM_Gallery/stm_schematic.html

Based on “tunneling current”• Starts to flow when sharp tip

approaches conducting surface at distance of ~ 1 nm

• Current extremely sensitive to distance

Tip mounted on a piezoelectric tube• Allows tiny movements by applying a

voltage at its electrodes

STM Operation

Electronics control tip position so tunneling current (tip-surface distance) is kept constant while scanning a small area of the sample

Movement recorded - displayed as an image of the surface topography

Under ideal circumstances individual atoms of a surface can be resolved

STM

STM – Moving Atoms

Modified STM can be used as a tool for picking up, moving, and putting down atoms

Imaging Atoms: Modified Scanning Transmission Electron Microscope

In 2002, IBM researchers and their collaborators modified an electron microscope; allowed clear images at the atomic scale to be madeModified electron microscope is second major instrument to provide images of atomsCan’t be used to move atoms like STM type instruments

http://physicsworld.com/cws/article/print/23440

Practice

Early & current theories of matter

Problems 1- 5, page 91

Problems 29 – 33, page 112

Chapter 4 (Partial)Structure of the Atom

4.1 Early Theories of Matter

4.2 Subatomic Particles & Nuclear Atom

4.2.5 Ultimate Structure of Matter – The Standard Model (Not in Book)

Section 4.2 Defining the Atom

• Define atom. • Distinguish between the subatomic particles in terms

of relative charge and mass.• Describe the structure of the atom, including the

locations of the subatomic particles and the relative sizes of the atom and the nucleus.

• Identify the scientists that contributed to the discovery of the nature of the atom and be able to describe their specific contribution and the experiment on which their discovery was based.

An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus.

Key Concepts

• An atom is the smallest particle of an element that maintains the properties of that element.

• Electrons have a 1– charge, protons have a 1+ charge, and neutrons have no charge.

• An atom consists mostly of empty space surrounding the nucleus; the size of the atom relative to the size is the nucleus is about 10,000.

Section 4.2 Defining the Atom

Crookes (Cathode Ray) TubeSee page 92, Figure 4-7

Effect of Electric and/or Magnetic Fields on Electron Trajectory

Discovering the Electron

From cathode ray tube experiments, it was determined that rays:

• Were actually stream of charged particles• Carried negative charge

J J Thomson

Discovering the ElectronThomson (1856-1940)

• Measured effect of electric and magnetic fields on cathode ray to determine ratio of charge to mass (q/m) for electron

• From comparison with known (q/m) values, concluded that electron mass much less than hydrogen atom must be a subatomic particle

• Did not determine actual value of mass

Discovering the Electron

Millikan (1868-1953)• Determined charge on electron

from oil drop experiment (see following)• From mass/charge ratio (previously determined by Thomson), calculated electron mass, me

me = 1/1840 mass of hydrogen atom

Millikan’s Oil Drop Experiment

Ions produced by energetic radiation (X-rays)

Some ions attach to oil droplets, giving them a net charge

Fall of droplet in electric field between the condenser plates is speeded up or slowed down, depending on the magnitude and sign of charge on droplet

Millikan’s Oil Drop Experiment

Electrically charged condenser plates

Atomizer

Millikan’s Oil Drop Experiment

Analyzed data from a large number of droplets

Concluded that the magnitude of charge (q) on a droplet is an integral multiple of electronic charge (e)

q = n e

(where n = 1, 2, 3, . . . ).

AKA “chocolate chip cookie dough” modelProposed by Thomson

Plum Pudding Atomic Model

Smeared out “pudding” of positive charge with negative electron “plums” imbedded in it

+

Electrons(negative)

+

+

Nuclear Atom (Rutherford)Rutherford devised test to distinguish between plum pudding and nuclear models

• Plum pudding – cloud of positive charge• Nuclear – concentrated positive charge

Plum pudding model advantage: + charges can avoid each otherAlpha particle deflection from gold foilConcluded that there must be nucleus

Rutherford’s Experiment

Lead Box

RadioactiveSample

GoldFoil

FluorescentScreen

Alpha ParticlesStriking Screen

Rutherford Scattering Experiment

Most go straight through

Some deflected

Some bounced back!

Rutherford Scattering ExperimentOver 98% of alpha particles went straight through

About 2% of alpha particles went through but were deflected by large angles

About 0.01% of alpha particles bounced off gold foil

“...as if you fired a 15” canon shell at a piece of tissue paper and it came back and hit you.”

Rutherford Scattering Experiment

Alpha particles should pass right through the atoms with minimum deflection

Expected Result(plum pudding)

Rutherford Scattering Experiment

Expected Result(plum pudding)

Actual Result(nuclear model)

Rutherford ConclusionsAtoms contain a positively charged, small core, called nucleus

Note: structure of nucleus (as protons) not yet known

Most of atom is empty space

Discovery of Protons

Protons (discovered 1920 – Rutherford)

• Nucleus contained positively charged particles called protons• Charge equal and opposite to that of electron

Missing Anything?

Shouldn’t protons repel each other?

Since electrons weigh nothing compared to protons…

If beryllium atom has 4 protons, mass should be ~ 4 amu

Actual mass 9.01 amu! Where is extra mass coming from?

Need more experiments!

Discovery of Neutron

Neutron (discovered 1932 – James Chadwick)

• Nucleus contained subatomic particles called neutrons

• No charge• Mass nearly equal to

that of proton

General Features of the Atom

Nuclear Atom – Relative Sizes

If entire atom were represented by a room, 5 m x 5 m x 5 m, the nucleus would be about the size of a period in the textbook

Nucleus diameter is ~ 1/10,000 diameter of an atom

Atom Components

See table page 97, table 4-1

Particle Symbol Relative mass

Electron e- 1/1840

Proton p+ 1.000

Neutron n0 1.001

Summary: key events in discovery of nature of matter for chemists

~400 BC Democritus’ Atomic Theory (not accepted)~350 BC Aristotle elements: earth, air, fire, & water

1803 John Dalton’s Atomic theory began forming1897 J. J. Thompson discovers electron

1910 Robert Millikan determines charge on electron1911 Ernest Rutherford discovers positive nucleus1919 Ernest Rutherford discovers proton - evidence for

proton as a constituent of nucleus

1932 James Chadwick discovers neutron

Practice

Subatomic particles & nuclear atom

Problems 6 - 9, page 97

Problems 34 - 46, page 112

Chapter 4 (Partial) - Structure of the Atom

4.1 Early Theories of Matter4.2 Subatomic Particles & Nuclear Atom4.2.5 Ultimate Structure of Matter – The Standard Model (Not in Book)

Section 4.2.5 Ultimate Structure of Matter – The Standard Model

• List and describe the fundamental particles of nature.• List the four fundamental forces and their relative

strengths; know that bosons are the carriers of force.• Describe hadrons, baryons, mesons, quarks and

leptons and be able to identify their component particles (if they are not themselves fundamental).

The Standard Model describes the fundamental particles of nature and the forces that act between particles.

Section 4.2.5 Ultimate Structure of Matter – The Standard Model

• List the 6 kinds of quarks and the 6 kinds of leptons.• Describe how the proton, the neutron and the electron

fit into the classification of matter under the Standard Model.

• Describe the nature of antimatter and the method by which is was both predicted and experimentally verified.

• Describe the role that large particle accelerators such as the Large Hadron Collider (LHC) play in discovering new information about the nature of matter.

4.2.5 Ultimate Structure of Matter – The Standard Model (Not in Book)

Standard Model Intro – Particles & ForcesThe Emptiness of MatterFundamental ForcesSub-structure of particlesMatter and Anti-MatterTracing Development of Ideas via Nobel PrizesTools of the Trade – Fermilab and CERN (LHC)

Beyond proton/neutron/electron Picture Textbook, page 114

“... scientists have determined that protons and neutrons have their own structures. They are composed of subatomic particles called quarks. These particles will not be covered in this textbook because scientists do not yet understand if or how they affect chemical behavior. As you will learn in later chapters, chemical behavior can be explained by considering only an atom’s electrons .”

Beyond proton/neutron/electron Picture (not in book)

To understand nucleus and how some nuclear radiation processes occur, need to examine both structure of nucleons (proton, neutron) and forces acting at nuclear distances

The standard model of physics attempts to describe all known forces and elementary particles

What Is Matter ?Matter is all the “stuff” around you!

Big picture (from standard model):

Hadrons

Matter

Leptons

Baryons Mesons Charged Neutrinos

Forces

Weak EM

StrongGravity

QuarksAnti-Quarks

Elementary Particles

Standard Model SummaryThe Standard Model (SM) is our current best description of the particles of which matter is made and the forces which govern these particles

SM describes 4 fundamental forces

SM describes 12 elementary particles: 6 kinds of quarks and 6 kinds of leptons (not counting anti-particles)

Particles come in two major categories: hadrons and leptons

Hadrons

Particles Built from Quarks - Hadrons

Hundreds of hadrons have been observed

Except for proton & neutron, they are unstable - half lives < 0.1 sFree neutron (outside nucleus) is unstable – half life 10.2 min

Particles in Standard ModelSix leptons are all elementary particles – includes the electron

All other particles (hadrons) are composed of combinations of quarks (6 kinds) – isolated quarks are not permitted

Class of hadrons called baryons composed of 3 quarks – includes proton & neutron

Class of hadrons called mesons composed of 2 quarks (quark + anti-quark)

“Ordinary” matter

Dimensions of Subatomic Particles

If protons and neutrons were 10 cm across, then quarks and electrons would be < 0.1 mm in size and entire atom would be ~ 10

km across

Structure Within the Atom

Space is mostly “empty space”

Atoms > 99.999% empty space

Electron

Nucleus

Protons & Neutrons are > 99.999% empty space

g

u d

u

Quarks make up negligiblefraction of

protons volume !!

Proton

The Universe

The universe and all the matter in it is almost all

empty space !

(YIKES)

Why does matter appear to be so rigid ?

Forces, forces, forces !!!!

Primarily strong and electromagnetic forces which give matter its solid structure

Strong force defines nuclear size

Electromagnetic force defines atomic size

Standard ModelFour Fundamental Forces

In order of decreasing strength:Strong – binds nucleons Electromagnetic – “opposites attract”Weak – involved in radioactive decay (beta decay)Gravity

Forces arise through exchange of a mediating field particle (a boson)

Four Fundamental Forces

?

Forces and ParticlesGravity and electromagnetic force act between all particles with mass and charge, respectively

Leptons not composed of quarks, so aren’t subject to strong force, but are subject to weak force

Quarks subject to all four forces

Attractive force between nucleons (protons, neutrons) is byproduct of strong force, since nucleons are composed of quarks

The Nucleus

Concentrated positive charge in nucleus

Nucleus should repel and blow apart

But nucleons have a deeper structure

Proton Neutron

Standard Model - ForcesNeutrons and protons in nucleus held together by strong force, which has a short rangeStrong force able to overcome strong electric repulsion of + charged protonsElectromagnetic (EM) force between charged particles (electrons attracted to nucleus) Weak force involved in neutron decay – involves changing one type of quark into 2nd type with electron emission Matter mostly empty space; forces, especially EM forces, make it seem like it isn’t

Forces In The AtomElectrons held in place by electromagnetic force

Nucleons held together by strong force

Force Carrier Particles (Bosons)

Strong Gluons

Electromagnetic Photons

Gravity Gravitons?

Getting weaker

Standard Model Fundamental Particles and Force Carriers

All 6 quarks and 6 leptons have corresponding antiparticles with opposite charge

Some particles are their own antiparticles

Standard Model - Generations

EM

Strong

Higgs Boson (gravitron) ?

?

Weak

Standard Model SummaryUp & down quarks (in the form of neutrons and protons) and electrons are constituents of ordinary matter

Individual quarks cannot be isolated

Other leptons and particles containing quarks can be produced in cosmic ray showers or in high energy particle accelerators; these particles are all short-lived

Each particle has corresponding antiparticle

Matter & Forces from Standard Model

Hadrons

Matter

Leptons

Baryons Mesons Charged Neutrinos

Forces

Weak EM

StrongGravity

QuarksAnti-Quarks

Proton & neutron in this group

Electron in this group

Gen

I

II

III

Each generation

is more massive –

takes higher

energy to create

Gen

I

II

III

Proton made of three quarks

One Down Quark

Two Up Quarks

Up quark has charge +2/3 and mass of (approximately) 1/3

Down quark has charge –1/3 and mass of (approximately) 1/3

Mass = 1/3 + 1/3 + 1/3 = 1Charge = 2/3 + 2/3 – 1/3 = +1

The Proton – Not Elementary

The Neutron – Not Elementary

Neutron also made of three quarks

Two Down Quarks

One Up Quark

Mass = 1/3 + 1/3 + 1/3 = 1

Charge = 2/3 – 1/3 – 1/3 = 0

Neutrons can decay

Matter - Elementary Particles

Proton & neutron are both baryons

Proton: 2 up quarks and 1 down quark

Neutron: 1 up quark and 2 down quarks

The three elementary particles that make up ordinary matter (atoms) are the up quark, the down quark, and the electron

Physicist’s perspective: ordinary matter is composed of 2 kinds of baryons and one type of lepton

neutron proton

Beta Decay In Neutron

electron neutrino

W– boson

Example of weak force, of which W– is the boson

Antimatter – Paul DiracIn 1928, wrote down equation which combined quantum theory (developed in 1920s by Schrodinger and Heisenberg) and special relativity (1900s, Einstein), to describe behavior of electron

Equation could have two solutions, one for electron with positive energy, and one for electron with negative energy

But in classical physics (and common sense!), energy of particle must always be a positive number!

http://livefromcern.web.cern.ch/livefromcern/antimatter/history/AM-history01.html

Antimatter – Paul DiracDirac interpreted this to mean that for every particle that exists there is a corresponding antiparticle, exactly matching the particle but with opposite charge

For electron, for instance, there should be an "antielectron" identical in every way but with a positive electric charge

In Nobel Lecture, Dirac speculated on existence of completely new Universe made out of antimatter!

http://livefromcern.web.cern.ch/livefromcern/antimatter/history/AM-history01.html

Antimatter – Carl Anderson1932, young professor at Caltech, studied showers of cosmic particles in cloud chamber; saw track left by "something positively charged, and with the same mass as an electron"

After nearly 1 year of effort and observation, decided tracks were actually antielectrons, each produced alongside an electron from impact of cosmic rays in cloud chamber

Called antielectron "positron", for its positive charge. discovery gave Anderson the Nobel Prize in 1936 and proved existence of antiparticles as predicted by Dirac http://livefromcern.web.cern.ch/livefromcern/antimatter/history/AM-history01-a.html

Anderson's cloud chamber picture of cosmic radiation from 1932 showing for first time the existence of anti-electron

Particle enters from bottom, strikes lead plate in middle and loses energy as can be seen from greater curvature of upper part of track

http://www.aps.org/publications/apsnews/200408/history.cfm http://livefromcern.web.cern.ch/livefromcern/antimatter/history/AM-history01-a.html

Antimatter – Carl Anderson

Standard Model DevelopmentDeveloped and verified by careful analysis of high energy physics experiments (particle accelerators and colliders) along with further development and refinement of quantum mechanics

Also requires improved experimental equipment, methods, analysis techniques

Current WorkLarge accelerator experiments at Fermilab (Illinois) [stopped operation Oct 2011] and at CERN (Switzerland/France) in the Large Hadron Collider (LHC) done to search for new particles and test Standard Model predictions

LHC Technology Review (MIT) May/June 2008 By Jerome Friedman

The recently completed Large Hadron Collider, the world's most powerful particle accelerator and most ambitious scientific instrument, is being readied to address some of the deepest questions in physics. Hundreds of feet below the surface of the earth, straddling the Swiss-French border near Geneva, it will smash counter-rotating, seven trillion-electron-volt beams of protons against one another in a 27-kilometer ring of superconducting magnets.

LHC With this immense energy, the LHC will be capable of producing new types of particles that are thousands of times heavier than the proton. And it will enable physicists to study phenomena at one-ten-billionth the scale of the atom.

The science will be carried out with five multisystem particle detectors, the most massive of which are Atlas and CMS. Atlas is comparable in size to a seven-story building, 135 feet long and 75 feet wide; CMS, a somewhat smaller but heavier detector, weighs more than one and a half times as much as the Eiffel Tower.

Compact Muon Solenoid CMS (high energy particle physics detector) at CERN lab (Geneva)

An example of one of the LHC particle detectors

END