Chapter 4 (Partial) Structure of the Atom

Chapter 4 (Partial) Structure of the Atom
4.1 Early Theories of Matter (& Early Chemistry – Alchemy and Related) 4.2 Subatomic Particles & Nuclear Atom 4.2.5 Ultimate Structure of Matter – The Standard Model (Not in Book)

Section 4.1 Early Ideas About Matter
The ancient Greeks tried to explain matter, but the scientific study of the atom began with John Dalton in the early 1800's. 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.

(Cont.) 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).

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.

Early Philosophers Thought matter formed of: Earth Air Fire Water

History: Development of Atomic Model
Atomic Theory Timeline Democritus Aristotle Boyle Lavoisier Dalton J. Dalton J. Proust A. Lavoisier R. Boyle Empedocles Aristotle Democritus Leucippus Zeno R. Bacon - 5 2 7 1 - 5 - 2 5 2 5 5 7 5 1 1 2 5 1 5 1 7 5 2

Democritus Greek, 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 Theory
To 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 died Believed 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 years 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 Idea 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)

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.”

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

Science During 1600’s to1800’s Scientists 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 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 Boyle – studied the relationship between pressure and volume of a gas Boyle still believed in alchemy – that metals could be converted into gold

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 Called the father of modern chemistry Beheaded for his political beliefs In truth, the Law of conservation of matter was first discovered in ~1750 by the Russian chemist Lomonosov.

Lavoisier Was sure that air contained > one element
Was 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 ( ) Used work of Lavoisier, Proust, and Gay-Lussac to revive Democritus’ idea that matter was made of atoms Based 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 Theory
Matter composed of extremely small atoms Atoms of given element are identical Atoms of different elements are different Can’t be created, divided, or destroyed Different atoms combine in whole number ratios to form compounds In chemical reactions, atoms are separated, combined or rearranged

Dalton’s Atomic Theory
Experimental 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 Theory
Slightly 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 Definition Smallest 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)

Schematic of STM

STM Operation 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 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 – 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 made Modified electron microscope is second major instrument to provide images of atoms Can’t be used to move atoms like STM type instruments

Practice Early & current theories of matter Problems 1- 5, page 91

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

Section 4.2 Defining the Atom
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.

Crookes (Cathode Ray) Tube See 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

Thomson ( ) 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

Millikan ( ) 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

Atom izer Electrically charged condenser plates

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, ).

Plum Pudding Atomic Model
AKA “chocolate chip cookie dough” model Proposed by Thomson 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 other Alpha particle deflection from gold foil Concluded that there must be nucleus

Rutherford’s Experiment
Alpha Particles Striking Screen Radioactive Sample Lead Box Fluorescent Screen Gold Foil

Rutherford Scattering Experiment
Some deflected Most go straight through Some bounced back!

Over 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.”

Alpha particles should pass right through the atoms with minimum deflection Expected Result (plum pudding)

Expected Result (plum pudding) Actual Result (nuclear model)

Rutherford Conclusions
Atoms 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 /1840 Proton p Neutron n

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 forming 1897 J. J. Thompson discovers electron 1910 Robert Millikan determines charge on electron 1911 Ernest Rutherford discovers positive nucleus 1919 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

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 4.2.5 Ultimate Structure of Matter – The Standard Model
The Standard Model describes the fundamental particles of nature and the forces that act between particles. 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).

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 & Forces The Emptiness of Matter Fundamental Forces Sub-structure of particles Matter and Anti-Matter Tracing Development of Ideas via Nobel Prizes Tools 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 Matter is all the “stuff” around you!
Big picture (from standard model): Matter Hadrons Forces Leptons Elementary Particles Gravity Strong Baryons Mesons Charged Neutrinos Weak EM Quarks Anti-Quarks

Standard Model Summary
The 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 s Free neutron (outside nucleus) is unstable – half life 10.2 min

Particles in Standard Model
Six 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

Structure Within the Atom
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

Space is mostly “empty space”

Atoms > 99.999% empty space
Electron Nucleus

Protons & Neutrons are > 99.999% empty space
Quarks make up negligible fraction of protons volume !! u d

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 Model Four 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 Particles Gravity 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
Proton Neutron Concentrated positive charge in nucleus Nucleus should repel and blow apart But nucleons have a deeper structure

Standard Model - Forces
Neutrons and protons in nucleus held together by strong force, which has a short range Strong force able to overcome strong electric repulsion of + charged protons Electromagnetic (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

Carrier Particles (Bosons)
Forces In The Atom Electrons 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
Higgs Boson (gravitron) ?? EM Strong Weak

Standard Model Summary
Up & 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 This brings us to the concept of “force carriers”. Force carriers are the particles which transmit a force from one particle to another. We will get into this in greater detail…

Matter & Forces from Standard Model
Hadrons Forces Leptons Gravity Strong Baryons Mesons Charged Neutrinos Weak EM Proton & neutron in this group Electron in this group Quarks Anti-Quarks

Each generation is more massive – takes higher energy to create
II III Each generation is more massive – takes higher energy to create

Gen I II III

The Proton – Not Elementary
Proton made of three quarks Two Up Quarks One Down Quark 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 = 1 Charge = 2/3 + 2/3 – 1/3 = +1

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

Beta Decay In Neutron neutron proton W– boson electron neutrino
Example of weak force, of which W– is the boson

Antimatter – Paul Dirac
In 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!

Antimatter – Paul Dirac
Dirac 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!

Antimatter – Carl Anderson
1932, 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

Antimatter – Carl Anderson
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

Standard Model Development
Developed 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 Work Large 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 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, seventrillion-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

Chapter 4 (Partial) Structure of the Atom

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