Evolution of the AtomEvolution of the Atom

Physics 335Prof. Don Ellis

David RodriguezMauricio M. Garcia

Francisco X. Toussaint

June 2, 2004

Northwestern UniversityNorthwestern University

Greek ViewsGreek Views

First “atomic theorists” (Fifth-century BC):Leucippus of Miletus (a town now in Turkey)Democritus of Abdera

Their theory: If you could look at matter on smaller and smaller scales ultimately you would see atoms. Atoms were objects that couldn’t be divided further.

The physical properties of atoms, such as color and taste, depended on shapes, arrangements and orientations of the atoms.

First Elemental DiscoveriesFirst Elemental Discoveries Galileo (1564-1642): atoms Galileo (1564-1642): atoms

were infinetely small, were infinetely small, vacuum suction between vacuum suction between infinitesimally small infinitesimally small surfaces would suffice to surfaces would suffice to hold solids togetherhold solids together

Newton (1642-1727): First Newton (1642-1727): First one to mention the one to mention the “electrical Attraction “electrical Attraction reaching to small reaching to small distances”, leaving the door distances”, leaving the door open for other short range open for other short range forces. Also realized that forces. Also realized that heat is a molecular motion.heat is a molecular motion.

Law of conservation of mass.Law of conservation of mass. Antoine Lavoisier (1743-1794):Antoine Lavoisier (1743-1794):

Took the first major step Took the first major step towards modern quantitative towards modern quantitative chemistry. Discovered that the chemistry. Discovered that the total final weight of all the total final weight of all the materials involved is exactly materials involved is exactly equal to the total initial weight, equal to the total initial weight, first step on thinking about first step on thinking about chemistry in terms of atoms. chemistry in terms of atoms.

Beginning the modern study of Beginning the modern study of chemistry: precise terminology chemistry: precise terminology and measurements.and measurements.

Classification of substances Classification of substances into elements and compounds. into elements and compounds. The atomic interpretation soon The atomic interpretation soon appeared.appeared.

Dalton’s ModelDalton’s Model In 1808 John Dalton published the first In 1808 John Dalton published the first

formal work on the atom in his book “A formal work on the atom in his book “A New System of Chemical Philosophy”.New System of Chemical Philosophy”.

Dalton’s simple insights was that at the Dalton’s simple insights was that at the root of all matter are exceedingly tiny root of all matter are exceedingly tiny irreducible particles.irreducible particles.

Dalton’s contribution was to consider the Dalton’s contribution was to consider the relative sizes and characters of these relative sizes and characters of these atoms and how they fit together. atoms and how they fit together.

ExEx He said that hydrogen was the He said that hydrogen was the lightest of all elements and he started lightest of all elements and he started giving atomic weight to elements.giving atomic weight to elements.

The Discovery of the First The Discovery of the First Subatomic ParticleSubatomic Particle

J.J. ThompsonJ.J. Thompson Discovered the electron in 1897 Discovered the electron in 1897

at the Cavendish Lab at at the Cavendish Lab at Cambridge University.Cambridge University.

Experiments with beams of Experiments with beams of negative particles.negative particles.

Einstein’s ContributionEinstein’s Contribution First incontrovertible evidence of First incontrovertible evidence of

the atoms’ existence in his paper the atoms’ existence in his paper on Brownian Motion (Robert Brown) on Brownian Motion (Robert Brown) in 1905. in 1905.

He published this paper along with He published this paper along with other four that included one that other four that included one that examined the photoelectric effect examined the photoelectric effect by means of Planck’s new quantum by means of Planck’s new quantum theory and the one that merely theory and the one that merely changed the world, his special changed the world, his special theory of relativity.theory of relativity.

Since he had so much work to do Since he had so much work to do with this last one, he forgot about with this last one, he forgot about the atom and other scientist the atom and other scientist started to work on it. started to work on it.

Rutherford’s ModelRutherford’s Model In 1911 Ernest Rutherford discovered that the atom had a In 1911 Ernest Rutherford discovered that the atom had a

nucleus surrounded by electrons. nucleus surrounded by electrons. By 1919 he also discovered the proton.By 1919 he also discovered the proton. He stated that the nucleus is one millionth of a billionth of He stated that the nucleus is one millionth of a billionth of

the full volume of the atom but contains all the mass.the full volume of the atom but contains all the mass. Describes the atom as having a central positive nucleus Describes the atom as having a central positive nucleus

surrounded by negative orbiting electrons. surrounded by negative orbiting electrons. Suggested that most of the mass of the atom was Suggested that most of the mass of the atom was

contained in the small nucleus, and that the rest of the contained in the small nucleus, and that the rest of the atom was mostly empty space.atom was mostly empty space.

Problems:Problems: Electrons should only orbit the atom for a small time Electrons should only orbit the atom for a small time

and then collapse to the nucleus with disastrous and then collapse to the nucleus with disastrous consequences.consequences.

How could Protons with positive charges be held How could Protons with positive charges be held together in the nucleus?together in the nucleus?

Subatomic matter behaved like nothing the imagined Subatomic matter behaved like nothing the imagined before.before.

The Experiment (Rutherford) The Experiment (Rutherford) In 1911 performed an experiment with beams of alpha In 1911 performed an experiment with beams of alpha

particles bombarded through thin gold foil. He observed particles bombarded through thin gold foil. He observed the deflections of the rays by the scintillation they caused the deflections of the rays by the scintillation they caused when they struck a fluorescent screen coated with zinc when they struck a fluorescent screen coated with zinc sulfide. Most of the particles passed straight with no sulfide. Most of the particles passed straight with no deflection, occasionally one was found to have been deflection, occasionally one was found to have been scattered at a large angle. scattered at a large angle.

He concluded that the scattering pattern was due to He concluded that the scattering pattern was due to concentrated, small, positively charged particles.concentrated, small, positively charged particles.

Bohr’s ModelBohr’s Model In 1913 Niels Bohr was an associate of In 1913 Niels Bohr was an associate of

Rutherford.Rutherford.

Since they couldn’t see these subatomic Since they couldn’t see these subatomic particles, they worked the structure of the atom particles, they worked the structure of the atom from how it behaved when they interacted with from how it behaved when they interacted with it.it.

Their main problem was with spectrum reading Their main problem was with spectrum reading of the wavelengths of hydrogen. These emitted of the wavelengths of hydrogen. These emitted energy at certain wavelengths but not others.energy at certain wavelengths but not others.

SolutionSolution Published in the paper “On the Published in the paper “On the Constitutions of Atoms and Molecules”. It Constitutions of Atoms and Molecules”. It explained how electrons could keep from falling explained how electrons could keep from falling into the nucleus by suggesting that they could into the nucleus by suggesting that they could occupy only certain well-defined orbits. An occupy only certain well-defined orbits. An electron will move between orbits without electron will move between orbits without visiting the space between them. “QUANTUM visiting the space between them. “QUANTUM LEAP”.LEAP”.

The Discovery of the NeutronThe Discovery of the Neutron James ChadwickJames Chadwick

Assistant for Rutherford.Assistant for Rutherford. After 11 years of research, in After 11 years of research, in

1932, he discovered the 1932, he discovered the neutron using Beryllium Rays.neutron using Beryllium Rays.

Scientists at the time said that Scientists at the time said that this delay was very good as this delay was very good as mastering the neutron was mastering the neutron was essential to the development of essential to the development of the atomic bomb.the atomic bomb.

Quantum model of the atomQuantum model of the atomLouis de BroglieLouis de Broglie (1924) (1924) Applied wave-particle theory to electronsApplied wave-particle theory to electrons electrons exhibit wave propertieselectrons exhibit wave properties

QUANTIZED WAVELENGTHS

Quantum MechanicsQuantum Mechanics Werner Heisenberg’s Uncertainty PrincipleWerner Heisenberg’s Uncertainty Principle

Impossible to know both the velocity and position of Impossible to know both the velocity and position of an electron at the same timean electron at the same time

Erwin Schrödinger’sErwin Schrödinger’s Wave EquationWave Equation (1926) (1926) Finite # of solutions -> quantizes energy levelsFinite # of solutions -> quantizes energy levels Probability of finding an electron: Probability of finding an electron:

Orbital Orbital (“electron cloud”)(“electron cloud”) Region in space where there is 90% probability of Region in space where there is 90% probability of

finding an electron.finding an electron.

σ3/2 Zπ

11s 0

eΨ a

Radial Distribution CurveRadial Distribution CurveOrbitalOrbital

Quantum numbersQuantum numbers They specify the address of each They specify the address of each

electron in an atomelectron in an atom Principal quantum number (n)Principal quantum number (n)

Energy levelEnergy level Size of the orbitalSize of the orbital n = # of sublevels per leveln = # of sublevels per level nn2 2 = # of orbitals in the energy = # of orbitals in the energy

levellevel Angular Momentum Quantum Angular Momentum Quantum

# (l)# (l) Energy SublevelEnergy Sublevel Shape of the orbtialShape of the orbtial

s

p

d

f

Sublevel sets: 1 s, 3 p, 5 d, 7 f

Magnetic Quantum Number (mMagnetic Quantum Number (mll)) Orientation of orbitalOrientation of orbital Specifies the exact orbital within each sublevelSpecifies the exact orbital within each sublevel

Orbitals combine to form a spherical shape:Orbitals combine to form a spherical shape:

Spin Quantum Number (mSpin Quantum Number (mss)) Electron spin Electron spin +½ or -½ +½ or -½ An orbital can hold 2 electrons that spin in opposite directions.An orbital can hold 2 electrons that spin in opposite directions.

Quantum numbersQuantum numbers

2s

2pz

2py

2px

Wolfgang Pauli’s Exclusion Wolfgang Pauli’s Exclusion Principle (1925)Principle (1925)

No two electrons in an atom can have No two electrons in an atom can have the same 4 quantum numbers.the same 4 quantum numbers.

Each electron has a unique addressEach electron has a unique address

1. Principal #1. Principal # energy levelenergy level

2. Ang. Mom. #2. Ang. Mom. # sublevel (s,p,d,f)sublevel (s,p,d,f)

3. Magnetic #3. Magnetic # orbitalorbital

4. Spin #4. Spin # electronelectron

1930’s1930’s Forces that kept atoms together: Strong Nuclear Force and Forces that kept atoms together: Strong Nuclear Force and

Weak Nuclear Force.Weak Nuclear Force. Strong; A short range force that attracts protons and Strong; A short range force that attracts protons and

neutrons to each other. It holds the nucleus together. It is neutrons to each other. It holds the nucleus together. It is actually a force that acts between quarks by the exchange actually a force that acts between quarks by the exchange of gluons. It keeps nucleus together. Only ranges in of gluons. It keeps nucleus together. Only ranges in 1/100,000 of diameter. This is why elements with large 1/100,000 of diameter. This is why elements with large nucleus are instable. nucleus are instable.

Weak: Weak: The weak nuclear force affects all leptons and quarks. The weak nuclear force affects all leptons and quarks. It is the only force affecting neutrinos (except for It is the only force affecting neutrinos (except for gravitation, which is negligible on laboratory scales). The gravitation, which is negligible on laboratory scales). The weak interaction enables all lepton and quark particles and weak interaction enables all lepton and quark particles and antiparticles to interchange energy, mass, electric charge antiparticles to interchange energy, mass, electric charge and flavor—effectively to change into each other. It keeps and flavor—effectively to change into each other. It keeps together electrons and is ten billion billion billion stronger together electrons and is ten billion billion billion stronger than gravity.than gravity.

Two governing laws. Quantum theory Two governing laws. Quantum theory For the very small. For the very small. RelativityRelativity for the very big. for the very big.

1940’s1940’s

Scientist had reached a point where they understood the Scientist had reached a point where they understood the atom at an extremely profound level.atom at an extremely profound level.

Demonstrated in 1945 by the atomic bombs.Demonstrated in 1945 by the atomic bombs.

Even More Elemental MatterEven More Elemental Matter C.T.R. WilsonC.T.R. Wilson

1911 Invented first particle detector while building an artificial 1911 Invented first particle detector while building an artificial cloud chamber to study cloud formations. When accelerating cloud chamber to study cloud formations. When accelerating alpha particles through the chamber to seed the clouds, it left a alpha particles through the chamber to seed the clouds, it left a visible trail.visible trail.

Ernest LawrenceErnest Lawrence 1930 invents the cyclotron, the first “particle smasher” (U.C. 1930 invents the cyclotron, the first “particle smasher” (U.C.

Berkeley).Berkeley). Lead to discovery of particle families like muons, pions, hyperons, Lead to discovery of particle families like muons, pions, hyperons,

mesons, K-mesons, Higgs bosons, intermediate vector bosons, mesons, K-mesons, Higgs bosons, intermediate vector bosons, baryons, and techyons.baryons, and techyons.

Getting Down to QuarksGetting Down to Quarks In 1960 Murray Gell-Mann invented a new In 1960 Murray Gell-Mann invented a new

class of particles to restore simplicity to class of particles to restore simplicity to hadrons (protons, neutrons, etc…), the hadrons (protons, neutrons, etc…), the Quark.Quark.

Hadrons are made of still smaller even more Hadrons are made of still smaller even more fundamental matter. In other words, Quarks fundamental matter. In other words, Quarks are particles that make up particles.are particles that make up particles.

Someone will need trillions of volts of Someone will need trillions of volts of electricity and the budget of a small central electricity and the budget of a small central American county to get to them.American county to get to them.

Six categories: Up, Down, Strange, Charm, Six categories: Up, Down, Strange, Charm, top, and bottom.top, and bottom.

Further divided in their “flavors”: red, green, Further divided in their “flavors”: red, green, and blue.and blue.

The Standard ModelThe Standard Model Essentially a “tools kit” to the subatomic world.Essentially a “tools kit” to the subatomic world.

Consist of six quarks, six leptons, five known bosons and a Consist of six quarks, six leptons, five known bosons and a postulated sixth (Higgs Boson), plus three of the four physical postulated sixth (Higgs Boson), plus three of the four physical forces (the strong and weak nuclear forces and forces (the strong and weak nuclear forces and electromagnetism).electromagnetism).

How it works: How it works: The basic building blocks are quarks. These are held together The basic building blocks are quarks. These are held together

by gluons to form protons and neutrons.by gluons to form protons and neutrons. Leptons are the source of electrons and neutrinos.Leptons are the source of electrons and neutrinos. Quarks and leptons together are called fermions.Quarks and leptons together are called fermions. Bosons are particles that produce and carry forces, and Bosons are particles that produce and carry forces, and

include photons and gluons. The Higgs boson may or may not include photons and gluons. The Higgs boson may or may not exist; it was invented simply as a way of endowing particles exist; it was invented simply as a way of endowing particles with mass.with mass.

Drawbacks: It doesn’t take into account gravity and it fails to Drawbacks: It doesn’t take into account gravity and it fails to explain, this is why they introduces the notional Higgs bosonexplain, this is why they introduces the notional Higgs boson

Further StudiesFurther Studies Superstring TheorySuperstring Theory

All particles are “strings”, vibrating strands of energy All particles are “strings”, vibrating strands of energy that oscillate in eleven dimensions. Three of these we that oscillate in eleven dimensions. Three of these we know and the fourth is time. The other seven are know and the fourth is time. The other seven are unknowable to us.unknowable to us.

This enables physicist to pull together quantum and This enables physicist to pull together quantum and gravitational laws.gravitational laws.

M theoryM theory Incorporates surfaces known as membranes.Incorporates surfaces known as membranes. Theory: The process begins in the indefinite past with Theory: The process begins in the indefinite past with

a pair of flat empty membranes sitting parallel to a pair of flat empty membranes sitting parallel to each other in a warped five dimensional each other in a warped five dimensional space…………space…………

Questions?Questions?