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Northwestern University
Evolution of the Atom
David Rodriguez
Mauricio M. Garcia
Francisco X. Toussaint
Physics 335
Prof. Don Ellis
June 2, 2004
Greek 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 Discoveries


Galileo (1564-1642): atoms
were infinetely small,
vacuum suction between
infinitesimally small
surfaces would suffice to
hold solids together
Newton (1642-1727): First
one to mention the
“electrical Attraction
reaching to small
distances”, leaving the door
open for other short range
forces. Also realized that
heat is a molecular motion.
Law of conservation of mass.

Antoine Lavoisier (1743-1794):
 Took the first major step towards
modern quantitative chemistry.
Discovered that the total final
weight of all the materials
involved is exactly equal to the
total initial weight, first step on
thinking about chemistry in terms
of atoms.
 Beginning the modern study of
chemistry: precise terminology
and measurements.
 Classification of substances into
elements and compounds. The
atomic interpretation soon
appeared.
Dalton’s Model

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In 1808 John Dalton published the first formal
work on the atom in his book “A New System
of Chemical Philosophy”.
Dalton’s simple insights was that at the root of
all matter are exceedingly tiny irreducible
particles.
Dalton’s contribution was to consider the
relative sizes and characters of these atoms
and how they fit together.
Ex He said that hydrogen was the lightest of
all elements and he started giving atomic
weight to elements.
The Discovery of the First
Subatomic Particle

J.J. Thompson


Discovered the electron in 1897 at
the Cavendish Lab at Cambridge
University.
Experiments with beams of negative
particles.
Einstein’s Contribution



First incontrovertible evidence of the
atoms’ existence in his paper on
Brownian Motion (Robert Brown) in
1905.
He published this paper along with other
four that included one that examined
the photoelectric effect by means of
Planck’s new quantum theory and the
one that merely changed the world, his
special theory of relativity.
Since he had so much work to do with
this last one, he forgot about the atom
and other scientist started to work on it.
Rutherford’s Model
 In 1911 Ernest Rutherford discovered that the atom had a
nucleus surrounded by electrons.
 By 1919 he also discovered the proton.
 He stated that the nucleus is one millionth of a billionth of
the full volume of the atom but contains all the mass.
 Describes the atom as having a central positive nucleus
surrounded by negative orbiting electrons.
 Suggested that most of the mass of the atom was
contained in the small nucleus, and that the rest of the
atom was mostly empty space.
 Problems:
 Electrons should only orbit the atom for a small time
and then collapse to the nucleus with disastrous
consequences.
 How could Protons with positive charges be held
together in the nucleus?
 Subatomic matter behaved like nothing the imagined
before.
The Experiment (Rutherford)
 In 1911 performed an experiment with beams of alpha particles
bombarded through thin gold foil. He observed the deflections of
the rays by the scintillation they caused when they struck a
fluorescent screen coated with zinc sulfide. Most of the particles
passed straight with no deflection, occasionally one was found to
have been scattered at a large angle.
 He concluded that the scattering pattern was due to concentrated,
small, positively charged particles.
Bohr’s Model

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In 1913 Niels Bohr was an associate of
Rutherford.
Since they couldn’t see these subatomic
particles, they worked the structure of the atom
from how it behaved when they interacted with
it.
Their main problem was with spectrum reading
of the wavelengths of hydrogen. These emitted
energy at certain wavelengths but not others.
Solution Published in the paper “On the
Constitutions of Atoms and Molecules”. It
explained how electrons could keep from falling
into the nucleus by suggesting that they could
occupy only certain well-defined orbits. An
electron will move between orbits without
visiting the space between them. “QUANTUM
LEAP”.
The Discovery of the Neutron

James Chadwick
 Assistant for Rutherford.
 After 11 years of research, in 1932,
he discovered the neutron using
Beryllium Rays.
 Scientists at the time said that this
delay was very good as mastering
the neutron was essential to the
development of the atomic bomb.
Quantum model of the atom
Louis de Broglie (1924)
 Applied wave-particle theory to electrons
 electrons exhibit wave properties
QUANTIZED WAVELENGTHS
Quantum Mechanics


Werner Heisenberg’s Uncertainty Principle
 Impossible to know both the velocity and position of an
electron at the same time
Erwin Schrödinger’s Wave Equation (1926)

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Finite # of solutions -> quantizes energy levels
Probability of finding an electron:
1
Z
Ψ 1s 
π
 
a0
3/2
e σ
Orbital (“electron cloud”)

Region in space where there is 90% probability of finding an
electron.
Orbital
Radial Distribution Curve
Quantum numbers


They specify the address of each
electron in an atom
Principal quantum number (n)

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p
Energy level
Size of the orbital
n = # of sublevels per level
n2 = # of orbitals in the energy level
Angular Momentum Quantum # (l)


Energy Sublevel
Shape of the orbtial
d
s
Sublevel sets: 1 s, 3 p, 5 d, 7 f
f
Quantum numbers

Magnetic Quantum Number (ml)

Orientation of orbital
Specifies the exact orbital within each sublevel

Orbitals combine to form a spherical shape:

2s
2px
2py

2pz
Spin Quantum Number (ms)

Electron spin  +½ or -½

An orbital can hold 2 electrons that spin in opposite directions.
Wolfgang Pauli’s Exclusion
Principle (1925)

No two electrons in an atom can have the
same 4 quantum numbers.
Each electron has
1. Principal #
2. Ang. Mom. #
3. Magnetic #
4. Spin #

a unique address

energy level

sublevel (s,p,d,f)

orbital

electron
1930’s
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Forces that kept atoms together: Strong Nuclear Force and Weak
Nuclear Force.
 Strong; A short range force that attracts protons and neutrons to
each other. It holds the nucleus together. It is actually a force
that acts between quarks by the exchange of gluons. It keeps
nucleus together. Only ranges in 1/100,000 of diameter. This is
why elements with large nucleus are instable.
 Weak: The weak nuclear force affects all leptons and quarks. It
is the only force affecting neutrinos (except for gravitation,
which is negligible on laboratory scales). The weak interaction
enables all lepton and quark particles and antiparticles to
interchange energy, mass, electric charge and flavor—effectively
to change into each other. It keeps together electrons and is ten
billion billion billion stronger than gravity.
Two governing laws. Quantum theory  For the very small.
Relativity for the very big.
1940’s

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Scientist had reached a point where they understood the atom at an
extremely profound level.
Demonstrated in 1945 by the atomic bombs.
Even More Elemental Matter


C.T.R. Wilson
 1911 Invented first particle detector while building an artificial cloud
chamber to study cloud formations. When accelerating alpha particles
through the chamber to seed the clouds, it left a visible trail.
Ernest Lawrence
 1930 invents the cyclotron, the first “particle smasher” (U.C. Berkeley).
 Lead to discovery of particle families like muons, pions, hyperons, mesons,
K-mesons, Higgs bosons, intermediate vector bosons, baryons, and
techyons.
Getting Down to Quarks
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In 1960 Murray Gell-Mann invented a new class
of particles to restore simplicity to hadrons
(protons, neutrons, etc…), the Quark.
Hadrons are made of still smaller even more
fundamental matter. In other words, Quarks are
particles that make up particles.
Someone will need trillions of volts of electricity
and the budget of a small central American
county to get to them.

Six categories: Up, Down, Strange, Charm, top,
and bottom.

Further divided in their “flavors”: red, green, and
blue.
The Standard Model
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Essentially a “tools kit” to the subatomic world.
Consist of six quarks, six leptons, five known bosons and a
postulated sixth (Higgs Boson), plus three of the four physical forces
(the strong and weak nuclear forces and electromagnetism).
How it works:
 The basic building blocks are quarks. These are held together by
gluons to form protons and neutrons.
 Leptons are the source of electrons and neutrinos.
 Quarks and leptons together are called fermions.
 Bosons are particles that produce and carry forces, and include
photons and gluons. The Higgs boson may or may not exist; it
was invented simply as a way of endowing particles with mass.
Drawbacks: It doesn’t take into account gravity and it fails to
explain, this is why they introduces the notional Higgs boson
Further Studies
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Superstring Theory
 All particles are “strings”, vibrating strands of energy that
oscillate in eleven dimensions. Three of these we know and
the fourth is time. The other seven are unknowable to us.
 This enables physicist to pull together quantum and
gravitational laws.
M theory
 Incorporates surfaces known as membranes.
 Theory: The process begins in the indefinite past with a pair
of flat empty membranes sitting parallel to each other in a
warped five dimensional space…………
Questions?