Atomic Physics

Prelude to Modern Science

Dr Gary Stilwell

History and Discovery

What are atoms

Early models

Problems to solve

The modern Atom

Some definitions

Seeing the atom

Structure of matter and force

Power of the Atom - E=mc2

March 1, 1954 H-bomb test at Bikini Atol

Matter and Black Holes

Later we will be discussing black holes.

At the center of a black hole matter is squeezed into

zero volume.

Let us now discuss this matter. . .

So, Just What Is This Matter that can be so Squeezed in a Black Hole?

The Easy Answer is -- Atoms:

Democritus (c. 430 BCE) a)/tomoj

"By convention sweet is sweet . . . But in reality nothing exists but atoms and the void". What we perceive by the senses is not reality! Sizes & shapes determine material properties.

- - -

Pierre Gassendi (ca. 1624) atoms not exist forever, made by God

John Dalton (ca. 1803) elements by atomic weight

JJ Thompson (ca. 1898) a 'plum pudding' model of the atom

Ernest Rutherford (ca. 1910) a 'solar system' model of the atom

- had major problems -- detailed later

electrons should collapse ("crash") through energy loss

not explain light colors - atomic spectra

not explain photoelectric effect

Neils Bohr (1913) a new 'quantized' model which would solve those problems

A Few Key Dates in Atomic History

1803Dalton postulates atomic principles1895Roentgen discovers X-rays1897Becquerel discovers radioactivity (named such by Curie) in uranium

1897JJ Thomson verifies existence of electron - plum pudding model1898Curies separate radioactive elements radium and polonium

1899Rutherford discovers alpha and beta radiation1900Quantum theory of radiation presented by Planck

1905Einstein publishes Quantum explanation of photoelectric effect, Brownian motion, special theory of relativity, and E=mc2

1911Rutherford proposes solar system model (based on 1909 gold foil)

1913Bohr proposes quantum theory to modify Rutherford's model - ad hoc and ugly but explained problems

Evolution of the Atomic Model

Democritus and early Greeks - sizes and shapes

Dalton - ball with hooks

next

Two Early Models of the Atom

Plum pudding --Thompson

Rutherford thenBohr --Solar system

Rutherford's Gold Foil

Early View of Atom - Rutherford's "Solar System" Model in 1911

Rutherford discoverednucleus in 'gold foil' bombardmentexperiment - some of thebullets ricocheted.

Rutherford's Model looks good butwill not work -- too many problems!

Atomic Models

Why so Many Models?

Because there are major problems with earlier models:

- Accelerating electrons should radiate continuous electromagnetic waves and lose energy and crash and into the nucleus instantaneously

- There is no method for explaining atomic spectra (discrete colors of light)

- No explanation for photoelectric effect

Models improve with better data

Remember - a model is not reality but an

attempt to explain and predict what we perceive

The "Crash" Problem

So, accelerating electrons should radiate continuous electromagnetic waves and lose energy and crash and into the nucleus instantaneously

Quantum physics to the rescue! They don't . . .

Because only certain orbits are allowed. Movement from one orbit to another is a quantum jump releasing one quantum of energy -- a photon.

The Crash (problem)

Atomic Spectra (problem)

Light from a light bulb has a continuous spectrum when shown through a prism - visible light from red to violet

However --Light from any particular element has discrete spectral lines of color when shown through a prism - next slide shows various elements

NASA photos

Atomic Spectra - cont.why are the lines there?

Dark linesabsorb

Bright linesemit

The Electromagnetic Spectrum

Visible Light

Low energy

High energy

Spectrum - absorbtion & emission

like the sunor stars

like interstellar gasor a stars atmosphere

dark line

bright line

Lines are caused by quantum energy levels unique to a particular element

Photoelectric Effect (Problem)

The electrons were called cathode raysand are the basis of the CRT

Why doesn't alllight cause this?

Why does it happen at all?

The Photoelectric Effect

Einstein to the rescue again:

In 1905 he explained that light is not a wave with the energy

spread out, but rather a concentrated particle-like 'bundle' of

energy - photons

A photon hits an atom and can knock an electron out

For high energy photons, the ejection occurs at once

For low energy photons, an electron will not be ejected

counter intuitively, color (high or low energy) matters,

intensity (brightness) does not

We have now described the quantum and the quantum jump

Bohr's Atom

Bohr's 1913 Atomic Model

Bohr's model allowed only discrete orbits - no spiraling

Electrons radiate light waves onlywhen jumping from one orbit toanother - solving the 'crash' andspectrum problems

It worked - but why?1924 DeBroglie's matter waves explained

There particles were unknow - just nucleus and electron

A Summary of Model of Atom - so far...

Electrons absorbing energyand getting excited jumpout.

Photons are of differing wave frequencies and energies (color).This explains the spectraof different elements. Each element has a unique spectrum - like fingerprints.

Electrons give up or absorb energyin discrete amounts so they quantum jump from one orbit to another instantaneously - not spiraling , solving the 'crash' problem.

Light shining on an atomcan knock off electronsHigh energy photons do somore easily than low energyphotons.A photons energy comesfrom its color NOT itsintensity (quantity of them).This explains the photo-electric effect problem.

Electrons giving up energy(a photon) jump in.

There particles were unknow - just nucleus and electron

1919Rutherford transmutes nitrogen to oxygen, verifies proton

1925de Broglie proposes wave nature of electrons (why Bohr works)1927Heisenberg formulates uncertainty principle1928Dirac predicts positron, discovered in 1932 by Anderson1930Eddington explains where sun's energy originates (fusion)1932Chadwick discovers neutron1933Szilard proposes chain reaction by neutrons, 1934 patents atomic bomb, transfered to British navy in 19361934Fermi transmutes elements to transuranics1938Hahn, Strassmansplit atom1939Meitner and Frisch explain Hahn's result (fission)1940s42-Manhattan, 45-Trinity tests, Japan bombed1964Gell-Mann suggests quarks as solution to particle explosion1995Top quark discovered (the last in the theory)

A Few Key Dates in Atomic History - cont.

Sub Atomic Matter

2H

Particle physicsparticle discoveredElectron 1897Proton 1919Neutron 1932Quarks 1964

o

Electron

Structure of Ordinary Matter

The Structure of the Atom

Atom = 100,000 times Nucleus10-10 / 10-15 = 105The atom is almost EMPTY void!!

End of Atomic Development

lots of good definitions and pictures follow

for further discussion . . .

END

The Atom's Size and Particles first approximation

(2) Ionization

Loss or gainof a singleelectron ionizesthe atom

total ionization

Scanning Tunneling Microscope

Simplification - Quarks and Leptons and the Fundamental Forces of Nature

By the mid-twentieth century, there was a veritable zoo of particles and forces found by atom smashers

A simplified scheme was eventually found that reduced all matter particles to just two types - quarks and leptons

and

all forces to just four types - gravity, electromagnetic, weak and color (strong) forces

Sub-atomic Particles and Dates of Discovery cont.

Sub Atomic Matter

2H

Standard Model of Sub-Atomic Particles all known matter and force

Ordinary matter

Force

Exotic matter

Fermions

Anti-matter

Anti-matter cont.

It works both ways

Forces - breaking symmetry at cosmic origin - temperature and time

The Atomic Nucleus

Radioactivity

- a spontaneous disintegration of atomic nuclei

- nucleus emits

alpha particles which are Helium nuclei

beta particles which are electrons

gamma particles which are electromagnetic rays

and others

- these react with other particles causing additional

release of energy

Types of Radioactive Decay

Fusion Reaction - H-bomb and Stars

Neutron provides4/5 of energyAlpha particle1/5

Problem:

confine long enoughto get more energy outthan went in to start upthe process

Neutrons do most of the energy production

Niels Bohr

was born over 120 years ago

7th October 1885

Niels Bohr is best known for the investigations of atomic structure and also for work on radiation, which won him the 1922 Nobel Prize for physics. One of the 'fathers' of quantum theory

ComponentApprox. SizeRelativeSize

Atom10 - 10m1,000,000,000

Nucleus10 -15 m 10,000

Neutron10 -16 m1,000

Proton10 -16 m

1,000

Electron10 -18 m10

Quark10 -19 m1