PreLecture: Rutherford SimulationTurn on the “Show Traces” option, and simply show the Phet:http://phet.colorado.edu/en/simulation/rutherford-scattering

Atomic and Nuclear Physics (7)

Humans have been thinking about atoms for thousands of years. The biggest surprise about

atoms is that they do not behave at all like regular-sized objects. The objects in your world

operate according to familiar patterns of Newton, but the particles that make up your

world do not operate by those rules.

Mr. KlapholzShaker Heights

High School

Atoms, Pre-Rutherford In the year 1900, people had evidence that atoms existed, and that there was positive and negative charge, but there was almost no idea about how atoms were built of charge.

Raisin Bun

http://kristinasjollyhockeysticks.blogspot.com/2009_03_01_archive.html

Plum Pudding or

Raisin Bun Model of the atom

One idea was that the electrons were in a positive goop, like plums in pudding, or like raisins in a pastry.

http://mnhs.wikispaces.com/School+Forum

Rutherford• Then came one of the biggest experiments in

human experience. • In short, it told us that: the atom’s mass was

concentrated in an ultra-dense speck of a positive nucleus, the negative electrons formed the outer part of the atom, and in between the electrons and the nucleus was… nothing.

• This was nothing like plum pudding or raisin bread.

http://elements.vanderkrogt.net/element.php?sym=Rf

How did Rutherford figure out so much?

Rutherford, Geiger, Marsden

http://reich-chemistry.wikispaces.com/stephen.gasecki.timothy.graham.atomichistory.fall.2009

of alpha particles

The alpha(positive, massive, fast)

Data

• Almost every alpha particle goes through the gold foil unaffected by the gold.

• Those very few alpha particles that are affected by the foil are deflected a lot.

Gold atom and alpha particles

http://www.clemson.edu/caah/history/facultypages/PamMack/lec122sts/invention14.html

Conclusions of Rutherford’s experiment

• Most of the alpha particles are not affected by the gold foil. This tells us that most of an atom is empty.

• The few particles that bounced back must have been forced by a positive massive object. That’s what we now call the nucleus.

• And that’s how things stayed for 4 years: atoms had a massive positive part, and a negative part, but we didn’t know how it all fit together.

Niels Bohr was able to come up with a model that made sense of just about everything:

• The electrons were light, negative particles that orbit the nucleus.

• The reason that atoms are mostly empty is that electrons zing around in paths that are much bigger than a nucleus.

• Like the planets orbiting the sun, the electrons were attracted to the nucleus.

• For any circular orbit, the force is toward the center. For electrons, it’s the attractive force between positive and negative that provides that force.

Electron Orbits

http://wisp.physics.wisc.edu/astro104/lecture6/lec6_print.html

More on Bohr

• Bohr predicted energy orbits (“orbitals”). An electron could only exist in particular orbits.

• When an electron went from high energy (say n = 4) to lower energy (say n=1), the electron would emit light.

• The greater the energy drop, the greater the frequency of the light: hf = DE.

• h = 6.6 x 10-34 m2 kg s-1 = 6.6 x 10-34 J s

How to measure emission spectrum

http://www.tutornext.com/help/emission-spectra

Absorption spectrum apparatus.The white light starts on the right, goes through

the gas, and is observed on the left.

http://physics.kenyon.edu/EarlyApparatus/Optics/Sodium_Absorption_Spectrum/Sodium_Absorption_Spectrum.html

Emission spectrum is on the top. Absorption spectrum is on the bottom.

http://www.cbu.edu/~jvarrian/252/emspex.html

Why are only specific colors emitted / absorbed by hydrogen?

How do the emitted colors relate to the absorbed colors?

Hydrogen

Hydrogen

Hydrogen

Hydrogen

Photon Basics• A photon is a particle of light.• A photon is a bundle of energy with a specific

frequency. High frequency goes with high energy: E = hf.

• Light acts as a wave. Light acts as a particle. Particles are very different from waves. “Duality.”

• Bright light has more photons than dim light.

Photons and Atoms• When an electron drops from a high energy level

to a low level, the atom emits a photon; the energy of the photon equals the energy difference of the electron levels.

• When an atom absorbs light, an electron goes from a low energy level to a high level. The energy of the absorbed photon equals the energy difference of the electron levels. The only photons that an atom can absorb are the ones that match the differences in electron energy levels.

Ions

• If an atom loses an electron, then the atom is charged. Will it be positive or negative?

• A charged atom is called an ion.• The energy required to remove an electron is

called the ionization energy.

The Electron VoltHow much energy would an electron gain if we

let a 1-Volt battery give it a boost? • Voltage = Energy / Charge• Energy = Voltage x Charge• Energy = (1 V) x (1.602 x 10-19 C)• E = 1.602 x 10 x 10-19 J • 1 electron-Volt = 1.602 x 10-19 J …1 eV = 1.602 x 10-19 J (It is a unit of energy)

Planck’s constant

• As you know, h = 6.626 x 10-34 J s • Also, h = 4.136 x 10-15 eV s

Easy Problem Solving• If an electron is in the 2nd orbital (19 eV) and

drops to the first orbital (10 eV), then what is the frequency of the photon that is emitted?

Solution:DE = 9 eVDE = hf

f = DE ÷ h f = (9 eV) ÷ (4.136 x 10-15 eV s )

f = 2 x 1015 Hz

Spot the nucleus

http://student.ccbcmd.edu/~cminnier/radioact1/121radio3.htm

The Nucleus (1 of 2)• There are protons in the nucleus– Charge = +1.602 x 10-19 C– Mass = 1.673 x 10-27 kg

• There are neutrons in the nucleus– Charge = 0– Mass = 1.675 x 10-27 kg

• Notice that the charge of the proton is equal (in size) to the charge of the electron.

• Notice that neutrons are just a wee bit more massive than protons. (It is almost as if neutrons had a proton and an electron inside.)

The Nucleus (2 of 2)• Nucleon – the name for protons and neutrons.

Protons are nucleons, and neutrons are nucleons. The number of nucleons is symbolizes by A.

• Atomic Number (Z) – This is the number of protons in the nucleus. This is the only thing that makes an atom be a particular element. For example, all atoms of oxygen have 8 protons.

• Isotopes – Two atoms of the same element can have different numbers of neutrons (N) and therefore different masses.

A problem

• Hey, what holds the nucleus together in such a small space?

• What holds a neutron to another neutron?• And, … WHAT COULD POSSIBLY HOLD A

PROTON SO CLOSE TO ANOTHER PROTON?

The Strong Force

• The third force that we know of is the force that holds nucleons together in the nucleus.

• This force is extremely short range, and unbelievably intense.

• Because of the strong force, it is pretty tough to get the nucleons in a nucleus to come apart.

• Once a nucleon is out of the nucleus, this force has no effect on it.

Mass of a Nucleus

• If you add up the masses of the nucleons before they are together, you get a slightly greater value than if you measure the mass of the whole nucleus.

• The difference is called the mass defect.

Binding Energy

• If you multiply the mass defect by the speed of light squared, you get the energy required to break apart the nucleus.

• E = mc2.• The bigger the nucleus, the greater the

binding energy. (It takes more energy to separate all those nucleons.)

• But what if you graphed B.E. per nucleon vs size of nucleus?

The binding energy curve

http://library.thinkquest.org/3471/mass_binding.html

Why do nuclei move toward iron (56)…?

http://www.google.com/imgres?imgurl=http://www4.nau.edu/meteorite/Meteorite/Images/BindingEnergy.jpg&imgrefurl=http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryB.html&h=459&w=567&sz=37&tbnid=2l4x3t2tTYh6pM:&tbnh=108&tbnw=134&prev=/images%3Fq%3Dbinding%2Benergy%2Bcurve&zoom=1&q=binding+energy+curve&hl=en&usg=__yHj4cmQEXQXAHu4tw_7fByoPI3s=&sa=X&ei=CXHdTM--Kse1nweDorCDDw&ved=0CCQQ9QEwBA

Big nuclei get smaller, and small nuclei combine.

http://www.google.com/imgres?imgurl=http://www4.nau.edu/meteorite/Meteorite/Images/BindingEnergy.jpg&imgrefurl=http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryB.html&h=459&w=567&sz=37&tbnid=2l4x3t2tTYh6pM:&tbnh=108&tbnw=134&prev=/images%3Fq%3Dbinding%2Benergy%2Bcurve&zoom=1&q=binding+energy+curve&hl=en&usg=__yHj4cmQEXQXAHu4tw_7fByoPI3s=&sa=X&ei=CXHdTM--Kse1nweDorCDDw&ved=0CCQQ9QEwBA

Radioactive Decay (big nuclei get smaller)

• Big nuclei are not stable. They come apart naturally.

• There are three kinds of radiation, named for the first three letters of the Greek alphabet:– Alpha– Beta– Gamma

Meet the Alpha (42a = 4

2He) again

Alpha particles…

• are the nuclei of helium atoms.• have a lot of energy, so even one alpha can

ionize many atoms.• are a health hazard, due to their ability to ionize

atoms in living tissue.• are easy to detect, due to their ability to ionize.• are easy to stop, due to the amount of energy

that they lose when ionizing atoms.

Alpha decay

• Makes the nucleus drop in mass by about 4 mass units.

• Makes the nucleus drop in atomic number (Z) by 2 charge units. The atom literally changes from one element to another.

Beta particles (b- = 0-1b = 0

-1e = e-)

• are pretty much the same as electrons• have low mass, so they are not very effective

at ionizing atoms• have a lot of energy so they are easy to

measure.• are difficult to stop.

Beta Decay

http://www.green-planet-solar-energy.com/nuclear-power-information.html

Beta Decay

http://education.jlab.org/glossary/betadecay.html

The nucleus gets a wee bit less massive, and it becomes more positively charged. (An antineutrino is also released.)

Neutrinos and Antineutrinos

• Ultra low mass.• Almost never react with matter.• Hard to detect.

Gamma (g = 00g) radiation

• This is pure electromagnetic energy. (Light is an example, and so are x-rays.) E = hf.

• When a gamma ray leaves any object (including a nucleus) the object is left with less energy.

• The mass does not change.• The charge does not change.

Comparison tableAlpha Beta Gamma

Range in air About 5 cm About 30 cm About 100 cmPenetration Stopped by

paperStopped by thin layer of lead, or even aluminum foil

Stopped by thick layer of lead

Ionization ability

High Low Moderate

Detection GM tubeCloud chamberPhotographic paper

GM tubeCloud chamberPhotographic paper

GM tubeCloud chamberPhotographic paper

Nuclear Radiation and Health

• Good: X-rays, treatment of cancer• Bad: causes mutations and cancer (In

high doses radiation kills quickly).• Protection: distance (1/R2) and shielding

http://www.sciencedaily.com/articles/mind_brain/steroids/5/

Half Life (T½)• Which leaf will fall next?• Do the leaves fall more

often when there are more on the tree, or less on the tree?

• How often do the leaves fall?

• How much time will it take until all the leaves have fallen off?

http://www.istockphoto.com/stock-illustration-10466423-autumn-tree-with-falling-leaves.php

Half Life (T½)

• In a sample of uranium, which nucleus will decay next?

• Do the nuclei decay more often when there are more uranium nuclei, or when there are less?

• How often do the nuclei decay?• How much time will it take until all the nuclei

have decayed?

Half Life (T½)

• Although we do not know when all of the nuclei will have decayed, the time for half to decay is a reasonable quantity to measure. It works for trees, nuclear fission, and even the level of caffeine in a teacher’s blood.

• Any single urananium nucleus has a specific probability of decaying in the next second.

The rate of decay is proportional to the number of nuclei that

have not yet decayed.

Decay Curve1) When have all of the nuclei decayed?

http://www.tutorvista.com/physics/radioactive-decay-physics

2) Half life is defined as the amount of time for half of the nuclei to decay.

http://www.tutorvista.com/physics/radioactive-decay-physics

The rate at which the nuclei decay is the gradient of the decay curve. Try sketching it. It is

called “Activity”. What are the units?

http://www.tutorvista.com/physics/radioactive-decay-physics

Nuclear Reactions - Fission(big nuclei get smaller)

• Big nuclei (such as uranium and plutonium) are not as stable as medium-sized nuclei, so they come apart.

• During fission, the Binding Energy per nucleon will increase, leaving a more stable nucleus.

• This type of reaction happens in the nuclear power plant in Perry, OH, 38 miles from here.

http://watergatesummer.blogspot.com/2007_11_01_archive.html

Nuclear Reactions - Fusion(small nuclei get bigger)

• Small nuclei are not as stable as medium-sized nuclei, so these nuclei join together when given the chance.

• During fusion, the Binding Energy per nucleon will increase, leaving a more stable nucleus.

• This type of reaction happens in the sun. Fusion research is intense, with the hope of making cleaner nuclear energy here on earth.