Nuclear and Atomic Physics

Intro: Atomic Structure and a little history.

Print off the topic outline for this unit by tomorrow!

Early atomic structure…

JJ Thomson: “Plum pudding” model An atom is a mixture of positive and negative

charges

Image from: http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec11.html

Atomic Structure 1909: Rutherford—Worked with Geiger

and Marsden on the gold foil experiment A very thin gold foil was placed in the center

of a chamber that had photo detecting material on its inner surface.

Alpha particles (essentially a helium nucleus) were shot toward the gold foil

The particles were detected on the inner surface of the chamber, and any scattering was noted

Image from: http://bhs.smuhsd.org/science-dept/marcan/apchemistry/structure_ML_MD.html

Gold foil experiment… Results:

Most alpha particles were detected at very small scattering angles (essentially went through the foil, but were deflected)

Some larger-angle scattering occurred, sometimes large enough that the alpha particle seemed to reflect back nearly on its original path.

Conclusion: Atoms have a dense, positively charged

center (the nucleus) and the electrons must be in the space surrounding the nucleus

Rutherford’s Atom…

“Planetary model”:

Image from: http://abyss.uoregon.edu/~js/ast123/lectures/lec04.html

Planetary Model

Massive, positively charged nucleus Electrons orbited much like planets around

the Sun The Coulombic (electrostatic) force of

attraction between the positive protons and the negative electrons kept the electrons in orbit

Planetary Model—problems… According to a theory of

electromagnetism, accelerating charges will emit energy in the form of electromagnetic radiation

Radiating energy would cause the electron to have a lower total energy and therefore would cause it to orbit a smaller distance from the nucleus…

Electron would spiral in and crash into the nucleus

Neils Bohr—atomic postulates

Studied Hydrogen atom Determined that there are certain defined

energy states in which the electron can exist In one of these states, the electron will not

radiate its energy and will remain in a stable orbit

Energy can only be lost if the electron transitions into a state of lower energy

Emission and Absorption Spectra

All elements will emit light in characteristic colors when heated

Scottish physicist Thomas Melville—first to study emitted light (1726-1753) Heat source = flame Passed emitted light through prism Pattern produced was significantly different

than white light passed through spectrum Bright line spectrum

Emission and Absorption spectra give the same “fingerprint” for an element, but in different ways

Alpha particles directed at a thin gold foil will most likely...

A. pass directly through the foil with no deflection

B. be reflected straight back from the solid foilC. pass through the foil wil a small amount of

deflectionD. be deflected at large angles as it passes

through the foil

Large deflections of alpha particles in Geiger and Marsden's scattering

experiment suggested...A. atoms consist of a small negative nucleus

surrounded by protonsB. atoms consist of a small positive nucleus

surrounded by electronsC. atoms consist of a small neutral nucleus

surrounded by electrons and protonsD. atoms consist of a large positive mass with

embedded electrons

The Rutherford model of the atom has the positive charge...

A. spread uniformly throughout the atom's volume

B. circling the nucleus as positive electronsC. concentrated in a central nucleusD. none of the above

Isotopes Nuclear isotopes (a.k.a nuclides) have

specific nuclear notation:

Z = atomic number (= # protons)A = mass number ( Nucleon number)

(= #protons + # neutrons)X = chemical symbol of the element

AZ X

Isotopes Most elements have more than one isotope

(although not always a stable one!) Isotopes are atoms of the SAME ELEMENT with

DIFFERENT numbers of NEUTRONS Atomic number is ALWAYS the same for any

isotope—only the mass number (nucleon number) changes

Evidence for neutrons using isotopes: there is no other way to logically explain the difference in mass for various atoms of a particular element.

Isotope Practice:Isotope Mass

NumberAtomic Number

# of protons

# of neutrons

# of electrons

8939Y

Cl-1 37

16 19 16

60 27 25

Isotope Mass Number

Atomic Number

# of protons

# of neutrons

# of electrons

8939Y 89 39 39 50 39

Cl-1 37

16 19 16

60 27 25

Isotope Mass Number

Atomic Number

# of protons

# of neutrons

# of electrons

8939Y 89 39 39 50 39

Cl-1 37 17 17 20 18

16 19 16

60 27 25

Isotope Mass Number

Atomic Number

# of protons

# of neutrons

# of electrons

8939Y 89 39 39 50 39

Cl-1 37 17 17 20 18

3516S 35 16 16 19 16

60 27 25

Isotope Mass Number

Atomic Number

# of protons

# of neutrons

# of electrons

8939Y 89 39 39 50 39

Cl-1 37 17 17 20 18

3516S 35 16 16 19 16

Co+2 60 27 27 33 25

Nuclear Interactions Fundamental Forces

Type Relative Strength

Field Particle

Gravitational 1 Graviton (hypothetical)

Weak nuclear 1032 W+/- and Z0

Electromagnetic 1036 Photon

Strong nuclear 1038 Gluons

Fundamental Forces… Evidence:

Strong nuclear force: protons do stay together in stable nuclei, even

though the electromagnetic forces between them would suggest they would repel

Weak nuclear force: evidence suggested during beta decay (where a

neutron disintegrates into a proton and an electron…)

Radioactive Decay Discovered in 1896 by Antoine Henri

Becquerel Inspired by discovery of X-rays, wanted to

know connection between those and fluorescent or phosphorescent materials

Experiment: Photographic paper wrapped in black paper to

keep out light… Salt samples (such as Uranium) placed on the

covered paper Also exposed the wrapped paper to sunlight

for several hours…

Results: Photographic plate was NOT exposed due

to the sunlight Outlines of the uranium sample clearly

visible on plate: THEN manipulated:

Temperature Amount of light Other physicaland chemical changes

NO EFFECT! Ionizing Radiation—because rays could

ionize gas molecules

Radioactive Decay Marie Curie (and husband, Pierre)—followed

Becquerel’s experiments to look for other substances with the same properties as Uranium… Isolated Thorium (~1898) Discovered Radium and Polonium…won Nobel Prize

in Chemistry (1903) 1899—Rutherford discovered that Uranium

emits 2 kinds of radiation (“alpha and beta rays”) 1900—gamma rays discovered as a 3rd type of

radiation by Paul Villard

Types of Radiation Ionizing power: the ability of radiation to

knock electrons out of orbit when they collide with another atom.

Alpha particles (a) Helium nucleus Charge = +2e (the same as 2 protons) Mass = 4u (1u = mass of a nucleon) Type of energy: all kinetic

velocity ~ 0.05c Penetration: stopped by a sheet of paper Range: a few centimeters Ionizing power—largest of the 3 types of

radiation…very dangerous if ingested!

Beta Particles (b) Very fast moving electron—emitted from the

nucleus Charge = -1e Mass = 1/1850 u Energy = all kinetic

(velocity up to 99% speed of light) Penetration: will be stopped by a few mm of

aluminum Range: a few meters through air More penetrating than Alpha particles, but

less ionizing.

Gamma Rays (g) High energy electromagnetic radiation

(very high frequency, very short wavelength) Charge = neutral Mass = 0 Energy: Photon Energy (proportional to the

frequency of the ray) Velocity = speed of light (c) Penetration: can be stopped by several cm

of lead or by a meter or more of concrete Range: there is no maximum range Lowest ionizing power of the 3 types of

radiation

Particles can be identified based on how they interact with a magnetic field: Alpha particles will

curve slightly Beta particles will be

deflected significantly, and in the opposite direction from alpha

Gamma rays—no charge, so no deflection at all