Atomic and Nuclear Physics

1. Atomic and Nuclear Physics

2. Atomic Physics • The last 150 years have actually been a time of great advancement for physics. • Several important theories and ideas were postulated this time – ideas that you accept as commonplace. • One of the most pivotal of all of these ideas was Einstein’s Theory of Relativity.

3. Einstein • Einstein came up with his Theory of Relativity. • E = mc2. • The formula states there is a relationship between mass and energy. We will explore this idea further later.

4. The Electron • Another new discovery of modern physics is the presence of the electron. • The discovery of the electron was made by JJ Thomson and his cathode ray tube.

5. The Electron • The cathode ray tube has a magnetic coil around it which will cause charged particles path to change. • The cathode ray tube also has an electric field. • When only the electric field is on, the beam is deflected upwards. • When only the magnetic field is on, the beam is deflected downwards.

6. The Electron • These different paths suggested the presence of a charge opposite of a proton. • Using this idea, a charge to mass ratio was developed. • Another scientist, Millikan, computed the value of the charge and mass of an electron.

7. Quantum Theory • Another discovery has to deal with the fact that hot objects will emit a certain amount of radiation (think of the sun). • They called this Blackbody radiation.

8. Planck’s Equation • In 1900, Maxwell Planck entered the scene. • He looked at all the equations that came before and synthesized a new equation. • E = nhf • Where E is the energy emitted, n is the energy level, f is the frequency and h is Planck’s constant (6.626 x 10-34 Js).

9. Planck’s Equation • So what is this equation used for? • This equation says for any oscillating material, there will only be specific frequencies for which energy is emitted. • Another way to put it is that not every frequency will emit energy.

10. Planck’s Theory • This theory was made without ever really knowing about many of the quantities that we know about – like electrons, protons, photons, or atoms. • When viewed in terms of atoms, this theory makes more sense.

11. Photoelectric Effect • The photoelectric effect was also a pivotal discovery. • In the photoelectric effect, electrons are emitted from matter when they absorb energy from EM waves where the wavelength is short (example: UV light).

12. Compton Effect • A photon is a tiny particle in which light travels. • Using this idea, Compton came up with yet another theory! • Compton scattered x-rays (short wavelength light) and found that after the light was scattered, the frequency was lower than before it started.

13. Compton Effect • This meant that energy is being lost. • The energy is lost due to the collision of the molecules with the surface that scatters them. • The momentum of the particle has to be conserved. • p = E/c • Remember that a photon (which is light) will always travel at the speed of light.

14. Compton Effect • Remember that v= λf. • So we can change this to be: p=hf/c • And also : p = h/λ • Compton took these equations and derived the following: • λ’ = λ + h/moc (1-cos Θ) • m=mass of an electron

15. Wave-Particle Duality • All of these ideas lead to the overall conclusion that light is a wave with particle-like properties or vice versa. • Suffice it to say that light is more complicated than they originally thought.

16. de Broglie Wavelength • In the early 1920’s another scientist further explored this idea of the dual nature of light. • de Broglie argued that if light had both wave and particle like properties, then maybe particles (like electrons) also had wave like properties. • λ = h/mc

17. de Broglie Wavelength • Looking at the numbers, we notice that the wavelengths will be extremely small. • This makes sense if we think about the fact that a particle appears not to move.

18. Just For You • A lot of this stuff is confusing. • Just try to get the key ideas for each theory. • Look at example problems in your AP study guide.

19. Structure of the Nucleus • The basic structure of the atom was not solidified until the early 1930’s when it became what we know today. • Is this model 100% accurate? • No – we are still unsure of what an atom is made of. • The basic components of an atom are the protons, the neutrons and the electrons.

20. Structure • The two components found in the nucleus are often called nucleons. • Remember from chemistry that the atomic number is the number of protons and the atomic mass is the number of protons plus neutrons. • The symbol for the atomic number is a Z while the symbol for the atomic mass is an A.

21. Structure • This simple notation can also denote isotopes. • What is an isotope?

22. Size • The approximate size of the nucleus was determined by Rutherford from his gold-foil experiment. • He found that r = (1.2 x 10-15m)(A1/3 )

23. Nuclear Binding • If you look at the mass of a proton and neutron and add them up to create a specific example, you will notice that their total masses are greater than the mass of the element. • Why? • Energy is lost when binding these parts together. It is called binding energy.

24. Radioactivity • Radioactivity was discovered in 1896 when a scientist noticed that some of his sample happened to darken a photographic plate. • Marie Curie further explored this idea and is accredited with making the main discovery of radioactivity. • What is radioactivity?

25. Radioactivity • Radioactivity is the decay of an unstable nucleus. • This unstable nucleus will emit a variety of particles until it is no longer unstable. • These particles are as follows: • Alpha particles • Beta particles • Gamma particles

26. Alpha Particles • An alpha particle is nothing more than a helium atom. • It becomes clear that a new element is formed. This new element has 2 less protons.

27. Alpha Decay • Alpha decay occurs because strong nuclear forces are unable to hold very large nuclei together. • This means that the repulsive forces are too great. • When alpha decay occurs, energy is released. • Q = (Mp – Md – Malpha)c2

28. Beta Particles • A beta particle is nothing more than the transmission of an electron. • A neutrino is also released. A neutrino is a mass-less, charge-less particle.

29. Beta Decay • In a Beta Decay, an electron is released. • We can also release a positron. • A positron has the same mass as an electron but will have the opposite charge.

30. Electron Capture • Another related process is called electron capture. • In this process, a nucleus will absorb one of its electrons. This electron will become a neutron.

31. Gamma Particles • A gamma particle is nothing more than a high energy photon. • In fact, the emission of a gamma ray is much like the emission of photons from excited atoms.

32. Gamma Decay • How does a nucleus get excited? • Usually through a collision with another atom. • Realize that a gamma ray is chargeless which means that there is no change in the element.

33. Half-Life • So how do we figure out when an element will decay? • By knowing its half life! • A half life is the amount of time it takes for ½ of the substance’s atoms to decay. • Using this information, the rate of decay was formulated.