Unstable Nuclei & Radioactive Decay

1. Unstable Nuclei & Radioactive Decay

2. Radioactivity • Nucleus of an element spontaneously emits subatomic particles & electromagnetic waves. • Nucleus changes into a different element when it does this. • Original nucleus is called “unstable.” • Process is called “decay” or “transmutation.”

4. Rutherford & Radioactivity • 1898 – Rutherford began experiments with radioactivity. • 1899 – discovered alpha and beta “rays” from uranium.

5. Types of Radiation

6. Radioactivity – types

7. Characteristics of RadiationSee Table O 2 2 -1 -1 0 +1 +1

8. 2 Neutrons & 2 Protons. Charge = +2 Mass = 4

9. Beta Particle – fast moving electron.

10. Radioactive atom: Change occurs in nucleus.

11. Shielding

12. Can we predict exactly when an atom will decay? • NO! • For large #’s of atoms, we CAN predict how many will decay on average in a given amount of time.

13. Which elements are radioactive? • All elements past Bismuth in the periodic table. • If the atomic number is  83, it’s radioactive! • Other elements may have radioactive isotopes. • Stability depends on neutron/proton ratio. applet

14. What’s Going on in the Nucleus? • Electrostatic repulsions between protons. Want to fly apart. • But protons & neutrons all attracted to each other by nuclear strong force. • So having neutrons helps hold a nucleus together.

16. Strong Force • Works best if the nucleus isn’t too large. • As the nucleus gets larger, need to have more neutrons to help counteract the electrostatic repulsion between the protons. • Eventually, the nucleus is too large to be stable.

19. Balancing Act • Balance exists between electrostatic repulsive force & nuclear strong force. • Certain #’s of protons & neutrons make a stable nucleus. Other #’s of protons & neutrons are unstable. So the atom decays.

20. Beyond Element 83 • No amount of neutrons can hold a nucleus together once it has more than 83 protons. • Elements 84 & above are radioactive.

21. Stability and the n/p ratio • For atoms below atomic number = 20, best neutron/proton ratio  1. • As atomic number , atoms need more neutrons to be stable. • So n/p ratio for stable atoms increases to 1.5 for big atoms.

22. Stability Line

23. Type of radiation emitted depends on position relative to stable nuclei. Blue: too many neutrons. Yellow: not enough neutrons for the protons. Red/Orange: too many protons and neutrons

24. Natural Radioactivity – Unstable Nuclei Emit Radiation • Spontaneous nuclear change to attain good n/p ratio (high stability, low energy state). • Form a new kind of atom. • Each isotope or nuclide decays in a certain manner to get a better n/p ratio. The decay mode is named for the particle emitted. See Table N.

25. Nuclear Equations • Describe the decay process. • reactant or starting side (left)  product or ending side (right). •  separates two sides

26. Nuclear vs. Chemical • How is a nuclear change different from a chemical change?

27. Involve a change in an atom’s nucleus. Radioactive atoms spontaneously emit radiation and change into other kinds of atoms. Nuclear reactions involve 1,000,000 X more energy than ordinary chemical rxns. Involve changes in the outermost electrons. 1 or more substances changed into new substances. Atoms are rearranged, but their identities do not change. NUCLEAR CHEMICAL

28. Mass  Energy • In nuclear reactions, mass is converted into energy. • Mass Defect is the difference between the mass of a nucleus and the sum of the masses of its constituent particles. E = mc2

29. Binding Energy • Energy released when a nucleus is formed from its constituent particles. • It is a measure of the stability of an atom formed: • The higher the binding energy the more stable the nucleus. • The lightest and heaviest elements are the most unstable (low BE) • Intermediate elements are the most stable (highest BE).

30. Binding Energy Fe-56 Ni-62