Nuclear stability depends on the ratio of neutrons to protons, with most nuclei being stable if this ratio is close to one. Nuclei with even numbers of both protons and neutrons are the most stable. Unstable nuclei can undergo radioactive decay, releasing particles or electromagnetic waves to become more stable nuclei. Common types of decay include beta decay, electron capture, positron emission, alpha decay, and gamma emission. Nuclear equations must balance both mass and atomic numbers between reactants and products.

1. Rules of Nuclear Stability Except for hydrogen-1 & helium-3, all nuclei with = or more N than p+ are stable . Nucleons with a N/Z # that is too large or too small are unstable . See fig.6 page 646 Nuclei with even # of p+ & N are more stable . “ Magic numbers” of p+ & N tend to be more stable . Magic numbers are 2, 8, 20, 28, 50, 82, 126 Everything with a mass over 209 AMU or atomic number over 83 is unstable .

2. Nuclear Change 18.2

3. Radioactive Decay Changes an unstable nucleus to a stable nucleus. Causes a release of particles and/or electromagnetic waves.

4. Nuclear Particles H 1 1 n 0 1 -1 0 e e 0 +1 He 2 4 Particle Symbol Charge Proton p+ +1 Neutron n 0 Electron/Beta β - -1 Positron β + +1 Alpha particle α +2 Gamma ray 0

5. Beta Decay To become stable a N can be converted to a p+ by emitting a beta particle. Atomic # increases by 1 but the mass # does not change. Example: C 14 6 N 14 7 e 0 -1 +

6. Electron Capture Unstable nucleus absorbs an e- from the innermost orbital, s1. This changes a p+ to a N. Decreases atomic # by 1 but mass stays the same. Example: Cr 51 24 V 51 23 e 0 -1 + +

7. Positron Emission Unstable nucleus has too many p+. Emits a positron. Decreases atomic # by 1 but mass stays the same. Example: When a positron is given off it collides with an electron. Annihilation Cr 49 24 V 49 23 e 0 +1 +

8. Losing Alpha Particles Elements with N/Z # greater than 1 can stabilize by emitting an alpha particle. Mass is decreased by 4, atomic # decreased by 2. This can be repeated until it reaches stability. See figure 10 p. 651 Decay series. U 238 92 Th 234 90 He 4 2 +

9. Nuclear Equations must Balance Sum of the mass # on one side must = the sum of the mass # on the other. Sum of the atomic # on one side must equal the sum of the atomic # on the other. Example: Atomic # 92 Atomic # 90 + 2 = 92 Mass = 238 Mass = 234 + 4 = 238 U 238 92 Th 234 90 He 4 2 +