Unit 3 Chemistry

Types of Radioactive Decay

Radioactive decay occurs when an unstable nucleus loses energy by emitting radiation. The three main types are:

• Alpha Decay

  • Emits an alpha particle (2 protons and 2 neutrons; same as a helium nucleus) 

  • Decreases atomic number by 2 and mass number by 4

  • Low penetration power; stopped by paper

• Beta Decay

  • Emits a beta particle (an electron or positron)

  • A neutron changes into a proton (for decay) or a proton changes into a neutron (for decay)

  • Increases or decreases atomic number by 1 but does not change mass number

  • Moderate penetration power; stopped by aluminum foil

• Gamma Decay

  • Emits a gamma ray (high-energy electromagnetic wave)

  • No change in atomic number or mass number

  • High penetration power; requires lead or thick concrete to stop it

Nuclear Stability

• Atoms are stable when the ratio of neutrons to protons (N/Z ratio) is within a certain range.

• The belt of stability shows stable isotopes based on their neutron-to-proton ratio.

• Elements with atomic numbers greater than 83 are typically radioactive.

Half-Life and How to Calculate It

• Half-life: The time it takes for half of a radioactive substance to decay.

• Formula:

  • A_s= initial amount

  • A_E= remaining amount

  • t = total time elapsed

  • h = half-life duration

• Example Problem:

  • A sample has a half-life of 10 years. If you start with 80g, how much remains after 30 years?

Nuclear Reactions: Fission and Fusion

• Nuclear Fission

  • Splitting of a heavy nucleus into smaller nuclei

  • Releases a large amount of energy

  • Used in nuclear power plants (e.g., uranium-235 fission)

  • Produces radioactive waste

• Nuclear Fusion

  • Combining of two light nuclei to form a heavier nucleus

  • Releases even more energy than fission

  • Powers the sun and stars (e.g., hydrogen fusion into helium)

  • Requires extremely high temperature and pressure