Radioactivity and Nuclear Reactions

Section 1: Radioactivity

Let’s Review… Atoms are made of three kinds of particles: electrons, protons, and neutrons.

The 4 Forces

The Strong Force The strong force causes protons and neutrons to be attracted to each other. The strong force is very short-ranged, so the protons and neutrons in the nucleus have to be VERY close together for it to work.

Forces in a Large Nucleus Protons and neutrons are held together less tightly in a large nucleus. See page 538.

Radioactivity When a nucleus breaks down and gives off matter and energy. Radioactivity: When the strong force is not large enough to hold a nucleus together tightly, the nucleus can decay giving off matter and energy. All nuclei (plural of nucleus) that have more than 83 protons are radioactive. Many nuclei that have less than 83 protons are radioactive too!

Isotopes and Stability The ratio of protons to neutrons has an effect on the stability of the nucleus. In elements with small nuclei, a ratio of 1:1 is stable. In larger nuclei, a ratio of 3:2 (neutrons to protons) is stable. Nuclei with too many or too few neutrons are radioactive and unstable.

The Discovery of Radioactivity 1896 Henri Becquerel He left uranium salt in a drawer with a photographic plate. He later developed the plate and saw an outline of the uranium salt. He thought that the uranium salt had emitted some invisible rays that darkened the film.

Marie Curie Discovered 2 new elements: polonium and radium. She named polonium after Poland, where she was born. She came up with the term “radioactivity.” 2 Nobel Prizes: Physics and Chemistry Died of aplastic anemia-caused by radiation. Even today, her papers are considered too dangerous to handle, they are kept in lead-lined boxes, and anyone that reads them must wear protective clothing!

Section 2: Nuclear Decay

Nuclear Radiation When an unstable nucleus decays, particles and energy are emitted. 3 types of nuclear radiation: 1) alpha 2) beta 3) gamma Alpha and Beta radiation are particles. Gamma radiation is a wave.

Note: This is the atomic weight, which is the number of protons + neutrons Alpha Particles (a) Radium R226 Radon Rn222 + p n n p a (4He) 88 protons 138 neutrons 86 protons 136 neutrons 2 protons 2 neutrons The alpha-particle (a) is a Helium nucleus. It’s the same as the element Helium, with the electrons stripped off !

Damage from Alpha Particles Alpha particles are “large” respectively, and they lose energy quickly. Alpha particles can be stopped by a sheet of paper. Alpha particles are dangerous if they are released by radioactive atoms inside the human body. Damage from alpha particles can cause cells to not function properly, leading to illness and/or disease.

Transmutation Transmutation-the process of changing one element to another through nuclear decay.

Beta Particles (b) Carbon C14 Nitrogen N14 + e- 6 protons 8 neutrons electron (beta-particle) We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus. Note: the mass # does not change!!!

Damage from Beta Particles Beta particles travel much faster, and are more penetrating than alpha particles. They can pass through paper, but can be stopped by a sheet of aluminum foil. Beta particles can damage cells when they are emitted from radioactive nuclei inside the body.

Gamma Rays (g) Gamma rays are electromagnetic waves with the highest frequencies and shortest wavelengths. Neon Ne20 Neon Ne20 + 10 protons 10 neutrons (in excited state) 10 protons 10 neutrons (lowest energy state) gamma A gamma is a high energy light particle that is emitted from a nucleus when alpha or beta decay occurs. It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

Gamma Rays Neon Ne20 Neon Ne20 + The gamma from nuclear decay is in the X-ray/ Gamma ray part of the EM spectrum (very energetic!)

Uses of Gamma Rays Sterilizing medical and dental instruments. Treatment of cancer Sterilizing food.