Radioactivity and Nuclear Decay Test on Friday March 1

The Nucleus Protons and Neutrons are packed together tightly in the nucleus while Electrons swarm around the outside. The Nucleus is very small compared to the atom. – If the atom was a football field, the nucleus would be the size of a marble.

The Nucleus The Nucleus (Protons and Neutrons) is held together by the Strong Nuclear Force (AKA Strong Force) The strong force is an attractive force that is 100 times stronger than the electric force The strong force only acts over short distances. Since the nuclear force is so strong, there is lots of energy stored in the nucleus

Nuclear Size Bigger nuclei are held together less strongly than smaller nuclei. Therefore, bigger nuclei aren’t as stable as smaller nuclei.

Radioactivity Most of the time, the strong force is able to hold the nucleus together. If the nucleus is too big, the strong force can’t hold it together and the atom is unstable. Unstable atoms have a nucleus that decays and gives off matter and energy. The process of nuclear decay is called radioactivity.

Radioactive Elements All atoms that have an atomic number bigger than 83 are radioactive In other words, if a nucleus has more than 83 protons in it, the strong force cannot hold the nucleus together and it will decay. Elements that have more than 92 protons don’t exist naturally on earth. They are called synthetic elements and decay soon after being produced in a lab.

Isotope An atom is an isotope if the number of protons and the number of neutrons are different. Some elements have naturally occurring isotopes. (Example Carbon 12, Carbon 13, and Carbon 14)

Nuclear Ratios To determine the ratio of neutrons to protons: Divide the number of neutrons by the number of protons. For a normal atom (non-isotope), the ratio will always be 1:1

Radioactive Isotopes Isotopes are stable when the ratio of neutrons to protons is 3:2 If the ratio is much different from this, the isotope will be radioactive. In other words, nuclei with too few or too many neutrons compared to the protons is radioactive.

Nuclear Numbers A nucleus can be described by the number of protons and neutrons that it contains. The number of protons is called the atomic number The number of protons plus neutrons is called the mass number.

Nuclear Symbols Mass Number Atomic Number *To calculate the number of neutrons: subtract the atomic number from the mass number C

Discovery In 1896 Henri Becquerel left uranium on a film, later he saw an outline of the uranium. Two years later, Marie and Pierre Curie discovered two new elements (Polonium and Radium) that were radioactive. They developed a process to extract the radioactive elements from rocks.

Nuclear Radiation When a nucleus decays, particles and energy are emitted from it. These particles are called nuclear radiation. – Nuclear radiation consists of alpha, beta, and gamma particles.

Alpha Particles Made of two protons and two neutrons. Has a charge of +2 and a mass of 4. Leave charged ions behind when they travel through matter Because alpha particles have the biggest mass and the biggest charge of all radiation, they lose energy very quickly. They are the least penetrating of the radiation particles and can be stopped by a single sheet of paper.

Beta Particles When a neutron decays into a proton and emits one electron Has a charge of -1 and a mass of Caused by the weak force Much faster than alpha particles and more penetrating. They can be stopped by a sheet of aluminum foil.

Gamma Rays Electromagnetic Waves No charge and no mass Most penetrating form of nuclear radiation Travel at the speed of light Least damaging radiation Can be stopped by lead or concrete

Transmutation The process of changing from one element to another through nuclear decay. In Alpha radiation, an atom loses two protons. Its atomic number decreases by two. In Beta radiation, at atom gains a proton. Its atomic number increases by one.

Half Life A measure of the time it takes for half of a radioactive isotope to decay. The nucleus left after the isotope decays is called the daughter nucleus.

Radioactive Dating If we know the half life of an element and what percent of the sample remains unchanged (still radioactive) we can tell how long the sample has been there. Carbon-14 is used for living or once living things Uranium is used for rocks and non-living things.

Radiation Detectors Radiation can be detected with – Cloud Chambers – Bubble Chambers – Electroscopes These devices only detect if radiation is present, they don’t measure the amount. They detect ions formed when radiation passes through matter.

Cloud Chambers A sample is placed into a chamber filled with vapor. If the particle is radioactive, the radiation given off travels through the vapor and leaves a visible ion trail.

Bubble Chamber A chamber is filled with superheated liquid. As the radioactive sample decays, the ion trail causes the liquid to boil.

Electroscope The leaves of an electroscope spread apart in the presence of radiation. They are used to detect charged particles in the air.

Measuring Radiation The amount of radiation present can be measured with a Geiger Counter. A GC produces an electric current in the presence of radiation. The more radiation present, the bigger the current.

Background Radiation Low level radiation emitted by naturally occurring radioactive isotopes in the earth’s rocks, soil and atmosphere. The largest source of radiation is Radon Gas (from the decay of Uranium)

Nuclear Reactions There are two types of nuclear reactions: – Fission – Fusion

Nuclear Fission The process of splitting a nucleus into several smaller nuclei. Smaller nuclei and energy is released.

Chain Reactions Series of repeated fission reactions Critical Mass – amount of material required so that each reaction produces another reaction.

Nuclear Fusion When two nuclei with low masses are combined into a bigger nucleus. More energy released than Fission. Only occurs when nuclei are close together. Occurs only at very high temperatures (like in the sun)

Nuclear Fusion in the Sun As the sun ages, the hydrogen nuclei are converted to helium. Eventually there will be no hydrogen left and the sun will burn out.

Nuclear Reactions in Medicine Radioactive isotopes called Tracers are used to find or keep track of molecules. Radiation can also be used to treat certain types of cancer.