Radioactivity Changes in the nucleus

Forces within the atom Strong Nuclear Force: a short-range force that opposes repulsion of protons & holds p+ & n◦ together in the nucleus 2. Weak Nuclear Force: responsible for radioactive decay 3. Electromagnetic Force: attraction b/w opposite charges of (+) nucleus and (-) electrons; keeps e- in orbit. 4. Gravity: the weakest, little or no effect

History 1896 – Henri Becquerel Discovers radioactivity in Uranium minerals 1898 – Marie Curie Coined the term “radioactivity” Worked with pitchblende which contained U, Po and Ra; isolated Radium Only person ever to win two Nobel prizes 1911 – Ernest Rutherford Used alpha particles emitted from Ra in Gold Foil Experiment

Peaceful uses for Radioactivity Radio dating Sterilization Smoke detectors Medical diagnostics & treatment Electric power Synthesis of new elements

Chemical Reactions vs Nuclear Reactions CHEMICAL CHANGES Atoms interact only through their outer electrons New compounds form but not new elements Obey conservation laws (mass & energy) Some E changes take place as chemical bonds break & reform

Chemical Reactions vs Nuclear Reactions NUCLEAR CHANGES Involve only the nucleus of the atom. Isotopes are transformed into different isotopes! Can only take place under high-energy conditions of extreme temperature & pressure: inside stars, particle accelerators & nuclear reactors. Convert a small amount of mass to a large amount of energy according to E = mc2. We see this energy as sunlight!

Nuclear Binding Energy & Mass Defect When a nucleus if formed from nucleons, some mass changes into energy according to Einstein’s equation: E=mc2. The E released = the nuclear binding energy: the difference between the mass of an atom and the sum of the masses of the p+, n0, & e-. This difference is called the mass defect.

Nuclear Stability Question: What makes a nucleus unstable? Answer: Not enough or too many neutrons! Rule of Thumb: Isotopes that are much heavier or lighter than the most common isotope are likely to be radioactive. Probability: We know they will decay, we know how long it will take but we do not know when!

Predicting Nuclear Stability ATOMIC NUMBER RATIO OF NEUTRONS TO PROTONS Smaller atoms Almost equal (1 : 1) Larger atoms Need more n◦ to be stable (1.5 : 1) Greater than 83 All: more stable if….. All unstable =radioactive even numbers of nucleons

Four Types of nuclear changes: Radioactive Decay Fission Fusion Artificial Transmutation

Radioactive Decay The spontaneous decay of the nucleus of an atom by emission of particles and/or radiation. Includes: Alpha decay Beta decay Electron capture Positron emission

Fission Nuclear rxns in which a heavy nucleus splits into smaller nuclei when bombarded with neutrons. More n◦ are produced → split other heavy nuclei = a chain reaction An uncontrolled chain rxn = a nuclear bomb A controlled chain rxn = a nuclear reactor. Control rods are used to absorb the n◦. Moderators are used to slow the n◦.

Fission

Fusion Nuclear rxns in which two or more small nuclei combine to form one larger nucleus, releasing large amts of Energy. Think sunlight! Some of the mass ( < 1%) of the combining particles is converted to electromagnetic radiation according to E=mc2. The resulting nuclei are more stable.

Fusion

http://www.youtube.com/watch?v=WbBDoNivBZQ

WARM UP Find at least FIVE new terms in your notes that you need to learn…. Yellow PTs are for classroom use – you can leave them on the desk…. But please do not write on them! Thx.

Artificial Transmutation: Nuclear Bombardment Has taught us about the structure of atoms. Three things can happen: 1. Nucleus shatters, see what comes out. 2. Nucleus becomes metastable & gives off E, used in medical diagnostics. 3. Transmutation occurs, a new element forms.

Transmutation A change in the identity of the nucleus as a result of a change in the number of its protons.

Penetrating ability of radioactive particles

Penetrating ability of radioactive particles

Isotope Notation Helium – 4 OR He-4 hyphen notation for this nuclide

Atomic Number: p+ number, identifies element Mass Number: total of nucleons, identifies the isotope (or nuclide) Nucleons: Protons and neutrons.

Warm Up: For Plutonium-239…. The 239 is a _______ . The number of p+, n°, & e- is _______ . The nuclear symbol is _______ . What do you notice about the number of neutrons in larger atoms? What is the name of the force that holds nucleons together in the nucleus? What is diff b/w fission and fusion? Which happens in stars? Which in nuclear reactor?

Writing Nuclear Equations Represents a nuclear reaction The mass numbers must be equal on both sides of the arrow. The atomic numbers must be equal on both sides of the arrow.

Writing Nuclear Equations Some examples of nuclear decay equations are:

Radioisotopes All elements have one or more isotopes that are unstable. These isotopes may be natural or artificial; artificial radioisotopes are produced in nuclear bombardment rxns.

Cosmic Rays Streams of ionizing radiation of extraterrestrial origin. Consist mostly of protons and alpha particles (90%) but include small amounts of other atomic nuclei like C, N, and O. Enter the atmosphere and collide with atomic nuclei producing secondary radiation.

Carbon-14 Forms in the upper atmosphere when N-14 is bombarded with cosmic rays. C-14 → CO2 → living things (through photosynthesis!) Scientists measure the remaining amt. of C-14 in organic matter. Used to date specimens 500 to 500,000 years old.

Other isotopes useful for Radiodating Name t1/2 Use Tritium 12.3 years wines U-238 4.5 x 109 years rocks

Half-Life The time it takes for one-half the nuclei present in a sample to decay. Because half the sample becomes stable with each half-life that passes, the decay is exponential. Remember, we can’t predict when a nucleus will decay but we can predict how long it will take.

Predicting Nuclear Stability Even and odd numbers of p+ & n◦ Stable Even numbers of p+ & n◦ Slightly less stable Either p+ or n◦ are uneven Unstable Both p+ & n◦ are uneven