The Atomic Nucleus Remember Rutherford discovered the nucleus

The Atomic Nucleus

Remember… Rutherford discovered the nucleus using the gold foil experiments J. J. Thomson discovered the electron using cathode ray experiments The cathode ray led to the discovery of radioactivity

Discovery of Radioactivity Wilhelm Roentgen – 1896: German physicist discovered a “new kind of ray” “New rays” are NOT deflected and can pass through opaque materials Called these X-rays

Strong Nuclear Force Attractive force that acts between protons and neutrons; very strong but only at extremely short distances Atoms with large nuclei are not as stable as an atom with a very small nucleus Large atoms are more likely to fall apart or emit particles/energy All nuclei with more than 83 protons are radioactive

Radioactivity Radioactive Decay: when the nucleus of an unstable isotope emits particles, (releases energy) in order to become stable After decay, the element changes into a different isotope of the same element, or a completely different element Nuclear Radiation: The matter, or energy, that is emitted, during radioactive decay Nuclear atom radiation causes changes in the nucleus of an

Radioactive Rays Three types of nuclear radiation – Alpha, Beta, and Gamma Rutherford Rays discovered Alpha Rays and Beta

Alpha Particles Alpha Rays: positively charged particles he called alpha particles Consist of two protons and two neutrons that are ejected from the atom Do not easily penetrate solid material Relatively High large in size kinetic energy which can cause damage to the surface of a material

Beta Particles Beta Rays: identical to cathode rays; simply another name for an electron being ejected from a nucleus Faster than alpha particles Able to penetrate light materials such as paper and clothing Can penetrate deeply into the skin

Gamma Rays Gamma Rays: discovered by other scientists; carry no electric charge and no mass—they are an extremely energetic form of nonvisible light Pure energy Greater energy in a gamma ray than there is in visible light More harmful to us than either alpha or beta particles because they can cause structural damage to our cells

Transmutation The changing of one element to another When a radioactive nucleus emits an alpha or beta particle, the identity of the nucleus changes because the atomic number has changed

Transmutation If an element ejects an alpha particle result is a new atom with a mass number decreased by 4 and an atomic number decreased by 2 2 spaces back on periodic table because 2 fewer protons If an element ejects a beta particle result is a new atom with no change in mass number but atomic number increased by 1 1 space forward on periodic table because 1 more proton

Beta Decay

Radioactivity is Natural You are exposed to radiation every day, but small amounts are not really harmful. We have adapted to survive natural and man made sources of radiation sun, rocks, soil, computers, smoke detectors, x-rays Exposure varies from one location to another Places higher in elevation and lots of rocks get more radiation

Units of Radiation is measured in rems, lethal doses begin at 500 rems The radiation we receive from the environment is only a fraction of 1 rem Typically Radon exposed to 360 millirems per year is the leading source of natural radiation (seeps through cracks in the ground—why we check our basements!)

Medical Applications Radioisotopes and radiation have many applications in the field of medicine Radioactive tracers are used to diagnose diseases X-Rays, mammograms, CAT scans (all methods of seeing inside the body). Radiotherapy During (radiation for chemo) radiation therapy, a patient may receive doses of 200 rems each day for a period of weeks

Half-Life Time it takes for half the atoms in a sample of radioactive material to break down by radioactive decay Every time a half-life passes, half of what you started with has decayed Example: radium-226 has a half-life of 1, 620 years; half of a sample of radium will decay to other elements after 1, 620 years! The shorter the half-life, the more radioactive the substance

Half-Life Half-life is exponential – as the number of parent isotope atoms decreases, the number of daughter isotopes increases

Isotopic Dating Carbon-14 is the most common isotope of carbon (6 protons and 8 neutrons) Carbon-14 is radioactive and can form carbon dioxide, which is used by plants for photosynthesis all plants have a small amount of radioactive carbon-14, and all plants are consumed in some way, therefore all living things contain a small amount of radioactive carbon-14

Carbon-14 Dating Carbon-14’s half-life is 5, 730 years If a plant has 20 atoms of carbon-14, it will have 10 atoms of carbon-14 in 5, 730 years! We can calculate the age of carbon-containing things by measuring their amount of radioactive carbon-14 We can predict the age with about 15 percent uncertainty If we say something is 500 years old, it could be between 425 (low) and 575 (high) years old

Nuclear Fission Strong nuclear force between neutrons and protons dominates the repulsive force between the protons When very large atoms are bombarded with a neutron, the nucleus splits into many fragments Nuclear Fission: the splitting of the nucleus Discovered by German scientists Otto Hahn and Fritz Strassmann

Uranium-235 Uranium has a mass number 235 and is one of the most common atoms to undergo fission When it is hit with a neutron, three neutrons are released Those three neutrons can cause the fission of more uranium atoms, releasing more neutrons, which causes more fission Known as a chain reaction—a self-sustaining reaction in which the products of one reaction cause more reaction events.

Uses of Nuclear Fission Energy released by the fission of one uranium-235 nucleus is about 7 million times the energy released by the explosion of one TNT molecules Energy produced by fission was introduced to us in the form of nuclear bombs About 20% of electrical energy in the U. S. is generated by nuclear fission reactors

Nuclear Fusion Nuclear Fusion: small nuclei fuse (or combine) together Nuclei must be travelling at extremely high speeds in order to combine High speeds correspond to high temperatures found within the sun—thermonuclear fusion ~657 million tons of hydrogen is fused to 653 million tons of helium each second

Use of Nuclear Fusion Thermonuclear bomb, or the hydrogen bomb— first detonated in 1952 Typical thermonuclear bomb made by the U. S. today could be about 1000 times more destructive than the atomic bomb that was detonated over Hiroshima at the end of WWII Potential positive uses of fusion could be the controlled releases of vast amounts of clean energy