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NATS 1760- ATOMS & NUCLEI .docx
NATS 1760- ATOMS & NUCLEI .docx

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School
York University
Department
Natural Science
Course
NATS 1760
Professor
Darrin Durant
Semester
Winter

Description
NATS 1760 DARRIN DURANT/JAMES ELWICK Monday, January 28, 2013 ATOMS & NUCLEI (pg. 195-204) Richard Wolfson  Nuclear processes release over a million times as much energy as the more familiar happenings of our everyday world  All things on Earth and throughout most of the known universe are made from combinations of just 3 simple building blocks: 1) Neutron 2) Proton 3) Electron NEUTRONS & PROTONS SIMILARITIES DIFFERENCES - Would take 13 trillion of them, lined up, - Neutrons carry no electric charge to span an inch - Protons carry one unit of positive electric - Same mass (weight) charge  a pound would contain 270 trillion trillion particles  A particle with charge has either one unit of positive charge or one unit of negative charge  Together, neutrons and protons = nucleons  Electron would take about 2,000 electrons to equal the mass of a neutron or proton  electron carries one unit of negative electric charge  Nature provides what appear as three fundamental ways called forces by which pieces of matter can interact and stick together 1) Gravitational Force  Keeps you rooted to the Earth, makes you fall, and hold the Moon in its orbit around the Earth and the Earth in its orbit around the Sun o BUT gravity is the weakest of the forces and is significant only for larger objects 2) Electric Force  A simple interaction between electrically charged particles o Two particles with the same charge are repelled from each other by the electric force, while two particles with opposite charge are attracted o The strength of the attractive or repulsive force depends on the distance between the particles; if they are farther apart the force is weakened  Long-range force 3) Nuclear Force  Only acts between nucleons; between protons and protons, neutrons and neutrons, or between protons and neutrons  Short- range force  An atomic nucleus is simply a group of nucleons; neutrons and protons bound together by the nuclear force  A single proton is known as a hydrogen nucleus  Combination of a proton and a neutron is known as a deuterium nucleus  A combination of one proton and two neutrons is known as a tritium  Helium-3; formed of two protons and one neutron  Helium-4; containing two protons and two neutrons  All nuclei are positively charged meaning they attract negatively charged electrons  When a nucleus surrounds itself with a number of electrons equal to the number of protons it contains = an atom  Distance between nucleus and surrounding electrons is over 10,000 times the diameter of the nucleus  An element is a substance that behaves chemically in a unique and identifiable way and who‟s basic particle is a single atom  „Chemical behavior‟ means how they interact with other atoms, forming multitude of different substances that make up our world o The number of electrons it contains determines its chemical behavior  The number of protons in a nucleus is called the atomic number  Each element also has a unique one or two letter symbol (i.e. hydrogen= H)  The total number of nucleons- protons and neutrons is called the mass number of a nucleus  At the level of nuclear reactions, we must distinguish nuclei of the same element that have different numbers of neutrons and there different mass numbers  Isotopes  The range of possible nuclei is distinctly limited  Nuclei consists of protons and neutrons held together by the nuclear force- a strong but short-range force o But the electric force is also present, and acts to repel the protons in a nucleus  Particular isotopes are the only nuclei of helium, carbon and oxygen you can make and have stick together forever = stable nuclei vs. unstable RADIOACTIVITY: WHEN THINGS COME APART (pg. 205-214)  The coming apart process is called radioactive decay  Unstable nuclei or materials containing them are said to be radioactive  Some elements may have both stable and unstable isotopes; radioisotopes  The term radioactivity was coined by the physicist Marie Curie who won Nobel Prizes in both physics and chemistry for her pioneering work on radioactive decay  How does a radioactive nucleus come apart? 1) Alpha Decay  A nucleus spits out two protons and two neutrons bundled together as a helium-4 nucleus o Alpha particle o Need to rid themselves of excess protons 2) Beta Decay  A neutron turns itself into a proton and an electron o Beta particle leaves the nucleus with one more proton and one fewer neutron; but total number of nucleons remains the same, so the mass number is unchanged o A free neutron one that is not part of a nucleus will do so in less than an hour 3) Gamma Decay  a nucleus with excess energy is said to be excited , it may store this energy for a while= gamma ray o A gamma ray is a high- energy version of ordinary visible light, and is yet another manifestation of the electric force o Once the nucleus has shed its excess energy by the process of gamma decay  Each of the radioactive decays result in a modified nucleus and a much smaller entity  Alpha particle (helium -4 nucleus) o Relatively little penetrating power; can be stopped by a sheet of paper, a layer of clothing or an inch of hair  Beta particle (electron) o Much lighter than alpha particles and travel much faster; they can penetrate a fraction of an inch in solids and liquids (human body) and several feet in air  Gamma ray o Electrically neutral and therefore highly penetrating  The energetic particles emitted in radioactive decay= nuclear radiation  Nuclear radiation can cause serious damage to atoms and molecules in its path due to high energy o Also the concern of exposing humans to radiation  A simple way to characterize the level of radioactivity is to state how many decays occur in a given time  #= Activity  Activity is measured in curies a unit named in honor of Marie Curie o One curie is a substantial level of radioactivity (i.e. consumer products, Three Mile Island nuclear accident, Detonation of a medium-sized nuclear weapon)  The Becquerel = 1 decay/second  The effect of radiation on an object depends on the amount of energy the radiation deposits in that object= rad; a unit that describes energy o Rem measures radiation dose in terms of its effect on humans o Rad and Rem replaced with the gray and the Sievert  If you start with 1,000 radioactive nuclei, you will find that after a certain time about 500 will have decayed to 250 nuclei etc. process will continue till all nuclei have decayed  Half-life o But how much time does it take?  Dependent upon duration of the half-life; excited states of some nuclei decay with half lives around a thousandth of a trillionth second whereas uranium-238 has a half-life of 4.5 billion years  Given equal quantities of different radioactive materials, those with the shorter half-life will therefore be more high radioactive  Where do radioactive materials come from?  Explosive deaths of stars were the events called supernovas; materials which became our solar system  The Earth & Sun formed about 5 billion years ago ; so radioactive nuclei that were incorporated into our planet have had plenty of time to decay  Why should we find radioactive isotopes with half-lives much less than the age of our planet? = Decay chain 1) Isotopes such as uranium-238 form other nuclei which become radioactive and whose half-lives aren‟t long  Uranium-238 decays by emitting an alpha particle giving rise to thorium- 234 (but decays by beta emission with a half life of 24 days) 2) Beta Decay turns a neu
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