5What holds nucleus together? The answer was proposed by Hideki Yukawa in 1935, he called it a STRONG NUCLEAR FORCE.It is exerted by the p+ and n0 on each otherSNF overwhelms the electrostatic charge (++)
6SNFThe SNF acts over very short distance, so larger nuclei the force does not reach far enoughIf the SNF is not enough, the electrostatic forces will dominate and the nucleus will break apart.Instability occurs when nucleus is too big
7Band of StabilityPredicts whether or not an atom’s nucleus will be stable.For atoms less than 20 p+, n0:p+ are 1:1n0/p+ ratio greater than 1.5/1 will be unstable
8Nuclear symbolsChapter 25 requires that you understand nuclear symbols and how to manipulate them.AXZ
9Nuclear equations & Laws Notice that in the equation, atomic mass (A) and atomic # (Z) are the same on both sides of the arrow.Law of conservation of Mass!
10Nuclear vs Chemical Reactions In a chemical reaction:Atoms are rearranged to form new substancesIn nuclear reactions:We are creating new elements!ALCHEMY!
11Radioactive DecayRadioactive – something is radioactive if the nucleus spontaneously decomposesWhen it does this a different nucleus is formedExtra particles are usually formed alsoExample:The symbol represents a beta (b) particle
12Types of Radioactive Decay Alpha (a) particle: An alpha particle is a Helium Nucleus.The nuclear symbol is:
13b particle productionThe mass of a beta particle is 0, and the atomic number is –1:So, beta particles change neutrons to protons.
14Where did the e- come from? A decaying neutron!!!
15Gamma Rays (g) A gamma ray is a high energy photon of light. Released along with other types of radiation
16PositronsA particle with the same mass as an e- but a positive charge:Positrons are anitmatter!Example:
17Electron capture A nucleus CAPTURES an inner electron Gamma rays are always produced.Explanation:NOTE that in electron capture the beta particle is always on the LEFT of the arrow!!
18Decay SeriesAn unstable nucleus undergoes a series of changes until it becomes a stable nucleus
19Changing one element into another Radioactive decay is when a nucleus decomposes and turns in to another nucleusBombarding nuclei with particles can form new nuclei also.
20Making new elementsScientists make new elements by bombarding existing nuclei to create elements with larger and larger atomic numbers.
21Transuranium elements & Radioactivity Transuranium elements are just elements # (anything after Uranium!)All elements after Bismuth (Z = 83) are radioactive
22Half Life ½ life: the time for ½ of a sample of nuclei to decay For U-238, it is 4.5 billion yearsPa-234 has a ½ life of 1.2 minutes!!
23Carbon Dating (Radioactive Dating) Based on the radioactivity of carbon-14, which decays through b-particle production.Carbon-14 is continuously made in the atmosphere when neutrons from space hit the nitrogen in the air.
24More on carbon dating:Living things always have the same amount of C-14 because they take it in through the air.As soon as a living thing dies it stops taking in carbon from the atmosphere. The C-14 decays.The ½ life of C-14 is 5730 years.If a piece of wood has ½ the C as a living tree, it is 5730 years old.What are some of the problems with this method?
25Fission and FusionCombining two light nuclei to make a heavier nucleus is called fusionSplitting a heavy nucleus to form two nuclei with smaller masses is called fission.
26Fission Releases a lot of energy! 26 million times that of burning methaneCan cause chain rxns
27Chain RxnsAt least one neutron must go on to split another nucleus
29Nuclear Fusion Produces more energy than fission! Takes very high temperatures to get protons close enough to “stick” to each otherWhy is “cold fusion” a hollywood fad?
30Effects of Radiation Different particles penetrate in different ways The energy they haveThe mass they haveHow easily they make ionsChemical propertiesThese things make radiation more or less harmful
31Detecting RadiationHigh energy particles from radioactive decay make ions when they travel through matter.The fast particles “knock” electrons off the atoms they hit.A geiger counter can detect these ions through the flow of electricity