Chemistry_Grade_10-12 (1).pdf

Nuclei could also have been formed by the fusion of 4

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nuclei could also have been formed by the fusion of 4 He and 3 H . 7.10.2 Age of Ions ( 10 3 s - 10 13 s) However, at this time the universe was still very hot and the electrons still had too much energy to become bound to the alpha particles to form helium atoms . Also, the nuclei with mass numbers greater than 4 (i.e. greater than 4 He ) are very short-lived and would have decayed almost immediately after being formed. Therefore, the universe moved through a stage called the Age of Ions when it consisted of free positively charged H + ions and 4 He ions, and negatively charged electrons not yet bound into atoms. 7.10.3 Age of Atoms ( 10 13 s - 10 15 s) As the universe expanded further, it cooled down until the electrons were able to bind to the hydrogen and helium nuclei to form hydrogen and helium atoms. Earlier, during the Age of Ions, both the hydrogen and helium ions were positively charged which meant that they repelled each other (electrostatically). During the Age of Atoms, the hydrogen and helium along with the electrons, were in the form of atoms which are electrically neutral and so they no longer repelled each other and instead pulled together under gravity to form clouds of gas, which evetually formed stars. 7.10.4 Age of Stars and Galaxies (the universe today) Inside the core of stars, the densities and temperatures are high enough for fusion reactions to occur. Most of the heavier nuclei that exist today were formed inside stars from thermonu- clear reactions! (It’s interesting to think that the atoms that we are made of were actually manufactured inside stars!). Since stars are mostly composed of hydrogen, the first stage of thermonuclear reactions inside stars involves hydrogen and is called hydrogen burning . The process has three steps and results in four hydrogen atoms being formed into a helium atom with (among other things) two photons (light!) being released. The next stage is helium burning which results in the formation of carbon. All these reactions release a large amount of energy and heat the star which causes heavier and heavier nuclei to fuse into nuclei with higher and higher atomic numbers. The process stops with the formation of 56 Fe , which is the most strongly bound nucleus. To make heavier nuclei, even higher energies are needed than is possible inside normal stars. These nuclei are most likely formed when huge amounts of energy are released, for example when stars explode (an exploding star is called a supernova ). This is also how all the nuclei formed inside stars get ”recycled” in the universe to become part of new stars and planets. 7.11 Summary Nuclear physics is the branch of physics that deals with the nucleus of an atom. 122
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CHAPTER 7. ATOMIC NUCLEI - GRADE 11 7.11 There are two forces between the particles of the nucleus. The strong nuclear force is an attractive force between the neutrons and the electromagnetic force is the repulsive force between like-charged protons.
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