Ch+2+-+Atoms+and+Molecules

Ch+2+-+Atoms+and+Molecules - A toms and Molecules The...

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Atoms and Molecules: The Chemical Basis of Life Chapter 2 Elements are substances that cannot be broken down into simpler substances by chemical reaction Carbon, hydrogen, oxygen, and nitrogen are the most abundant elements in living things (about 96% of mass) Ca2+, phosphorus, K+ and Mg2+ in smaller quantities Element Functions Carbon backbone of organic molecules Hydrogen and oxygen components of water Nitrogen component of proteins and nucleic acids Atom An atom is defined as the smallest portion of an element that retains its chemical properties Nucleus protons (positive) neutrons (uncharged) Electrons encircle the nucleus Negative In an electrically neutral atom – the # of e- is equal to the # of protons Atomic Number and Mass Each atom is a particular element
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identified by number of protons ( atomic number ) A tomic mass sum of protons and neutrons Therefore: if the oxygen nucleus contains 8 protons and 8 neutrons: Oxygen has an atomic number of 8 Oxygen has an atomic mass of 16 AMU, or daltons AMU Atomic Mass Unit Mass of a single proton or neutron Most elements consist of atoms with different # of neutrons and therefore different masses Mass of an electron about 1/1800 amu Fig 2-2: Isotopes Unstable isotopes break down (decay) to stable isotopes: Emit radiation as beta particles Capture with autoradiography (dark grains on a photographic plate) Can be used for: Radiotherapy: interferes with cell division Diagnosis Uptake of 3H - thymidine during S-phase to measure actively synthesizing cells Fig 2-3 Autoradiography Orbitals
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Electrons move rapidly in electron orbitals Outside the nucleus Electron shell Electrons in orbitals are at the same principal energy level Electrons in shells far from nucleus have more energy than electrons in shells close to nucleus Valence Electrons Electron in the outer shell most energetic electrons Number and arrangement of an atom’s valence electrons determine its chemical properties Therefore - Chemical properties of an atom are determined by its highest-energy (valence)
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Ch+2+-+Atoms+and+Molecules - A toms and Molecules The...

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