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Chapter 2

Chapter 2: Small Molecules and the Chemistry of Life.docx

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CAS BI 108
Francis Monette

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BI108 Chapter 2 Notes: Small Molecules and the Chemistry of Life All matter is composed of atoms • 98% of mass of living organisms: C, H, O, P, N, S • Sodium/potassium-nerve function, calcium-biological signal, iodine-vital hormone, magnesium-chlorophyll • Atomic Weight: relative atomic mass; ratio of average mass per atom • Radioisotopes: unstable and spontaneously give off energy in the form of alpha, beta, or gamma radiation from the nucleus; radioactive decay transforms the original atom (release energy) Behavior of electrons determines chemical bonding and geometry • Orbital: region of space where electron is found at least 90% of the time o First Shell: innermost electron shell; s orbital o Second Shell: four orbitals (s orbital and three p orbitals); shape of a sphere o Additional Shells: elements with more than 10 electrons have three or more electron shells • Valence Shell: outermost shell; determines how the atom combines with other atoms o Reactive atoms can attain stability either by sharing electrons with other atoms or by losing or gaining more electrons Bonding/Molecules • Chemical Bond: an attractive force that links two atoms together in a molecule • Covalent Bond: two atoms attain stable electron numbers in their outermost shells by sharing pairs of electrons; strong—takes a lot of energy to break • Unequal sharing of electrons o The attractive force that an atomic nucleus exerts on electrons in a covalent bond is called electronegativity; depends on how many positive charges it has and distance between nucleus and electrons in valence shell o Nonpolar Covalent Bond: two atoms close to each other in electronegativity, share equally o Polar Covalent Bond: when electrons are drawn to one nucleus more than the other; one atom more electronegative than the other o Polar Bond: opposite charges are separated at two ends (poles) • Compound: pure substance made up of two or more different elements bonded together in a fixed ratio Ionic attractions form by electrical attraction • Difference in electronegativity results in a complete transfer of one or more electrons • Ions: electrically charged particles that form when atoms gain or lose one or more electrons o Cations: positively charged o Anions: negatively charged • Ionic Bonds: result of electrical attraction between ions; when dispersed in water distances become very large, and thus, the strength of attraction is reduced • Water is polar; biological solvent BI108 Chapter 2 Notes: Small Molecules and the Chemistry of Life Hydrogen bonds may form within or between molecules with polar covalent bonds • Weaker than most ionic attractions; formation is due to partial charges • May be many of them within a single molecule or between two molecules • Play important role in determining and maintaining 3D shapes of giant molecules (DNA/proteins) Hydrophobic interactions bring together nonpolar molecules Hydrophilic: water loving; polar Hydrophobic: water hating Hydrocarbons-nonpolar; aggregate with each other rather than with polar water molecules Van der Walls forces involve contacts between atoms • Interactions between nonpolar substances when atoms are in close proximity • Random variations in electron distribution create weak, temporary +/- attraction • The sum of many interactions over the span of a large nonpolar molecule can result in substantial attraction Chemical Reactions • Matter is never created nor destroyed • Energy: defined as the capacity to do work, but in chemical rxns it can be thought of as the capacity for c
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