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Life’s Chemical Basis
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Start With Atoms Atoms Fundamental building blocks of matter Nucleus Positively charged protons Uncharged neutrons (except for hydrogen) Electrons move around the nucleus Negatively charged
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electron nucleus
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Elements Element A pure substance consisting of atoms with the same number of protons (atomic number) Isotopes Atoms of the same element that differ in number of neutrons (atomic weight)
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Periodic Table of Elements
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Putting Radioisotopes to Use Radioisotopes are radioactive isotopes They are not stable Emit particles and energy as they decay spontaneously into other elements
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Why Electrons Matter Electrons occupy orbitals (volumes of space) around the nucleus Up to two electrons occupy each orbital Shell model represents orbital energy levels as successively larger circles, or shells Used to view an atom’s electron structure
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Shell Models
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Electron Interactions Atoms with unpaired electrons in their outermost shell tend to interact with other atoms They donate, accept, or share electrons to eliminate vacancies vacancy no vacancy
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Electrical Charge An atom with equal numbers of protons and electrons has no net charge Ions (positive or negative) Atoms that have gained or lost electrons
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Ion Formation
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What Happens When Atoms Interact? Common interactions in biological molecules: Ionic bond Covalent bond Hydrogen bond
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Ionic Bonds Strong association between a positive ion and a negative ion (attraction of opposite charges)
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Covalent Bonds Two atoms share a pair of electrons Nonpolar covalent bond Atoms share electrons equally Polar covalent bond Electrons are shared unequally One end slightly negative, other slightly positive Polar molecule has a separation of charge
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Covalent Bonds
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Hydrogen Bonds Form between a hydrogen atom and an electronegative atom Each with separate polar covalent bonds Are not chemical bonds Do not make atoms into molecules Individually weak Collectively stabilize structures of large molecules
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Hydrogen Bonds
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Water Molecules Water molecules are polar Form hydrogen bonds with other polar molecules Hydrophilic substances (water-loving) Hydrophobic substances (water-dreading)
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Liquid Water: Hydrogen Bonds
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Water’s Life-Giving Properties Polarity gives liquid water unique properties that make life possible: Resistance to temperature changes Internal cohesion Dissolves polar and ionic substances
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Water Temperature: From Ice to Evaporation
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Water’s Solvent Properties Solvents dissolve solutes (spheres of hydration)
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Water’s Cohesion
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Key Concepts: NO WATER, NO LIFE Life originated in water and is adapted to its properties Water has temperature-stabilizing effects, cohesion, and a capacity to act as a solvent for many other substances These properties make life possible on Earth
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Acids and Bases pH scale Indicates hydrogen ion (H + ) concentration of a solution Ranges from 0 (most acidic) to 14 (most basic or alkaline) At pH 7 (neutral) H + and OH – concentrations are equal
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A pH Scale
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Acids and Bases Acids donate H + in water More H + than OH - Bases accept H + in water More OH - than H +
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Salts Compounds that dissolve easily in water, and release ions other than H + and OH - Form when an acid interacts with a base Example: NaCl HCl + NaOH NaCl + H 2 O
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Buffer System A set of chemicals (a weak acid or base and its salt) that keeps the pH of a solution stable One donates ions, the other accepts them Example: bicarbonate (HCO 3 - ) OH - + H 2 CO 3 HCO 3 - + H 2 O HCO 3 - + H + H 2 CO 3
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