lecture 2 biao ding - Biol 113 AU2010 Lecture 2 The Chemical Context of Life A B C D Chemical bonds The simple chemistry of life Water The molecule that

1 Biol 113 AU2010 Lecture 2 The Chemical Context of Life A. Chemical bonds - The simple chemistry of life B. Water - The molecule that supports all of life C. Carbon - The backbone of life D. Functional groups - The chemistry that runs life • Valence electrons are those in the outermost shell, or valence shell • Elements with a full valence shell are chemically inert The chemical behavior of an atom is mostly determined by the valence electrons Fig. 2.8 (a) A ball bouncing down a flight of stairs provides an analogy for energy levels of electrons Third shell (highest energy level) Second shell (higher energy level) Energy absorbed First shell (lowest energy level) Atomic nucleus (b) Energy lost Electron Configuration and Chemical Properties • The chemical behavior of an atom is defined by its electron configuration and distribution • The periodic table of the elements shows the electron distribution for all the elements Fig. 2.9 Hydrogen 1 H Lithium 3 Li Beryllium 4 Be Boron 5 B Carbon 6 C Nitrogen 7 N Oxygen 8 O Fluorine 9 F Neon 10 Ne Helium 2 He Atomic number Element symbol Electron- distribution diagram Atomic mass 2 He 4.00 First shell Second shell Third shell Sodium 11 Na Magnesium 12 Mg Aluminum 13 Al Silicon 14 Si Phosphorus 15 P Sulfur 16 S Chlorine 17 Cl Argon 18 Ar Strong bonds: - Covalent bonds Weak bonds: - Ionic bonds - Hydrogen bonds - Van der Waals Interactions • Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms • These interactions usually result in atoms staying close together, held by attractions called chemical bonds A. Chemical Bonds - The Simple Chemistry of Life Covalent Bonds • A covalent bond is the sharing of a pair of valence electrons by two atoms • In a covalent bond, the shared electrons count as part of each atom’s valence shell Fig. 2.11 Hydrogen atoms (2 H) Hydrogen molecule (H 2 )

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2 • A molecule consists of two or more atoms held together by covalent bonds • A single covalent bond, or single bond , is the sharing of one pair of valence electrons • A double covalent bond, or double bond , is the sharing of two pairs of valence electrons Fig. 2.12 Name and Molecular Formula Electron- distribution Diagram Lewis Dot Structure and Structural Formula Space- fil ing Model (a) Hydrogen (H 2 ) (b) Oxygen (O 2 ) (c) Water (H 2 O) (d) Methane (CH 4 ) Covalent Bonds Nonpolar and polar covalent bonds • In a nonpolar covalent bond , the atoms share the electron equally • In a polar covalent bond , one atom is more electronegative, and the atoms do not share the electron equally • Unequal sharing of electrons causes a partial positive or negative charge for each atom or molecule • Electronegativity is an atom’s attraction for the electrons in a covalent bond • The more electronegative an atom, the more strongly it pulls