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Chemistry of Life Part I
Common Constituents and Bonds
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Covalent Bonds
• the sharing of a pair of valence electrons by two atoms
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LE 2-10
Hydrogenmolecule (H2)
Hydrogen atoms (2 H)
e- e-
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• 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
• Covalent bonds can form between atoms of the same element or atoms of different elements
• A molecule is two or more covalently bonded atoms
Strong bond
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LE 2-11b
Oxygen (O2)
Name(molecularformula)
Electron-shell
diagram
Structuralformula
Space-fillingmodel
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LE 2-11c
Water (H2O)
Name(molecularformula)
Electron-shell
diagram
Structuralformula
Space-fillingmodel
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• Electronegativity
– an atom’s attraction for the electrons in a covalent bond
Examples: Oxygen and nitrogen are highly electronegative
• The more electronegative an atom, the more strongly it pulls shared electrons toward itself
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• Nonpolar covalent bond– the atoms share the electron equally
-molecule has no charge (neither positive nor negative)
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LE 2-11b
Oxygen (O2)
Name(molecularformula)
Electron-shell
diagram
Structuralformula
Space-fillingmodel
Molecular Oxygen
Nonpolar covalent moleculeNeutral
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• Polar covalent bond– one atom is more electronegative
– the atoms do not share the electron equally
– Partial negative and positive charges
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LE 2-12
H
O
H
H2O+ +
–
H2O: Polar covalent molecule
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Ionic Bonds
• Formed by the transfer of electrons from one atom to another
• After transfer, both atoms charged
• A charged atom (or molecule) is called an ion
• Weaker than covalent bond
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• Anion – negatively charged ion
• Cation – positively charged ion
• Ionic bond – attraction between an anion and a cation
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LE 2-13
Na
Sodium atom(an uncharged
atom)
Cl
Chlorine atom(an uncharged
atom)
Na+
Sodium ion(a cation)
Cl–
Chlorine ion(an anion)
Sodium chloride (NaCl)
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• Ionically bonded atoms– ionic compounds, or salts e.g. NaCl
• often crystals
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LE 2-14
Na+
Cl–
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Hydrogen Bonds
• when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom
• Example: water (H2O)
• Weak, but many together are strong
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LE 2-15
–
Water(H2O)
Ammonia(NH3)
Hydrogen bond
+
+
–
+
+
+
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Van der Waals Interactions
• Attraction between adjacent atoms by fleeting charge differences
• Very weak
• Collectively, can be strong, as between molecules of a gecko’s toe hairs and a wall surface
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Autumn K, Sitti M, Liang YA, Peattie AM, Hansen WR, Sponberg S, Kenny TW, Fearing R, Israelachvili JN, Full RJ. 2002. Evidence for van der Waals adhesion in gecko setae. Proc Natl Acad Sci USA: 99(19):12252-6.
Department of Biology, Lewis & Clark College, Portland, OR 97219, USA. [email protected]
Kellar Autumn
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Order of Relative Bond Strength
Covalent >ionic> hydrogen> Van der Waals
In biological systems, often many weak bonds collectively are strong and help stabilize structures.
Example: DNA double helix: held together through H-bonds
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Molecular Structure of Molecules is Crucial
For example
Between hormone and hormone receptor
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LE 2-17a
Naturalendorphin
Morphine
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygen
Structures of endorphin and morphine
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LE 2-17b
Naturalendorphin Morphine
Brain cell
Endorphinreceptors
Binding to endorphin receptors