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CHEMISTRY REVIEW CONTINUED

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CHEMISTRY REVIEW CONTINUED Powered By Docstoc
					STUDYING THE INTERACTIONS
OF MOLECULES
n   Recall that a molecule is composed of 2 or
    more atoms chemically joined together.
n   There are different types of bonds that can
    hold elements together.
n   Biological molecules are organic or carbon
    based where carbon is bonded to carbon or
    hydrogen and may also include atoms of
    nitrogen, oxygen, phosphorus, and sulfur
    VALENCE ELECTRONS:            Electrons in
    the outermost orbital that determine an
    atoms chemical behaviour.

n   Noble gases – have full outer orbitals/thus they
    do not gain, lose, or share electrons (i.e. don’t
    participate in chemical reactions.)
n   All other elements attempt to gain, lose, or
    share valence electrons to achieve stable
    electron configurations like noble gases.
n   HENCE chemical reactions occur and chemical
    bonds form.
CHEMICAL BONDING
(Intramolecular Bonding = Forces that hold atoms
together within a molecule.)
n   IONIC
n   transfer e-
n   electrostatic attraction between metal cation and non
    -metal anion
n   Form ionic solids
     n Ionic solids form crystals that can pull apart in
       water.
     n All ionic compounds are water soluble.
n   Recall – Ions = atoms with a charge.
Cation(metals) = when atoms lose e- they become positive.
Anion(non-metals) =when atoms gain e- they become negative.
Ionic Bond = attraction between a cation and an anion

Example:         Na à Na1+ + 1 e- (oxidation= LEO loss of electrons)
                 O + 2 e- à O2- (reduction = GER gain of electrons)
CHEMICAL BONDING
2.       Covalent Bonding
     n    Share electrons
     n    Two non-metal nuclei attract e- at the
          same time and thus they are shared.
     n    Form molecules
Electrons can be shared equally forming a non-polar
     covalent bond or unequally forming a polar covalent
     bond.
ELECTRONEGATIVITY
n   Electronegativity – the measure of the
    tendency of an atom to attract electrons.
n   See figure 7 on page 14 for table of electro-
    negativities
n   Describe the electronegativity of the
    biological atoms (C, H, O, N, S, P)
    n   O, N, and Cl are atoms with high
        electronegativities
    n   C, H, and P are examples of atoms with lower
        electronegativities
Polar Covalent Bonds
n   When 2 atoms with significantly different
    electronegativities share electrons those electrons are
    more strongly attracted to the atom with the higher
    electronegativity and cause that atom to have a
    partial negative charge.(δ- )
n   The atom with the lower electronegativity becomes a
    partial positive charge (δ+)
Non-polar covalent bonds
n   When bonds are formed between atoms that
    have similar electronegativities the electrons
    are shared fairly equally between the atoms.
n   Examples: diatomic molecules or the bonds
    between carbon and hydrogen atoms.
ELECTRONEGATIVITY AND
CHEMICAL BONDING
Electronegativity difference (∆En) rule:
NONPOLAR        POLAR COVALENT IONIC
0à 0.5        0.6 à 1.6            > 1.7
e- shared      e- shared            e-
equally       unequally          transferred
n   Polarity or charge of biological
    molecules greatly affects their behaviors
    and functions within a cell.
INTERMOLECULAR BONDING
n   Intermolecular bonding
       n   “between” molecules (attractive forces that allow
           molecules to associate together)
       n   weaker than intramolecular
       n   Determine the state of substances at given temperature
           and pressures.
       n   Collectively called vanderWaals forces.
       n   VanderWaal has “3 children” collectively known as:
            n   London’s Forces
            n   Hydrogen Bonding
            n   Dipole dipole interactions (hydrophobic interactions)
INTERMOLECULAR BONDING
     The two intermolecular interactions that are
     important for biological systems are:
1.   Hydrogen bonding – strong – partial positive
     (δ+) hydrogen atoms are attracted to the
     partially negative (δ-) N, O, or F of another
     atom. (H-F, O-H, N-H).
       This is extremely important in biological systems
       as water molecules form hydrogen bonds with
       one another. All cells are aqueous environments.
       So hydrogen bonds between biological molecules
       and water are very important!
INTERMOLECULAR BONDING
2.        Dipole-dipole Forces (Hydrophobic Interactions)
          weak – occurs between polar molecules (δ+
          side of one molecule is attracted to the δ- side
          of another molecule. Remember opposites
          attract!!). Thus polar molecules have a
          tendency to mix and interact! (Like dissolves
          like)
     1.       Since water is polar, all polar molecules are
             considered “water loving” or hydrophilic.
     2.      Non-polar molecules don’t mix with water and are
             called “water hating” or hydrophobic.
     3.      The natural clumping together of non-polar
             molecules is known as the hydrophobic effect. This
             plays a central role in how cell membranes form or
             the 3-dimensional shapes of proteins.
MOLECULAR POLARITY
Molecular polarity depends on
1. The polarity of individual bonds (which
   depends in part upon functional
   groups for biological molecules)
   and
2. the shape of the molecule (this
   depends upon the distribution of
   electron orbitals)
    ELECTRON ORBITALS
n   It is impossible to know
    exactly where electrons
    are at any given time
n   Scientists can determine
    locations where
    electrons are most likely
    to be found
n   These spaces are called
    orbitals. (s,p,d,f) see fig.4
    pg.11
Hybridization of orbitals
n   When atoms form
    covalent bonds the
    orbitals hybridize.
Hybridization of orbitals
Hybridization of orbitals
n   Hybridization pushes bonding and non-
    bonding electrons as far apart as
    possible
n   Each hybridized orbital will hold 2 e-
n   To simplify we will assume that
    biological molecules will only possess up
    to 4 electron pairs (4 sp3 hybrid orbitals)
Shape depends on the number of
bonding and non-bonding pairs of
electrons. (see table 5 on page 15)
See table 5 on page 15
n   If the molecule has a symmetrical shape
    (linear, trigonal planar, or tetrahedral) then if
    all bonds are the same (can be polar or non-
    polar) the molecule will overall be non-polar
n   If the molecule has an asymmetrical shape
    (bent, or pyramidal) and the bonds are polar
    then the molecule will be polar. However if
    the bonds are non-polar then the molecule
    will be non-polar
REMEMBER
 The polarity of molecules greatly affects
 their behavior and function within a cell.

				
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posted:4/30/2014
language:English
pages:22