U1_2 basic chemistry read before class

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U1_2 basic chemistry read before class Powered By Docstoc
					    Chapter 2

•
  Chemical Elements and Compounds
• All organisms are composed of matter (anything that has mass
  and takes up space).

• Mass is a measure of the amount of matter an object contains and
  it remains the same regardless of location.

• Weight is a measurement of gravity’s pull on an object and varies
  with distance from the Earth’s center.

• There are 4 states of matter known to man:
    - liquids
    - gases
    - solids
    - plasma
Chemical Elements and Compounds
                                   • An element is a substance that
                                     cannot be broken down into other
                                     substances by chemical reactions
                                   • 25 of the 92 naturally occurring
                                     elements are essential to life
                                   • A compound is a pure substance
                                     of 2 or more elements combined

                                   Most common elements in living
                                   organisms:




Trace elements are elements required in small quantities for life
(they make up less than 0.01% of elements found in living things)
  Atomic structure determines the behavior
                of elements
• An atom is the smallest possible unit of matter
  that retains the physical and chemical properties
  of element.
• Atoms are made up of subatomic particles,
  namely neutrons, protons, and electrons. Each
  has a specific charge, location and mass.
  Neutron                 Proton                        Electron
    No Charge         +1 Charge (Positive)          -1 Charge (Negative)

Found in dense core   Found in dense core    Orbits around nucleus and held by
  called nucleus        called nucleus       electrostatic attraction to positively
                                                       charged nucleus
   1.009 Dalton          1.007 Dalton         Mass so small usually not used in
1 Atomic Mass Unit    1 Atomic Mass Unit                  atomic mass
 Atomic Number and Atomic Mass
Atoms of various elements differ in their number of subatomic particles.


  • Atomic number = # of             • Mass number = number of
    protons in an atom of a            protons and neutrons in atom
    particular element               • Written as superscript on left
  • All atoms of an element have       side of element’s symbol
                                           23
    same atomic number                        Na or 12 C
  • Written as a subscript to the    • # neutrons = atomic mass -
    left of the element’s              atomic number
    symbol:      11
                   Na or 6 C         • Number of neutrons can vary
  • Neutral atom- as follows, #        in element (isotope) but
    protons = # electrons              number of protons cannot
  Remember, it is the number of protons that determines the element.
        Electrons give the element its chemical properties.
  Atomic Number and Atomic Mass
             (cont)
                        # of protons     # of protons
 23                                                                  14
                              11               6
  Na                                                                     C
                        # of electrons   # of electrons
 11                                                                  6
                              11               6
                        # of neutrons    # of neutrons
                              12               8
                                         This atom of
                                         Carbon is an
                                         Isotope*.


 * Isotope = atom of element with same atomic number but different atomic
mass number because of different # of neutrons. Isotopes still react chemically
                         in same way as element.
    Energy Levels of Electrons
•   Electrons orbit around nucleus
•   Atoms are mostly empty space (if the nucleus is size of golf ball, atom
    diameter of 1 kilometer
•   When two atoms approach each other, the nuclei do not come close to each
    other. Only the electrons are directly involved in chemical reactions between
    atoms.
•   Electrons store potential energy because of their position and distance away
    from the nucleus--electrons with most potential energy are found in energy
    level farthest from nucleus and those with the lowest potential energy orbit in
    energy level closest to nucleus
•   Electrons can move from one energy level to another by gaining or losing
    energy equal to the amount of potential energy necessary to do so
                    Electron Orbitals
  • Orbital = 3D space where an electron will most likely be found 90% of
    time but can never know the exact path of an electron within that space
    or probability cloud.
  • No more than 2 electrons can ever occupy the same orbital.
  • First energy level has one spherical s orbital (1s orbital) holds
    maximum of two electrons




Second energy level holds a maximum of 8 electrons, 2 electrons each in one of
                                  its 4 orbitals
  The orbitals consist of a spherical orbital called 2s (similar to 1s but larger
         diameter) and 3 dumbbell shaped p orbitals (2px, 2py, 2pz)
            Electron Configuration
• Electron Configuration = distribution of electrons in atom’s electron
  shells
• Atom’s electron configuration determines its chemical behavior
• Electrons must first occupy lower shells before the higher shells can be
  filled
• Electrons are added to each of the p orbitals singly before they can be
  paired in the orbital
• If an atom does not have enough electrons to fill the shells, the outer
  shell will be the only energy shell partially full


                          1s (2e-)
                          2s (2e-)
                          2px (2e-)
                          2py (1e-)
                          2pz (1e-)
         Valence and the Octet Rule
•   Valence electrons = electrons in the outermost energy shell, or valence
    shell
•   Chemical properties of atom depend on the number of valence electrons
•   Octet rule = rule that a valence shell is complete when it contains eight
    electrons.




•   An atom with a complete valence shell (8 electrons) is unreactive (inert).
    However, atoms with incomplete valence shell is chemically reactive
    (forms chemical bonds) until it has 8 valence electrons in its valence shell.
    Number of chemical bonds depends on how many electrons it needs to
    fulfill octet rule
•   VALENCE ELECTRONS ARE RESPONSIBLE FOR AN ATOM’S
    BONDING CAPACITY
         Atoms Combine by Chemical
          Bonding: Covalent Bonds
                                       • Atoms with incomplete valence
                                         shells tend to fill shells by
                                         interacting (bonding) with other
                                         atoms
                                       • Covalent bonds = chemical
                                         bond between atoms formed by
                                         sharing pair of valence electrons
                                       • Strong chemical bond
                                       • Molecule = 2 or more atoms of
Each atom that can share valence         same element bound together
electrons has a bonding capacity
corresponding to the number of                       H2
electrons needed to fill the valence   • Compound = 2 or more atoms of
shell. This bonding capacity is          different elements bound together
called an atom’s valence.                           NaCl
      Nonpolar/Polar Covalent Bonds
• Electronegativity = atom’s ability to
  attract and hold electrons
• More electronegative atom is, the
  more strongly it attracts shared
  electrons towards itself
• Generally, the larger the atom the
  more electronegative it is              •Hydrogen’s electron is
                                          attracted   more   toward
• Nonpolar covalent bond = bond in        oxygen because of its
  which electrons shared equally (equal   electronegativity.
  time with both), both atoms equally
  electronegative                         • For most of the time,
• Polar covalent bond = bond which        oxygen will have a negative
  electron unequally shared, spending     charge (2 extra e-) and each
  more       time      around    higher   hydrogen a postive (1 less
  electronegative atom                    e- each) charge
           Hydrogen Bonds (weak)
• Compared to covalent bonds, hydrogen bonds are weak bonds (20
  times weaker than covalent)
• Play an important role in chemistry and life: causes water tension as
  well as help shape proteins and DNA structure
• Formed by the charge attraction of the + charged hydrogen atom of a
  polar bond to the - charged atom of another electronegative atom found
  in a different polar bond (attraction between oppositely charged
  portions of polar molecules)


                       Hydrogen Bond
                                              N
          O       H                                  H

                                              H               Postive Polar
          H
                                                              Electronegative
                Hydrogen Bonds (cont)
  •Hydrogen bonds are very fragile
  •They form, break, and reform with great frequency
  •Each hydrogen bond lasts 1/trillionth of a second but the molecules are
  constantly forming new bonds with a succession of partners
  •Hydrogen bonds hold substances together, a phenomenon called cohesion
           Van der Waals Interactions
Even a molecule with nonpolar covalent bonds may have positively and negatively
charged regions. Because e- are in constant motion, they are not always
symmetrically distributed in the molecule. At any moment, e- may accumalate in
one part of the molecule or another creating “hot spots” of positive and negative
charge and atoms and other molecules could adhere to them. This is a Van der
Waal Interaction.
Van der Waal interactions are very weak (more so than a hydrogen bond) and only
occur between molecules in very close proximity.
                       Bond, Ionic Bond
In some cases, atoms are so unequal in      • Ion = charged atom or molecule
their attraction for atoms that one atoms
                                            • Cation = positively charged ion
strips the other completely of its
electron, giving each atom a permanent        because lost electrons
charge!                                     • Anion = negatively charged ion
                                              because gained electrons
                                            • Ionic bond = bond formed by
                                              electrostatic attraction between
                                              anion and cation (one atom
                                              gives an electron to another,
                                              making one an anion and the
                                              other a cation)
                                            • Anion        molecule       more
                                              electronegative than donor atom
The chlorine has a high affinity for        • Ionic bonds are salts and are
electrons, and the sodium has a low           strong in crystal form but
ionization potential. Thus the chlorine       fragile when in water
gains an electron from the sodium atom.
     A molecule’s biological function is
            related to its shape
A molecule has a characteristic size and shape. The precise shape of a molecule is
usually very important to its function in the living cell.


                                        They found that the human brain's
                                        neurons had specific receptor sites for
                                        opiate drugs: opium, heroin, codeine and
                                        morphine. But then there was the
                                        obvious question.
                                        Why would nature put in our brains a
                                        receptor for a plant? After all, humans
                                        beings didn't evolve over millions of
                                        years eating opium or shooting heroin.
A molecule’s biological function is
   related to its shape (cont)

                The scientists reasoned there must be
                some other function for these receptors
                sites. They soon figured out that the
                active ingredient in all these opiates -
                morphine - had a chemical structure
                similar to endorphins, a class of
                chemicals present in the brain .
                Endorphins are feel-good chemicals
                naturally-manufactured in the brain when
                the body experiences pain or stress. They
                are called the natural opiates of the body.
A molecule’s biological function is
   related to its shape (cont)
             Endorphins flood the space between nerve cells
             and usually inhibit neurons from firing, thus
             creating an analgesic effect. On a lower level they
             can excite neurons as well. When endorphins do
             their work, the organism feels good, high, or
             euphoric, and feels relief from pain [analgesia].
             Logically, endorphin levels go up when a person
             exercises, goes into labor, or is stressed out.
             Although they seem to be triggered by stress,
             endorphins can do more than relieve pain, they
             actually      make        us        feel     good.

             Like an evil twin, the morphine molecule locks
             onto the endorphin-receptor sites on nerve
             endings in the brain and begins the succession of
             events that leads to euphoria or analgesia.
       Chemical Rxns Make/Break
                 Bonds
• Chemical reactions = process of making and breaking chemical bonds
  which leads to changes in the composition of matter
• Reactants              Products
• Matter is always conserved so all reactant atoms only rearranged to
  form products



  Because matter can not be destroyed, each side of the chemical
  equation must have the same number and types of atoms
  It must all add up
           CaCO           3
                              CaO + CO          2
     Reactants                           Products
  1 Calcium, 1 Carbon, 3 Oxygen       1 Calcium, 1 Carbon, 3 Oxygen

				
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posted:12/19/2010
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