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  • Atoms, Molecules, and LifeThe Importance of Water

    Chapter 2


    The basic structural unit of matter.





    electron shell





    1 3





    1 - Atomic Number = # of protons2 - Element3 - Symbol4 - Molecular Mass ~ # protons + # neutrons

    6 CCarbon


    1 3


  • Properties of Elements Element: a substance that cannot be broken

    down nor converted to another substanceby ordinary chemical means.

    Isotope: atoms of an element that containdifferent numbers of neutrons 12C has 6 protons and 6 neutrons (normal) 14C has 6 protons and 8 neutrons (heavy carbon)

    Some isotopes are radioactive, they spontaneously break apart, releasing energy and forming new elements.

    Electrons: the key to bonding

    Negatively charged Repel each other Attracted to the positively-charged nucleus Can become excited

  • Excited Electrons:Disobeying Electron Shells

    Electrons orbit through different energy levels 1st shell: up to 2 electrons (e-) 2nd shell (and each larger shell): up to 8 e-

    Always fill up the lowest energy level first

    More stable when shells are full

    Free Radicals Atoms with one or more unpaired electron in

    their outer shell. Very unstable. Capture electrons from neighboring

    molecules, creating new free radicals. Can lead to cell death and a variety of

    diseases. Antioxidants react with free radicals, rendering them harmless to the body.

    Interacting Elements An atom whose outer electron shell is full

    cannot interact with other atoms and is calledinert.

    Atoms is reactive when its outer electron shellis only partially full and it can react with otheratoms.

    Molecules Two or more atoms of the same or different

    elements held together. Molecules formed from different elements

    are compounds. Atoms can form three main types of bonds:

    Ionic, Covalent, and Hydrogen bonds

  • Ionic Bonds

    An electron is transferred, creatingpositive and negative ions that attractone another.

    Sodium (11 p+, 11 n) Chlorine (17 p+, 18 n)

    Example: MgCl2

    MgCl2: Total # protons:

    12 + (17)*2 = 46

    Total # electrons: 12 + (17)*2 = 46

    Combined MolecularMass:

    24.31 + (34.45)*2 = 93.21 g/mol







    Chlorine: 17 protons 18 neutrons 17 electrons (7 e- in the outer shell)

    Magnesium: 12 protons 12 neutrons 12 electrons (2 e- in the outer shell)

    Ionic bonding, with one electron from Magnesiumdonated to each Chlorine atom

    Covalent Bonds Electron pairs are shared.

    Nonpolar covalent bonds: equal sharing. Two hydrogen atoms share electrons to form uncharged H2. Electrons spend the same amount of time

    around each nuclei.

    Covalent Bonds, continued

    Polar covalent bonds: unequal sharing. Electrons spend more time around one atom

    than the other.

    - -


    Oxygen is slightly negative

    Hydrogen is slightly positive

    Water, H2O

  • Example: NH3,Ammonia

    Polar, covalent bonding, sharingthe electrons of Hydrogen to fillNitrogens outer e- shell. NH3:

    Total # protons: 7 + (1)*3 = 10

    Total # electrons: 7 + (1)*3 = 10

    Combined MolecularMass:

    14.01 + (1.008)*3 = 17.034 g/mol




    Hydrogen: 1 proton 0 neutrons 1 electrons (1 e- in the outer shell)

    Nitrogen: 7 protons 7 neutrons 7 electrons (5 e- in the outer shell)




    Covalent bonds are the most common bondfound in biologic systems.

    Hydrogen Bonds

    The slightly attraction between atomsinvolved in polar covalent bonds fromdifferent molecules.

    - -


    - -


    - -


    Properties of Water

    Life likely began underwater. All living organisms contain 60-90% H2O. All life depends on water.

    (examples: Photosynthesis and Digestion)

  • Properties of Water

    1. Very good solvent. It is able to dissolve a wide range of


    Solvent: Water

    Solute: Salt

    Solution:Salt Water

    Properties of Water

    Because of itspolarity, waterattracts bothpositive andnegative ions,breaking ionicbonds.

    Properties of Water

    2. Hydrophobic and Hydrophilic qualities. Hydrophobic (water-fearing)

    molecules are non-polar anduncharged.Water cannot dissolve them.

    Lipids, Fats and Oils

    Hydrophilic (water-loving)molecules are polar.Water dissolves the covalent bonds.

    Sugars, Salts, and Amino Acids

    Properties of Water

    3. Water molecules tend to stick together.

    Because of their hydrogen bonds, watermolecules have high cohesion(stickiness).

    Cohesion of water molecules at thesurface creates surface tension. The tendency for the water surface to resist


  • Examples of SurfaceTension and Cohesion

    Properties of Water4. Water can be acidic, basic, or neutral.

    Water molecules are easily ionized:

    Pure water has equal concentrations ofhydroxide (OH-) and hydrogen (H+) ions.

    Acids Acidic solutions have a higher concentration of H+

    than OH-. Acids release hydrogen ions when dissolved in

    water. Many acidic solutions taste sour (vinegar, lemon

    juice, etc).

    Bases Basic solutions have a higher concentration of OH-. Bases combine with hydrogen ions, reducing their

    number. Many basic solutions taste bitter, and are very

    harmful if consumed (ammonia, bleach, etc).

    pH Scale (0-14)

    Measures the ratio of H+ to OH-. Neutrality (equal concentrations

    of H+ and OH-) is set to 7. Acids have a pH below 7. Bases have a pH above 7.







  • Buffers A complex molecule that helps maintain a

    solution at a constant pH. Can combine with H+ to increase pH. Releases H+ to combine with OH- when needed to decrease pH.

    Example: Carbonate Buffering of Blood

    CO32- HCO3- H2CO3 pH ~ 6 pH ~ 9.5

    Properties of Water

    H20 heats slowly Takes a lot of energy to break H-bonds

    H20 moderates high and low temperatures H20 freezes slowly

    A large amount of energy must be removed toform ice crystals

    Water moderates the effects oftemperature changes.

    Water forms an unusual solid:Water forms an unusual solid:IceIce

    Less dense than liquid water!Less dense than liquid water!

    LiquidLiquid SolidSolid

  • HomeworkChapter 2

    The lizard in the beginning of this lectureappears to be running on water.

    Aside from the speed at which the lizard runs,what properties of water allow the lizard tomove like this? How does this property work?(i.e. describe the arrangement of molecules that giveswater this property)

    Give two other examples where these propertiesof water are used in nature.


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