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Atoms, Molecules, and LifeThe Importance of Water
Chapter 2
Atoms
• The basic structural unit of matter.
proton
neutron
electron
nucleus
electron shell
Hydrogen
Helium
Carbon
2
1 3
4
1H
Hydrogen
1.008
1 - Atomic Number = # of protons2 - Element3 - Symbol4 - Molecular Mass ~ # protons + # neutrons
6 CCarbon
12.012
1 3
4
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
12Mg
Magnesium
24.31
17Cl
Chlorine
34.45
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 fillNitrogen’s 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
7N
Nitrogen
14.01
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)
1H
Hydrogen
1.008
Covalent bonds are the most common bondfound in biologic systems.
Hydrogen Bonds
• The slightly attraction between atomsinvolved in polar covalent bonds fromdifferent molecules.
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++
- -
++
- -
++
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
substances.
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
breaking.
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.
Neutral
Basic
Acidic
14
0
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.