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Do you remember?

• What are the two types of bonds?

• How do you predict which type will form?

• What will O and H form? Why?– covalent bonds between H and O.

– Because they are both nonmetals

– But they have a special type of covalent bond called…POLAR.

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Polarity of Water• A water molecule is a polar molecule with opposite ends of the

molecule with opposite charges.– oxygen is more electronegative so it pulls on the electrons more– So…the region around oxygen has a partial negative charge.– The region near the two hydrogen atoms has a partial positive charge.

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Hydrogen bonding

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– The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond.

– Each water molecule can form hydrogen bonds with up to four neighbors.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.1

OPPOSITES ATTRACT

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HYDROGEN BONDS• The hydrogen bonds

joining water molecules are weak, about 1/20th as strong as covalent bonds.

• They form, break, and reform with great frequency

BUT Like charges repel each other…

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H bonds give water some cool properties…

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Organisms Depend on Cohesion

• Cohesion among water molecules plays a key role in the transport of water against gravity in plants

• Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels.

Hydrogen bonds hold the substance together, a phenomenon called cohesion

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Surface tension

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Density of WaterThe density of water:

1. Prevents water from freezing from the bottom up.

2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.

3. Makes transition between season less abrupt.

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Ice, water, vapor

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Hydrogen bonding (electric attraction)

Ice Liquid

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Are you afraid of water?

• What do you think the following two words mean?

– Hydrophilic– Hydrophobic

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“Like Dissolves Like”

• Polar Solutions (like water) are able to dissolve other polar molecules as well as ionic compounds. Why?

• Likewise, nonpolar solutions (like oil) will not dissolve polar and ionic compounds and will instead dissolve NONPOLAR molecules.

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So what about oil and water?

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Solvent for Life• Solution

– Solute– Solvent

• Hydrophilic– Ionic compounds

dissolve in water– Polar molecules

(generally) are water soluble

• Hydrophobic– Nonpolar compounds

There are three types of bonds you must know:

1. Ionic

2. Nonpolar Covalent

3. Polar Covalent

How do you know which type will form???

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There are three types of bonds you must know:

1. Ionic1. Metal with a nonmetal

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There are three types of bonds you must know:

Nonpolar Covalent:

-Nonmetal with a nonmetal

-If two of the same element

-or two elements that are NOTNOT Nitrogen, Oxygen or Fluorine.

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There are three types of bonds you must know:

Polar Covalent:

-Nonmetal with a nonmetal

-If Nitrogen, Oxygen or Fluorine are one of the

elements, they are VERY electronegative and

will hog the electrons…

Therefore NOF’s create

POLAR COVALENT

BONDS!

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pH

pH = – log [H+]

Log scale means 10X change per unit!

[H+]= 10-1M

[H+]= 10-9 M

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pH Scale• Measures the degree of acidity (0 – 14)

• Most biologic fluids are in the pH range from 6 – 8

• Each pH unit represents a tenfold difference (scale is logarithmic)– A small change in pH actually indicates a

substantial change in H+ and OH- concentrations.

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Buffers• A substance that eliminates large sudden changes

in pH.• Buffers help organisms maintain the pH of body

fluids within the narrow range necessary for life. – Are combinations of H+ acceptors and donors

forms in a solution of weak acids or bases– Work by accepting H+ from solutions when

they are in excess and by donating H+ when they have been depleted.

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Acid Precipitation• Rain, snow or fog with more strongly acidic than pH of

5.6• West Virginia has recorded 1.5• East Tennessee reported 4.2 in 2000• Occurs when sulfur oxides and nitrogen oxides react

with water in the atmosphere– Lowers pH of soil which affects mineral solubility

– decline of forests– Lower pH of lakes and ponds – In the Western

Adirondack Mountains, there are lakes with a pH <5 that have no fish.

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How would carbon bond with H?

• CH4

• How would it look?

• CH4 contains nonpolar covalent bonds because although C and H are different elements, they pull on the electrons equally.