Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies The Nature of Molecules Chapter 2 Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Atoms Electrons - Atoms with the same number of protons and electrons are electrically neutral.  Electrons are maintained in orbits by their attraction to the positively charged nucleus. - Ions - Atoms in which the number of electrons does not equal the number of protons.  Cation - Net positive charge.  Anion - Net negative charge.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Chemical Behavior of Atoms Moving an electron to a higher orbital requires an input of energy, while moving an electron closer to the nucleus releases energy.  Oxidation - Loss of an electron.  Reduction - Gain of an electron.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Ionic Bonds Ionic bonds form when atoms of opposite electrical charges attract each other.  Donation of an electron. +/- e −  Form crystals

Covalent Bonds Covalent bonds form when two atoms share two or more valence electrons.  Results in no net charge  Give rise to discrete molecules.  Form complex structures  Strength of covalent bond depends on # of shared electrons

Covalent Bonds Bond Strength  depends on the number of shared electrons. - Single Bond - Double Bond - Triple Bond More energy is required to break these bonds.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Chemical Reactions Chemical Reactions occur because of the formation and breaking of chemical bonds.  Reactants - Original molecules.  Products - Resultant molecules. 6H2O + 6CO2 → C6H12O6 + 6O2 reactants products Chemical reactions influenced by:  Temperature  Concentration of Reactants and Products  Catalysts

Chemistry of Water Water molecule consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies 1. Hydrogen Bonding – weak bonds; form weak assoc. between partially neg. O atoms and Partially Pos. H atoms of two water molecules Characteristics of Water:

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Characteristics of Water: 2. Polar Molecule - Has distinct ends, each with a partial charge; Polarity of water causes it to be attracted to other polar molecules.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Characteristics of Water: 3. Cohesion property - Attraction to other water molecules; responsible for surface tension.

Characteristics of Water: 4. Adhesion property - attraction to other substance; is adhesive to any substance with which it can form hydrogen bonds.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Charcteristics of Water Heat Storage - Because of the large number of hydrogen bonds, a large input of thermal energy is necessary to break apart water molecules. 5. High Specific Heat (extent to which a substance resists changing its temperature as it absorbs or loses heat); polarity 6. High Heat of Vaporization (energy required to change liquid water to gas); Sweating a great amount of energy (586 calories to chage 1g of liquid water into a gas.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Characteristics of Water 7. ice is less dense than liquid water because of the hydrogen bonds 8. Solvent - Water is an effective solvent because of its ability to form hydrogen bonds.  Water molecules gather around any substance bearing an electrical charge (ion or polar molecule) forming hydration shell.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Characteristics of Water: 9. Organizes Nonpolar Molecules - Water molecules act to exclude nonpolar molecules.  Hydrophobic - Nonpolar molecules shrink from contact with water. - (Water fearing)  Hydrophilic - Polar molecules readily form hydrogen bonds with water. - (Water Loving)

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Water Ionizes Ionization - Spontaneous ion formation. H 2 O OH - + H + pH Scale - Measures negative logarithm of hydrogen ion concentration in the solution. pH = -log [H + ] = -log(10 -[10] )  Acids - Substances that dissociate in water to increase concentration of H + ions. - pH values below 7.  Bases - Substances that dissociate in water to increase concentration of OH - ions. H + concentration is low. - pH values above 7.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Water Ionizes Buffer - A substance that acts as a reservoir for hydrogen atoms.  Donates hydrogen atoms to the solution when their concentration falls (pH is high), and removes them from the solution when their concentration rises (pH is low). - Minimize change in pH.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Buffers In organisms buffers consists of acid-base pairs In human blood the key buffer is carbonic acid (H 2 CO 3 ) and bicarbonate ion (HCO 3 - ) If an acid adds H+ ions to blood (pH low), the bicarbonate (HCO 3 - ) acts as a base and removes the excess H+ to form carbonic acid (H 2 CO 3 ) If a base removes H+ ions from blood (pH high), the carbonic acid (H 2 CO 3 ) dissociates, releasing more H+ into blood.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Copyright © McGraw-Hill Companies Permission required for reproduction or display The End