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 Any substance that has mass and occupies space is known as matter.  Composed of atoms. - Orbiting cloud of electrons. - Dense nucleus formed of protons (+) and neutrons (neutral).  Number of protons determines chemical character because it dictates number of electrons available for chemical activity.

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

4 Atomic Number & Atomic Mass Atoms of the various elements Differ in their number of subatomic particles The number of protons in the nucleus = atomic number The sum of the masses of protons + neutrons = atomic mass Neutral atoms have equal numbers of protons & electrons (+ and – charges)

Fig. 2-9 Hydrogen 1 H Lithium 3 Li Beryllium 4 Be Boron 5 B Carbon 6 C Nitrogen 7 N Oxygen 8 O Fluorine 9 F Neon 10 Ne Helium 2 He Atomic number Element symbol Electron- distribution diagram Atomic mass 2 He 4.00 First shell Second shell Third shell Sodium 11 Na Magnesium 12 Mg Aluminum 13 Al Silicon 14 Si Phosphorus 15 P Sulfur 16 S Chlorine 17 Cl Argon 18 Ar

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Atoms Atomic Mass  Measured in daltons. - Protons and neutrons both weigh approximately 1 dalton, while electrons weigh 1/1840 of a dalton million million billion daltons (6.02 X 10 to the 23 rd power) = 1 gram!!!

7 Energy  Is defined as the capacity to do work causing change Potential energy - Is the energy that matter possesses because of its location or structure (the electrons) Kinetic Energy - Is the energy of motion

8 Energy Levels of Electrons An atom’s electrons vary in the amount of energy they possess Electrons further from the nucleus have more energy Electron’s can absorb energy and become “excited” Excited electrons gain energy and move to higher energy levels or lose energy and move to lower levels

9 Isotopes Different forms of the same element Have the same number of protons, but different number of neutrons Most elements is nature exist as a mixture of different isotopes. May be radioactive spontaneously giving off particles and energy May be used to date fossils, in medicine, etc. (Carbon-14, radiation therapy, etc.)

10 Example of Isotope Use  Can be used in medicine to treat tumors Cancerous throat tissue Figure 2.6

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 Orbital - Area around a nucleus where an electron is most likely to be found. Because electrons are attracted to the nucleus, it takes energy to maintain them in orbitals.  Contain potential energy due to relative position.

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

14 Elements 92 naturally occurring elements. Can’t be broken down into simpler substances by chemical reaction Composed of atoms

15 Properties of Matter An element’s properties depend on the structure of its atoms Each element consists of a certain kind of atom that is different from those of other elements An atom is the smallest unit of matter that still retains the properties of an element

Essential Elements of Life About 25 of the 92 elements are essential to life Carbon, hydrogen, oxygen, and nitrogen make up 96% of living matter Most of the remaining 4% consists of calcium, phosphorus, potassium, and sulfur Trace elements are those required by an organism in minute quantities (less than.01% of body’s mass) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Table 2-1

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Kinds of Atoms For most atoms, the outer energy level contains a maximum of eight electrons.  Octet Rule - Atoms tend to establish completely full outer energy levels. - Inert - Elements possessing a full complement of electrons in the outer shell. - Reactive - Atoms lacking a full complement of electrons in the outer shell.

19 Molecules and Compounds Sodium + Chlorine yields Sodium Chloride + Molecules: Atoms held together by energy (chemical bonds) Ex. H-H, O-O Compounds: substances consisting of two or more elements combined in a fixed ratio Compounds have characteristics different from those of their elements Figure 2.2

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Chemical Behavior of Atoms Oxidation-Reduction (Redox) Reactions.  Oxidation - Loss of an electron.  Reduction – Gain of an electron.

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

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Covalent Bonds Covalent bonds form when two atoms share one or more pairs of valence electrons

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Covalent Bonds Bond Strength  Strength of a bond depends on the number of shared electrons. - Single Bond - Double Bond - Triple Bond

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 leading to a change in the composition of matter  Reactants - Original molecules.  Products - Resultant molecules. Chemical reactions influenced by:  Temperature  Concentration of Reactants and Products  Catalysts

25 Chemical Reactions Chemical reactions  Convert reactants to products  Synthesis reactions convert reactants into a new product (energy required)  Decomposition reactions break complex substances into simpler ones (energy released)  Reversible reactions proceed in both directions

26 Chemical Reactions Photosynthesis  Is an example of a chemical reaction Figure 2.18

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Chemistry of Water Water molecules consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds (water, oxygen and carbon dioxide are examples of inorganic molecules necessary for life) Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Hydrogen Bonds In a water molecule, both oxygen and hydrogen atoms attract shared electrons in the covalent bonds (electronegativity).  Shared electrons are thus more likely to be found near the oxygen nucleus than near the hydrogen nuclei. - Polar Molecule - Has distinct ends, each with a partial charge.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Emergent Properties of Water Polarity of water causes it to be attracted to other polar molecules.  Cohesion - Attraction to other water molecules. - Responsible for surface tension.  Water is adhesive to any substance with which it can form hydrogen bonds.  Adhesion - Attraction to other substance.

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

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Properties 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.  High Specific Heat (extent a substance resists changing temperature as it absorbs or loses heat).  High Heat of Vaporization (energy required to change liquid water to gas). - Sweating

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Properties of Water Solvent - Water is an effective solvent because of its ability to form hydrogen bonds.  Water molecules gather around any substance bearing an electrical charge. Solute: substance (molecules) dissolved in the solvent Solution: end-product of dissolving a solute in a solvent

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

Properties of Water Organization of Nonpolar Molecules - Water molecules act to exclude nonpolar molecules.  Hydrophobic - Nonpolar molecules shrink from contact with water. - (Water Hating)  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. pH Scale - Measures negative logarithm of hydrogen ion concentration in the solution.  Acids - Substances that dissociate in water to increase concentration of H + ions. - pH values below 7.  Bases - Substances that combines with H + ions when dissolved in water. - pH values above 7.

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

Water Ionizes Buffer – Weak acids or bases that act as a reservoir for hydrogen atoms to minimize change in pH pH of the fluids within most cells in a human is 6.5 – 7.5  Donates hydrogen atoms to the solution when their concentration falls, and removes them from the solution when their concentration rises.  Ex. Acid-base pair carbonic acid and bicarbonate is the key buffer in blood