1 The Nature of Molecules Chapter 3

2 Atoms

3 Kinds of Atoms Ninety-two naturally occurring elements – Periodic table arranged by grouping atoms based on valence electrons (electrons in the outer energy levels). Octet rule Inert atoms have outer level filled. Reactive atoms do not have outer level filled.

4 Lewis dot diagram

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6 Chemical Bonds Ionic bonds are formed because ions of opposite charge attract one another. – table salt ( Na + and Cl - ) Covalent bonds are formed when two or more atoms share pairs of valence electrons. – Strength depends on number of shared electrons. – Bonds with in water are covalent

7 Water Chemistry Water consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds. – stable atom with no electrical charge most outstanding property is its ability to form weak chemical associations

8 Hydrogen Bonding In a water molecule, both the oxygen and hydrogen atoms attract the shared electrons in the covalent bond – Oxygen atom is more negative than the hydrogen atoms. distinct ends with partial charge (polar) hydrogen bonds

9 Water Chemistry Water clings to polar molecules. – Cohesion refers to attraction to other water molecules. responsible for surface tension: water strider leaves can pull water up the stem Seeds swell to germinate

10 – Adhesion refers to attraction to other substances. Water is adhesive to any other substance with which it can form hydrogen bonds. Capillary action on glass

11 Water Chemistry Water is an effective solvent as it can form hydrogen bonds. – Water clings to polar molecules causing them to be soluble in water. Hydrophilic - attracted to water( sugar/salt) – Water tends to exclude nonpolar molecules.

12 Water molecules Salt crystal Na + Cl – Na + Cl – Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

13 Hydrophobic - repelled by water- form their own layer Water and oil do not mix !

14 Water stores heat. H+ bonds absorb heat when they break Minimizes temperature change How water helps maintain homeostasis in organisms Water cools when it evaporates

15 pH pH scale expresses hydrogen ion (H + ) concentration in a solution. – logarithmic scale ranging from 0-14 neutral = 7

16 pH Acids dissociate in water to increase the concentration of H +. – pH values lower than 7 Bases combine with H + ions when dissolved in water, thus decreasing H + concentration. – pH values above 7

17 pH Neutral More H ion Less H ion OH

18 How Acid and bases affect the homeostasis of our bodies CO 2 is acidic: high levels in blood will send message back to brain to breathe faster: remove CO2 and return to normal pH `

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20 Chemical Reactions A chemical reaction occurs during the formation or breaking of chemical bonds. – Reactants refer to original molecules. – Products refer to resultant molecules. Chemical reactions can be influenced by: – temperature – concentration of reactants and products – catalysts