2. The chemical context of life

3. matter occupies space and has mass Matter is composed of chemical elements lelements cannot be broken down compound = two or more elements in a fixed ratio - such as NaCl (salt) Life is composed of matter sodium chlorine NaCl salt + =

4. “honc” Trace elements

5. May be needed in only small amounts but may cause harm if absent (iodine) goiter

6. Atoms zero charge; one dalton mass neutron proton +1 charge; one dalton mass electron -1 charge; minor mass NUCLEUSNUCLEUS

7. Atoms

8. He 2 Atomic Number and Weight atomic number 4 atomic weight (protons) (protons+neutrons) In a neutral atom, # protons = # electrons Helium

9. Examples Na 11 23 (11 prot, 11 elect, 12 neut) C 6 12 (carbon-12, stable isotope) C 6 14 (carbon-14, unstable and radioactive)

10. The number of neutrons vary among isotopes of an element

11. Radioactive isotope 1. The number of protons is unique to each element (C = 6) 2. Isotopes have different numbers of neutrons 3. The nucleus of a radioactive isotope decays and gives off energy

12. Radioactive isotopes MRI Very useful in biology and medicine but must be handled with extreme care

13. Electron shells Can be represented by a variety of shapes

14. Valence shell The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell

15. Chemical bonds Atoms combine to form molecules 1. Covalent bond - sharing a pair of valence electrons by two atoms nonpolar covalent polar covalent 2. Ionic bonds - one atom “steals” electrons from another

16. Covalent bonds share valence electrons

17. ...but they don’t always share evenly Nonpolar covalent - the atoms have the same electronegativity and do share electrons equally Example: methane

18. Polar covalent One atom is more electronegative than another and keeps the electrons near itself more of the time Ex. water

19. Non-polar covalent -vs- Ionic bonds Non polar covalent bonds Atoms still share electrons, even if they are not evenly distributed (  - ). Ionic bonds An atom “steals” electrons from another atom.

20. Ionic bonds often complete an atom’s valence shell

21. anion An ionic bond is formed between: - charge + charge cation

22. Sodium chloride Ionic compounds are salts

23. Water is the solvent of life

24. Unique properties of water Polar Molecule –Cohesive Stabilizes Temperature –Solvent of Life

25. Like no other common substance, water exists in nature in all three physical states: –as a solid –as a liquid –as a gas

26. Water molecules form hydrogen bonds Hydrogen atoms in one polar covalent molecule are attracted to an electronegative atom in another polar molecule This creates H-bonds Very important for function in biological molecules

27. Water’s polarity leads to hydrogen bonding and other unusual properties Hydrogen bonds are weak; forming and breaking constantly - but strong in high numbers If you could stop water in an instant most molecules would be H-bonded to another These characteristics (polarity & H-bonding) give water the properties of cohesion, temperature stability, and make it a universal solvent

28. Cohesion: Water molecules stick together surface tension

29. Cohesion: water molecules can move great distances

30. Adhesion- Unique ability of water to “stick” to other things.

31. Ice floats! Water molecules in ice (solid water) are farther apart than the molecules in liquid water

32. Ice is therefore less dense than liquid water, causing it to float.. Why is this a good thing? –If ice sank, it wouldn’t thaw –Creates a blanket of insulation

33. Water is the universal solvent Solution - a homogenous mixture of two or more substances : aqueous solutions Solvent - dissolving agent (water) Solute - substance dissolved Polar or charged solutes can “stick” to water molecules (i.e. Dissolve)

34. Na + Cl – – – – – – – – – – – – + + + + + + + + + + + + + + + + + +

35. The chemistry of life is sensitive to acids and bases A substance that releases H + ions in solution is an acid, and one that accepts H + ions in solution is a base Acidity is measured on the pH scale: –0-6 is acidic –8-14 is basic –pH of 7 is neutral: Pure water

36. The pH scale

37. pH  pH = -(log [H + ])  [H + ] = Concentration of Hydrogen ions  For [H + ] = 10 -2, pH = 2  Each change in pH represents 10x change in [H + ] and [OH - ]  pH of 7 is neutral; [H + ] = [OH - ]  pH [OH - ]  pH > 7 is basic; [H + ] < [OH - ]

38. buffers Buffers are substances that resist pH change –They accept H + ions when they are in excess and donate H + ions when they are depleted –carbonic acid in bloodstream to keep pH at 7.3 - 7.5 H 2 CO 3 HCO 3 - + H + Cells are kept close to pH 7 by buffers