Chapter 2 Life ’ s Chemical Basis

1. Chapter 2Life’s Chemical Basis

2. Structure of Atoms • Atomsare the building blocks of all substances • Made up of electrons, protons and neutrons • Chargeis an electrical property • Attracts or repels other subatomic particles

3. Characteristics of Atoms • Electrons (e-) have a negative charge • Move around the nucleus • Thenucleuscontains protons and neutrons • Protons(p+)have a positive charge • Neutrons (n) have no charge

4. Characteristics of Atoms • Atoms differ in number of subatomic particles • Atomic number(number of protons) determines the element • Elements consist only of atoms with the same atomic number • Mass number • Total protons and neutrons in a nucleus • Used to identify isotopes

5. Atoms proton neutron electron

6. The Periodic Table • Periodic table of the elements • An arrangement of the elements based on their atomic number and chemical properties • Created by Dmitry Mendeleev

7. Periodic Table of the Elements

8. Isotopes and Radioisotopes • Isotopes • Different forms of the same element, with different numbers of neutrons • Some radioactive isotopes – radioisotopes – are used in research and medical applications

9. Take Home Message: The basic building blocks of all matter • All matter consists of atoms, tiny particles that in turn consist of electrons moving around a nucleus of protons and neutrons • An element is a pure substance that consists only of atoms with the same number of protons. Isotopes are forms of an element that have different numbers of neutrons • Unstable nuclei of radioisotopes disintegrate spontaneously (decay) at a predictable rate to form predictable products

10. About Vacancies • Electrons move around nuclei in orbitals • Each orbital holds two electrons • Each orbital corresponds to an energy level • An electron can move in only if there is a vacancy • free radical • Atom with an unpaired electron

11. Why Atoms Interact • Theshell modelof electron orbitals diagrams electron vacancies; filled from inside out • First shell: one orbital (2 electrons) • Second shell: four orbitals (8 electrons) • Third shell: four orbitals (8 electrons) • Atoms with vacancies in their outer shell tend to give up, acquire, or share electrons

12. A The first shell corresponds to the first energy level, and it can hold up to 2 electrons. Hydrogen has one proton, so it has 1 electron and 1 vacancy. A helium atom has 2 protons, 2 electrons, and no vacancies. The number of protons in each model is shown. 1 proton 1 2 1 electron B The second shell corresponds to the second energy level, and it can hold up to 8 electrons. Carbon has 6 protons, so its first shell is full. Its second shell has 4 electrons, and four vacancies. Oxygen has 8 protons and two vacancies. Neon has 10 protons and no vacancies. hydrogen (H) helium (He) first shell 8 6 10 second shell carbon (C) oxygen (O) neon (Ne) C The third shell, which corresponds to the third energy level, can hold up to 8 electrons. A sodium atom has 11 protons, so its first two shells are full; the third shell has one electron. Thus, sodium has seven vacancies. Chlorine has 17 pro tons and one vacancy. Argon has 18 protons and no vacancies. 18 11 17 third shell sodium (Na) chlorine (Cl) argon (Ar) Stepped Art Figure 2-5 p26

13. Atoms and Ions • Ion • An atom with a positive or negative charge due to loss or gain of electrons in its outer shell • Examples: Na+, Cl- • Electronegativity • A measure of an atom’s ability to pull electrons from another atom

14. Combining Substances • Compounds • Molecules consisting of two or more elements whose proportions do not vary • Example: Water (H2O) • Mixture • Two or more substances that intermingle but do not bond; proportions of each can vary

15. The Water Molecule one oxygen atom two hydrogen atoms

16. electron loss electron gain Sodium atom Chlorine atom 11p+ 11e– 17p+ 17e– charge: 0 charge: 0 Chloride ion Sodium ion 11p+ 10e– 17p+ 18e– charge: –1 charge: +1 Figure 2-6 p27

17. Take-Home Message:Why do atoms interact? • An atom’s electrons are the basis of its chemical behavior • Shells represent all electron orbitals at one energy level in an atom; when the outermost shell is not full of electrons, the atom has a vacancy • Atoms with vacancies tend to interact with other atoms

18. Chemical Bonds • Chemical bond • An attractive force existing between two atoms when their electrons interact • Molecule • Two or more atoms joined in chemical bonds

19. Three Types of Bonds • The characteristics of a bond arise from the properties of the atoms that participate in it • The three most common types of bonds in biological molecules are ionic, covalent, and hydrogen bonds

20. Ionic Bond: Sodium Chloride ionic bond 11 17 Sodium ion 11p+, 10e– Chloride ion 17p+, 18e–

21. Covalent Bonds • Covalent bond • Two atoms with similar electronegativity and unpaired electrons sharing a pair of electrons • Can be stronger than ionic bonds • Atoms can share one, two, or three pairs of electrons (single, double, or triple covalent bonds)

22. Characteristics of Covalent Bonds • Nonpolar covalent bond • Atoms sharing electrons equally; formed between atoms with identical electronegativity • Polar covalent bond • Atoms with different electronegativity do not share electrons equally; one atom has a more negative charge, the other is more positive

23. molecular hydrogen (H2) molecular oxygen (O2) water (H2O) Figure 2-9 p29

24. Table 2-1 p29

25. Take-Home Message:How do atoms interact in chemical bonds? • A chemical bond forms between atoms when their electrons interact • A chemical bond may be ionic or covalent depending on the atoms taking part in it • An ionic bond is a strong mutual attraction between two ions of opposite charge • Atoms share a pair of electrons in a covalent bond; when the atoms share electrons unequally, the bond is polar

26. Polarity of the Water Molecule • Overall, water (H2O) has no charge • The water molecule is polar • Oxygen atom is slightly negative • Hydrogen atoms are slightly positive • Hydrogen bonds form between water molecules • Gives water unique properties

27. Polarity of the Water Molecule negative charge positive charge

28. Hydrogen Bonding • Hydrogen bond • A weak attraction between a highly electronegative atom and a hydrogen atom taking part in a separate polar covalent bond • Hydrogen bonds do not form molecules and are not chemical bonds • Hydrogen bonds stabilize the structures of large biological molecules

29. a hydrogen bond Figure 2-11a p31

30. Figure 2-11c p31

31. Water’s Special Properties • Living organisms are mostly water; all the chemical reactions of life are carried out in water • Water is essential to life because of its unique properties • The properties of water are a result of extensive hydrogen bonding among water molecules

32. Water Has Cohesion • Hydrogen bonds give water cohesion • Provides surface tension • Draws water up from roots of plants • Cohesion • Molecules resist separation from one another

33. Water Stabilizes Temperature • The surface temperature of water decreases during evaporation • Evaporation • Conversion of a liquid to a gas by heat energy • Ice is less dense than liquid water • Hydrogen bonds form a lattice during freezing

34. Water is a Solvent • Solvent • A substance (usually liquid) that can dissolve other substances (solutes) • Water is a solvent • The collective strength of many hydrogen bonds pulls ions apart and keeps them dissolved

35. Water is a Solvent • Water dissolves polar molecules • Hydrogen bonds form between water molecules and other polar molecules • Polar molecules dissolved by water are hydrophilic(water-loving) • Nonpolar (hydrophobic)molecules are not dissolved by water

36. 2.6 Acids and Bases • pHis a measure of the number of hydrogen ions in a solution • The more hydrogen ions, the lower the pH • pH 7 is neutral (pure water) • Most biological processes occur within a narrow range of pH, typically around pH 7 • Concentration refers to the amount of a particular solute that is dissolved in a given volume of fluid

37. — 0 battery acid — 1 gastric fluid acid rain — 2 lemon juice cola more acidic vinegar — 3 orange juice tomatoes, wine bananas — 4 beer bread black coffee — 5 urine, tea, typical rain corn — 6 butter milk pure water — 7 blood, tears egg white — 8 seawater baking soda detergents — 9 Tums toothpaste hand soap — 10 milk of magnesia — 11 household ammonia hair remover more basic — 12 bleach — 13 oven cleaner — 14 drain cleaner Figure 2-12 p32

38. Biological Reactions Occur In Water • Molecules in water (H2O) can separate into hydrogen ions (H+) and hydroxide ions (OH-) H20 ↔H+ + OH-

39. Acids and Bases • Acidsdonate hydrogen ions in a water solution • pH below 7 • Basesaccept hydrogen ions in a water solution • pH above 7

40. Acids: Weak or Strong • Acids and bases can be weak or strong • Gastric fluid, pH 2-3 • Acid rain • Example: Hydrochloric acid is a strong acid HCl ↔ H+ + Cl-

41. Salts • Salt • A compound that dissolves easily in water and releases ions other than H+ and OH- HCl (acid) + NaOH (base) → NaCl (salt) + H20

42. Buffers Against Shifts in pH • Buffer • A set of chemicals (a weak acid or base and its salt) that can keep the pH of a solution stable OH- + H2CO3 (carbonic acid) → HCO3- (bicarbonate) + H20 H+ + HCO3- (bicarbonate) → H2CO3 (carbonic acid)

43. Buffering Carbon Dioxide in Blood • Carbon dioxide in blood forms carbonic acid, which separates into H+ and bicarbonate H2O + CO2 (carbon dioxide) → H2CO3 (carbonic acid) → H+ + HCO3- (bicarbonate)

44. Take Home Message: Why is hydrogen important in biological systems? • pH reflects the number of hydrogen ions in a fluid. Most biological systems function properly only within a narrow range of pH • Acids release hydrogen ions in water; bases accept them • Salts release ions other than H+ and OH– • Buffers help keep pH stable. Inside organisms, they are part of homeostasis

45. Table 2-1 p34