Life’s Chemical Basis

1. Life’s Chemical Basis Chapter 2

2. Impacts, Issues:What Are You Worth? • Fifty-eight elements make up the human body

3. 1.1 Start With Atoms • The behavior of elements, which make up all living things, starts with the structure of individual atoms

4. Characteristics of Atoms • Atomsare the building blocks of all substances • Made up of electrons, protons and neutrons • Electrons (e-) have a negative charge • Move around the nucleus • Chargeis an electrical property • Attracts or repels other subatomic particles

5. Characteristics of Atoms • Thenucleuscontains protons and neutrons • Protons(p+) have a positive charge • Neutrons have no charge • 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

6. Characteristics of Atoms • Isotopes • Different forms of the same element, with different numbers of neutrons • Mass number • Total protons and neutrons in a nucleus • Used to identify isotopes

7. Atoms

8. proton neutron electron Fig. 2-2, p. 22

9. 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

10. Periodic Table of the Elements

11. 2.2 Putting Radioisotopes to Use • Some radioactive isotopes – radioisotopes – are used in research and medical applications

12. Radioisotopes • Henri Becquerel discovered radioisotopes of uranium in the late 1800s • Radioactive decay • Radioisotopes emit subatomic particles of energy when their nucleus breaks down, transforming one element into another at a constant rate • Example:14C →14N

13. Tracers • Tracer • Any molecule with a detectable substance attached • Examples: • CO2 tagged with 14C used to track carbon through photosynthesis • Radioactive tracers used in medical PET scans

14. PET Scanning

15. A A patient is injected with a radioactive tracer and moved into a scanner like this one. Detectors that intercept radioactive decay of the tracer surround the body part of interest. Fig. 2-4a, p. 23

16. Fig. 2-4b, p. 23

17. B Radioactive decay detected by the scanner is converted into digital images of the body’s interior. Two tumors (blue) in and near the bowel of a cancer patient are visible in this PET scan. tumors Fig. 2-4b, p. 23

18. Animation: PET scan

19. 2.1-2.2 Key Concepts:Atoms and Elements • Atoms are particles that are the building blocks of all matter; they can differ in numbers of protons, electrons, and neutrons • Elements are pure substances, each consisting entirely of atoms with the same number of protons

20. 2.3 Why Electrons Matter • Atoms acquire, share, and donate electrons • Whether an atom will interact with other atoms depends on how many electrons it has

21. Atoms and Energy Levels • 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 vacancy no vacancy

22. 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

23. Shell Models

24. Fig. 2-5a, p. 24

25. electron sodium 11p+, 11e– chlorine 17p+, 17e– argon 18p+, 18e− carbon 6p+, 6e– oxygen 8p+, 8e– neon 10p+, 10e– hydrogen 1p+, 1e– helium 2p+, 2e– Fig. 2-5a, p. 24

26. sodium 11p+, 11e– chlorine 17p+, 17e– argon 18p+, 18e− neon 10p+, 10e– oxygen 8p+, 8e– carbon 6p+, 6e– hydrogen 1p+, 1e– helium 2p+, 2e– C) Third shell. This shell corresponds to the third energy level. It has four orbitals with room for eight electrons. Sodium has one electron in the third shell; chlorine has seven. Both have vacancies, so both form chemical bonds. Argon, with no vacancies, does not. B) Second shell. This shell, which corresponds to the second energy level, has four orbitals—room for a total of eight electrons. Carbon has six electrons: two in the first shell and four in the second. It has four vacancies. Oxygen has two vacancies. Both carbon and oxygen form chemical bonds. Neon, with no vacancies, does not. A) First shell. A single shell corresponds to the first energy level, which has a single orbital that can hold two electrons. Hydrogen has only one electron in this shell, so it has one vacancy. A helium atom has two electrons (no vacancies), so it does not form bonds. Stepped Art Fig. 2-5a, p. 24

27. Animation: The shell model of electron distribution

28. 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

29. Ion Formation

30. Sodium ion 11p+ 11e– ______ no net charge Chlorine atom 17p+ 17e– ______ no net charge electron loss electron gain Sodium atom 11p+ 10e– ______ net positive charge Chloride ion 17p+ 18e– ______ net negative charge Fig. 2-6, p. 25

31. Sodium ion 11p+ 11e– ______ no net charge Chlorine atom 17p+ 17e– ______ no net charge electron loss electron gain Sodium atom 11p+ 10e– ______ net positive charge Chloride ion 17p+ 18e– ______ net negative charge A) A sodium atom becomes a positively charged sodium ion (Na+) when it loses the electron in its third shell. The atom’s full second shell is now the outermost, and the atom has no vacancies. B) A chlorine atom becomes a negatively charged chloride ion (Cl–) when it gains an electron and fills the vacancy in its third, outermost shell. Stepped Art Fig. 2-6, p. 25

32. Animation: How atoms bond

33. From Atoms to Molecules • Chemical bond • An attractive force existing between two atoms when their electrons interact • Molecule • Two or more atoms joined in chemical bonds

34. 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

35. A Compound: Water

36. 2.3 Key Concepts:Why Electrons Matter • Whether one atom will bond with others depends on the element, and the number and arrangement of its electrons

37. 2.4 What Happens When Atoms Interact? • 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

38. Different Ways to Represent the Same Molecule

39. Ionic Bonding • Ionic bond • A strong mutual attraction between two oppositely charges ions with a large difference in electronegativity (an electron is not transferred) • Example: NaCl (table salt)

40. Ionic Bonds

41. Fig. 2-7a, p. 26

42. A A crystal of table salt is a cubic lattice of many sodium and chloride ions. Fig. 2-7a, p. 26

43. Fig. 2-7b, p. 26

44. B The mutual attraction of opposite charges holds the two kinds of ions together in a lattice. Chloride ion B 17p+, 18e– Sodium ion. 11p+, 10e– Fig. 2-7b, p. 26

45. Animation: Ionic bonding

46. Covalent Bonding • 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)

47. 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

48. Polarity • Polarity • Separation of charge intodistinct positive and negative regions in a polar covalent molecule • Example: Water (H2O)

49. Covalent Bonds

50. Molecular hydrogen (H—H) Two hydrogen atoms, each with one proton, share two electrons in a nonpolar covalent bond. Molecular oxygen (O=O) Two oxygen atoms, each with eight protons, share four electrons in a double covalent bond. Water molecule (H—O—H) Two hydrogen atoms share electrons with an oxygen atom in two polar covalent bonds. The oxygen exerts a greater pull on the shared electrons, so it has a slight negative charge. Each hydrogen has a slight positive charge. Fig. 2-8, p. 27