Chapter 3

1. Chapter 3 The Evolution of Atomic Theory

2. 3.1 Dalton’s Atomic Theory • Atoms proposed around 400 B.C. • Took 2000 years to be accepted • Dalton’s atomic model (early 1800s) formulated explanations for a variety of laws.

3. 3.1 Dalton’s Atomic Theory (Continued) • Law of conservation of mass • When a reaction takes place, matter is neither created or destroyed.

4. 3.1 Dalton’s Atomic Theory (Continued) • Law of constant proportions • Multiple samples of any pure compound always contain the same percent by mass of each element making up the compound.

5. 3.1 Dalton’s Atomic Theory (Continued) • Percent by mass • Consider that 50.0 g of water (H2O) is decomposed into the component elements yielding 5.6 g of H and 44.4g of O2. What is the percent by mass of the components? • It is always the same for pure water.

6. 3.1 Dalton’s Atomic Theory (Continued) • Dalton’s atomic theory • All matter is made up of atoms. • Atoms can neither be created nor destroyed. • Atoms of a particular element are alike. • Atoms of different elements are different. • A chemical reaction involves the union or separation of individual atoms.

7. 3.1 Dalton’s Atomic Theory (Continued) • Ball-and-hook model • Different size balls represent different atom types. • Different types have different numbers of hooks representing bonding.

8. 3.1 Dalton’s Atomic Theory (Continued) • No point of the theory is entirely true, so updates have occurred. • Atoms are not the most fundamental unit. • Protons, electrons, and neutrons • Atoms can be created and destroyed in nuclear reactions. • Although updates were needed, it does not diminish the theory or its usefulness.

9. 3.2 Development of a Model for Atomic Structure • What do atoms look like? • Problem tackled by J.J. Thomson, James Chadwick, and others • Found atoms are comprised of even smaller particles on the inside.

10. 3.2 Development of a Model for Atomic Structure (Continued) • J.J. Thomson • In 1897, he discovered the electron. • The first subatomic particle • Very small and lightweight • 1/1836 mass of a hydrogen atom • Has a negative charge, which is referred to as negative one

11. 3.2 Development of a Model for Atomic Structure (Continued) • J.J. Thomson and E. Goldstein • In 1907, they discovered the proton. • Much heavier than the electron • Mass is roughly equal to a hydrogen atom. • Exhibits a positive charge referred to as positive one

12. 3.2 Development of a Model for Atomic Structure (Continued) • James Chadwick • 25 years later, he discovered the neutron. • Roughly same mass as a proton • Did not exhibit a charge • Was electrically neutral • Very difficult to study due to the absence of charge

13. 3.2 Development of a Model for Atomic Structure (Continued) • The first model proposed by J.J. Thomson is called the plum-pudding model. • Knew two basic facts: • Atoms contain small, negatively charged particles. • Atoms of an element behave as if they have no charge. • Reasoned that something must encapsulate the negative charge of the electron • Proposed a “cloud of positive electricity”

14. 3.2 Development of a Model for Atomic Structure (Continued) Plum-Pudding Model

15. 3.3 The Nucleus • Rutherford’s gold foil experiment • Fired alpha particles at metal foils • Alpha particle has two protons and two neutrons with a charge of +2. • Gold foil is only a couple of thousand atoms thick. • Used a glass substrate covered with zinc sulfide to monitor the alpha particles • Expected a straight flight path for the particles

16. 3.3 The Nucleus (Continued) • Results were almost the expected. • While most flew straight through, some bent or were “bounced” backward.

17. 3.3 The Nucleus (Continued) • This non-linear flight was very unexpected. • Rutherford’s quote: “It was … as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

18. 3.3 The Nucleus (Continued) • From the study, he knew two things: • Most of the alpha particleswent straight through the foil. • Most of the atom must be empty space. • A few of the alpha particles(about 1 in 20,000) were deflected from a straight-line path. • There must be a small and massive something inside the atom. • This massive unit must have a positive charge.

19. 3.4 The Structure of the Atom (Continued)

20. 3.3 The Nucleus (Continued) Rutherford’s Model of the Atom

21. 3.3 The Nucleus (Continued) • Why don’t opposite charges simply collapse into each other? • Rutherford knew more work was needed. • Ended in a new form of physics • Quantum physics was born.

22. 3.4 The Structure of the Atom • What we know: • The nucleus at the center of the atom contains: • Protons—relatively massive and positively (+1) charged • Neutrons—relatively massive and neutrally charged • Electrons orbit the nucleus with a -1 charge. • Positive and negatively charged atoms are drawn to one another.

23. 3.2 Development of a Model for Atomic Structure (Continued) Properties of Subatomic Properties

24. 3.3 The Nucleus (Continued) • Rutherford’s model of the atom • An atom is mostly empty space. • Contains the electrons spread throughout the atom • A nucleus is a tiny, massive, positively charged unit in the atom. • Placed in the center of the atom • Contains the protons and neutrons

25. 3.4 The Structure of the Atom (Continued) • Atomic number (Z) • The number of protons in the nucleus • Determines the identity of the atom • The atom with one proton is always hydrogen.

26. 3.4 The Structure of the Atom (Continued) • Mass number • Number of protons plus the number of neutrons

27. 3.4 The Structure of the Atom (Continued) Sketch a neutral carbon atom in the following three ways, showing the correct numbers of protons, neutrons, and electrons. (a) Make the mass number equal to 12. This would contain 6 protons, 6 neutrons, and 6 electrons. (b) Make the mass number equal to 13. This would contain 6 protons, 7 neutrons, and 6 electrons. (c) Make the mass number equal to 14. This would contain 6 protons, 8 neutrons, and 6 electrons.

28. 3.4 The Structure of the Atom (Continued) • Isotopes • Atoms with the same number of protons (same element), but different numbers of neutrons • Exhibit identical chemical properties

29. 3.4 The Structure of the Atom (Continued) • Isotope symbol • Shows both the mass number and the atomic number along with the element symbol • Often, the atomic number is omitted as it is implied by the element symbol.

30. 3.4 The Structure of the Atom (Continued) • Isotopes • All isotopes are not present in the same amounts. • 12C = 98.89%, 13C = 1.11%, and 14C = trace amounts • Nearly all elements haveisotopes. • Hydrogen has three isotopes with special names.

31. 3.4 The Structure of the Atom (Continued) • Identify an atom with a mass number of 16, containing 9 neutrons. • Give the full atomic symbol for an atom with 16 neutrons and an atomic number of 15.

32. 3.4 The Structure of the Atom (Continued) • Identify an atom with a mass number of 16, containing 9 neutrons. This would be a boron atom. 2. Give the full atomic symbol for an atom with 16 neutrons and an atomic number of 15. is the answer.

33. 3.4 The Structure of the Atom (Continued) • Atomic mass • The actual mass of any atom • Have units of atomic mass units (amu) or daltons (Da) • Relative atomic mass • Measures how massive an atom is in comparison to a 12C atom • 1H would be 1/12 the mass of 12C.

34. 3.4 The Structure of the Atom (Continued) • Weighted average of atomic mass • Values reported on the periodic table • Weighted average of the isotope masses • 14C is ignored due to its only having trace amounts present.

35. 3.5 The Law of Mendeleev—Chemical Periodicity • By 1860, nearly 70 elements had been isolated and studied. • As discovered, a means to organize the elements was needed. • Enter Mendeleev and his arrangements

36. 3.5 The Law of Mendeleev—Chemical Periodicity (Continued) • A major breakthrough occurred when the elements were ordered by increasing atomic mass • He noticed a regular repeating of properties • Every eighth element exhibited similar properties. • K, Na, and Li for example

37. 3.5 The Law of Mendeleev—Chemical Periodicity (Continued) • K, Na, and Li all: • React vigorously with water • Form oxides (K2O) and hydroxides (KOH) • Are conductive

38. 3.5 The Law of Mendeleev—Chemical Periodicity (Continued) • Law of octaves • Elements that are eight elements apart by mass react in similar manners. • Called chemical periodicity or periodic behavior

39. 3.5 The Law of Mendeleev—Chemical Periodicity (Continued) • Law of Mendeleev • Properties of the elements recur in regular cycles (periodically) when elements are arranged in order of increasing atomic mass.

40. 3.5 The Law of Mendeleev—Chemical Periodicity (Continued) • Gaps in Medeleev’s table • He left gaps for undiscovered elements in his table. • He predicted the properties of the missing elements.

41. 3.5 The Law of Mendeleev—Chemical Periodicity (Continued) • Predicted values matched those in the later found elements • Giving further support to his arrangement of elements

42. 3.6 The Modern Periodic Table • Current organization of the 113 elements • Elegant and simplistic in showing information • Relays both subatomic structures and chemical properties • Shows the atomic number and mass for each element

43. 3.6 The Modern Periodic Table (Continued) • Periods of the periodic table • The horizontal rows of the table • Numbered 1 through 7 as you descend

44. 3.6 The Modern Periodic Table (Continued) • Groups • The vertical columns of the table • Also called families • Numbered in a few different manners: • Using roman numerals • Using Arabic numbers • Groups with varying names are shown in violet.

45. 3.6 The Modern Periodic Table (Continued)

46. 3.6 The Modern Periodic Table (Continued) • Sections of the table • Main-group elements—groups 1, 2, and 13−18 • Much early chemistry based here • Shows strongest periodic nature • Transition metals—groups 3−12 • Lanthanides (rare earths) and actinides • In the lower section of the table, not numbered

47. 3.6 The Modern Periodic Table (Continued) • Transition-metal numbering • Numbered with roman numeral and B • I and II are at the high end of the B numbers. • This awkwardness leads to the adaption of the 1-through-18 numbering scheme.

48. 3.6 The Modern Periodic Table (Continued) • Phase of matter at room temperature • Solid, liquid, and gas • Group 18 elements are the noble gases.

49. 3.6 The Modern Periodic Table (Continued) • Metallic nature • Metal, nonmetal, or metalloid • Separated by stair-step starting at boron • Roughly 75% are metals. • All life is based on the nonmetal carbon.

50. 3.6 The Modern Periodic Table (Continued) • Metals • Shiny solids • Bendable and malleable • Nonmetals • Brittle • Do not conduct electricity or heat well • Semimetals • Can act as a metal or nonmetal