Chapter 8 Composition of the Atom

Transcription

1 Chapter 8 Composition of the Atom 8-1 Scientific Modelling Scientists use models to help explain things that cannot be observed. This happens all the time; consider a globe for example. Scientific models are only representations of how we perceive things to be and frequently our models change to fit new information. Good models can be used to make good predictions, propose experiments or bring understanding. 8-2 The Discovery of the Electron Crookes and Thomson s Cathode Ray Tubes Crookes Tube Thomson s Tube 1

2 Question: What did these experiments show? Answer: There is a component of the atom which has a negative electrical charge. Why? 8-3 The Proton Thomson s 2 nd Tube Question: What did this experiment show? Answer: There is a component of the atom which has a positive electrical charge. Why? 2

3 8-4 (A) Discovering Relative Sizes Thomson s experiments lead to a significant conclusion: Since the amount of deflection of a beam of particles is proportional to the ratio of it s charge (e) to mass (m), then the experimental value should give you some idea of how the two values compare. The experimental value was found to be: e = 1.76 X 10 8 C (coulombs) m gram What does this tell us? Why? 3

4 8-4 (B) Millikan s Experiment Questions: What did the ionizing radiation do? How would the charged metal plates affect a positively charged oil drop? How would the charged metal plates affect a negatively charged oil drop? What do you think would happen to a negatively charged oil droplet as you increased and decreased the charges on the plates (made them more or les +/-)? Okay, the beauty of the experiment If you know the rate of the rise in the oil droplet, it s mass and the charge on the plates, you can calculate the mass of an electron! 4

5 This turned out to be 9.11 X g (~ 1840 electrons = 1 proton) It was also determined that this value held for all gases which implied that the electrons from all gases were the same. Okay! One more question: How would you use this same apparatus to determine the mass of a proton? Turns out that all gases did not behave the same, that Hydrogen was the lightest with a value of 1.67 X g. Turns out that ionized hydrogen IS a proton! 5

6 8-6 / 8-7 Sir Earnest Rutherford s Contribution see handout Question: What is an alpha particle? What kind of charge does it have? Run simulation Question: What does the fact that most of the α particles went straight through tell us about the nature of the atom? What does the fact that some of the α particles were deflected tell us about the nature of the atom? 6

7 What does the fact that some of the α particles were deflected directly back at the source tell us about the nature of the atom? 8-8 The Neutron Irene Curie & Frederic Joliet bombard Be with α particles. A high penetrating powered beam is made. Only thing is the beam is not affected by electrical charges. Question: What does this tell us about the nature of the beam? The Evolution of the Atomic Model 1807 The Dalton Model. Solid Sphere 1903 The Thomson Model. Plum pudding 1911 The Rutherford Model. Planetary 7

8 Atoms positively charged dense nuclei composed of protons (+) and neutrons (0). negatively charged electrons (-) in cloud-like orbits or energy levels. amu charge symbol proton 1 ( + ) p neutron 1 ( 0 ) n electrons 1/1820* ( - ) e- The Nucleus and Isotopes figure * do not significantly add to the atomic mass of an element 8

9 Atomic Number. The identity of an element is determined by its atomic number - (THE NUMBER OF PROTONS) Atomic Mass. The atomic weight/mass of an element is determined by the - SUM OF PROTONS AND NEUTRONS in the nucleus. Isotopes are variations of an element having the same atomic number with a different atomic mass think (otherwise it would be a different element) * isotopes are produced by varying the number of neutrons Three isotopes of carbon figure 9

10 Why are there neutrons? figure figure without n - strong n - shield repulsive effects (+) - (+) repulsive force and act like nuclear would blow the nucleus glue (aka - nuclear apart force) As the number of protons increases, the number of neutrons does as well, but at a greater rate. When you get many protons in a nucleus, the number of possible (+) - (+) interactions increases dramatically so you need much more glue. Calculating atomic masses from isotopic data Atomic masses are commonly quoted in decimal numbers. This is because these numbers refer to the average atomic masses of all the isotopes as they occur in nature. Therefore, if a whole number is quoted on you periodic table, this means that there is only one version of this element: this means that it was manufactured it does not occur in nature!!! Suppose you were told that chlorine has two isotopes and is present in nature in the following proportions: 10

11 Chlorine-35 (75.77%) Chlorine-37 (24.23%) Find the average atomic mass (as would be expected on the Periodic Table). Calculate: Carbon and oxygen using the data on p237 of Heath. Electrons Electrons are important for two main reasons: 1) They balance the charge of the nucleus 2) They determine the reactivity of an element since chemical bonds are formed at the electron cloud. - They do not exist in planetary orbits, but in probability clouds at different levels (energy levels) from the nucleus. These energy levels are further sub-divided into sub-orbits each of which is occupied by a maximum of two electrons. 11

12 - Electrons load at the lowest energy level (shell) first and then into increasing energy levels in the most stable configuration possible. See handouts Mr. H s Handy-Dandy Electron Loading Memory Aid (phew!) 7s 7p 7d 7f 6s 6p 6d 6f 5s 5p 5d 5f 4s 4p 4d 4f 3s 3p 3d 2s 2p 1s 12

13 Example Isoelectronics - when one element (ionized) has the same electron configuration as another element. (Usually a step up or down on the periodic table). 13