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Unformatted text preview: 9/23/11 Atoms and Elements An essen�al feature of atomic theory is that atoms combine in whole-‐number ra�os to make compounds. Examples: NaCl H2O H2O2 Atoms are Constantly in Mo�on Atomic theory states that ma�er is made up of �ny atoms. Solids occupy deﬁnite loca�ons in materials; liquids and gases ﬂow easily from one place to another. A phenomenon called Brownian mo�on provides evidence for the movement of molecules. Brownian Mo�on The spores seemed to dance about on the water’s surface. Caused by collisions between the par�cles and the molecules of the ﬂuid in which they are suspended. Spores move about because they collide constantly with water molecules. Occasionally, a collision with a par�cularly fast-‐moving water molecule causes the spore to change its direc�on abruptly. Atoms and Elements Remember, chemists try to visualize chemical reac�ons at the molecular level. This molecular view is so ingrained that chemists o�en take it for granted. As you study chemistry, strive to a�ain this molecular point of view. Brownian Mo�on Robert Brown (1828) no�ced that spores moved about con�nuously and underwent irregular changes of direc�on. Dynamic Equilibrium Atoms and molecules are always moving. The condi�on of balanced mo�on is called dynamic equilibrium. 1 9/23/11 Dynamic Equilibrium Dynamic Equilibrium A system at equilibrium shows no change in its observable proper�es. A dynamic system contains objects that move con�nuously. Dinitrogen tetroxide forms an equilibrium mixture with nitrogen dioxide. Used as a rocket propellant to launch vehicles for spacecra�. Dynamic Equilibrium Electrons The ink example is dynamic because when the color of the liquid is uniform and unchanging, the water and ink molecules move about. Dynamic equilibria occur frequently in chemical systems. All chemical processes reach a state of equilibrium if allowed to con�nue for a suﬃcient �me but molecular ac�vity always goes on a�er equilibrium has been reached. Early experiments provided extensive informa�on on the composi�on of the atom. Gas discharge tube experiment. Electrons Cathode Ray Tube Experiment Two electrodes; at high charges a bolt shoots across the electrodes causing some atoms to break apart into charged par�cles; + charges move to the – electrode and – charges to the + electrode; exp. showed that atoms consisted of smaller fragments that are + and – charged. Hence, species are electrons. J. J. Thomson (1906 Nobel Prize). Discoverer of electron. Seven of his students won Nobel Prizes (Ernest Rutherford, William Bragg, Max Born, Charles Wilson, Owen Richardson, etc.). 2 9/23/11 Cathode Ray Tube Experiment Cathode ray is a beam of electrons. e-‐s are aﬀected by both electrical and magne�c forces. Measured amt. of B required to exactly counterbalance the deﬂec�on of the beam by a known E. Cathode Ray Tube Experiment Cathode ray tube and applied external
magnetic field. Cathode ray tube and applied
external electrical field. Cathode ray tube and both fields applied. Cathode Ray Tube Experiment Calculated the ra�o of the electrons charge to its mass Charge / mass = e / m = 1.76 X 1011 C / kg Oil Drop Experiment Schema�c view of Millikan’s oil drop exp. Inside the chamber an atomizer generated a ﬁne mist of oil droplets. Bombarding the droplets with x-‐rays gave some of them extra – charge. In the presence of suﬃcient E force, these nega�vely charged droplets could be suspended in space. Oil Drop Experiment Robert Millikan (Caltech) Charge of the electron Measured value of Planck’s constant using photoelectric emission graphs of various metals. Oil Drop Experiment In other words, the gravita�onal force and E force were counterbalanced. Knowing the E ﬁeld, the charge on the oil droplet could be determined. e = -‐1.6 X 10-‐19 C. It follows that since Charge / mass = e / m = 1.76 X 1011 C / kg Then, e / e / m = 9.1 X 10-‐31 kg, mass of electron 3 9/23/11 The Nucleus Ernest Rutherford’s Experiment By the early 20th century, scien�sts had discovered that atoms contain electrons and posi�vely charged par�cles. The nature of electrons had been elucidated by the exps. of Thomson and Millikan as just described, but the nature of the posi�ve par�cles was en�rely unknown. Also, it was not known how the par�cles ﬁt together to make up an atom. Atomic Building Blocks Name Symbol Charge Mass 9.1094 X 10-‐31 kg Electron e -‐1.6022 X 10-‐19 C
Proton p +1.6022 X 10-‐19 C 1.6726 X 10-‐27 kg Neutron n 0 1.6749 X 10-‐27 kg Our picture of atomic architecture is now complete. Three par�cles – electrons, protons and neutrons – combine in various numbers to make the diﬀerent atoms of all the elements of the periodic table. 4 ...
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