2 Introduction List as many things as you can think of that “glow” What do you have to do to make these “glowing” things “glow”?This chapter will introduce the chemistry needed to understand how glowing things work
3 Section 4.1: Development of Atomic Theory Objective:Describe the development of modern atomic theory
4 Ancient GreeceVarious philosophers considered the many mysteries of life.Aristotle concluded that matter was composed of 4 elements - earth, air, fire, and water – and that it could be divided endlessly into ever smaller pieces.Democritus was the first person to propose the idea that matter was not infinitely divisible. He believed that matter was composed of atomos or atoms; atoms were solid & indivisible.
5 John Dalton Aristotle's idea went unchallenged for 2000 yrs. John Dalton ( ), an English schoolteacher and chemist, revised Democritus’ ideas based on careful & accurate scientific research that he conducted himself.
6 Dalton’s Atomic Theory (1803) All matter is made of tiny particles “atoms”Atoms cannot be created, divided, destroyed or changed into other types of atomsAtoms of the same element have identical propertiesAtoms of different elements have different propertiesAtoms of different elements combine in whole-number ratios to form compoundsChemical changes join, separate or rearrange atoms in compounds(p. 124 in text)
7 J. J. ThomsonBecause of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate.
8 Thomson’s Cathode Ray Tube Thomson’s experiments used a vacuum tube (a tube that has had all the gases pumped out of it) called the cathode ray tube.
9 Cathode Ray Tubes When connected to a battery, electrodes at the end of the tube seemed to generate a “cathoderay.” The cathode ray looked like a ray of lighttraveling through the tube, from the cathode plateto the anode plate.Cathode rayMetal plate (cathode) releases streamMetal plate (anode) to which stream travels
10 Cathode Ray Tubes & Charge Next Thomson put charged plates outside the tube.He found that the rays bent towards a positivelycharged plate and away from a negative one.-Negatively charged platePositively charged plate+Ray is deflected away from negative plate and towards positive plate
11 Thomson’s conclusions Since like charges repel, Thomson’s knew that there was something negatively charged in the cathode ray.Since there were no particles in the tube, these negative particles had to come from the atoms of the metal plates.Since all types of metal produced the same result, the negative charge had to be in all types of atoms.In 1897, Thomson announced that the rays were electrons and they had a negative charge
12 Theories changeThomson’s evidence showed Dalton’s idea of solid, uniform atoms was incorrect.In addition, since atoms themselves are not negatively charged but neutral, scientists believed there had to be other particles in the atom, especially positively charged ones.
13 The Plum Pudding ModelEugene Goldstein conducted experiments to find the positive parts (protons) and determined they had the opposite charge as the electron but were 1837 times heavier!Thomson developed a model of the atom called the “plum pudding” model.
14 Also Called The Cookie Dough Model The “chips” are the negative electrons.The “dough” is the positive portionThe “chips” are stationary and don’t move within the “dough”
15 Gold Foil ExperimentIn 1911, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the plum pudding model of the atom.This team included Geiger and Marsden.
16 Gold Foil ExperimentGeiger and Marsden (under Rutherford’s direction) bombarded very thin gold foil with radioactive particles (alpha particles “”)They were to observe the direction the particles took as a result of passing through the foil.
18 Gold Foil ExperimentIt was believed that if the plum pudding model was correct, the alpha particles would pass straight through the gold atoms.Instead, researchers found that some alpha particles were deflected at very wide angles.
19 Observations & Conclusions Most of the alpha particles passed straight through the foil with no deflectionThese particles did not run into anything; they traveled through empty space.Some particles had slight deflectionsThese particles ran into something much smaller than themselves.A few particles were deflected at wide angles – some came straight back!These particles ran into something very dense
20 Rutherford’s Nuclear Model To explain the results, Rutherford proposed a new model of the atom – the nuclear model.In this model, atoms are nearly all empty spaceThere is a small area of the atom that contains most of the mass. This area caused the wide deflections seen.This area is called the nucleus. Protons are found here.Electrons (the smaller particles), the cause of the small deflections, are found in the space outside the nucleus.
21 The NeutronThe protons (+) and electrons (-) could explain the charges of the various parts of the atom that were observed.They could not explain the total mass of atoms.Neutrons were proposed in 1920’s but not confirmed until 1932 by James Chadwick.Neutrons had mass similar to protons and no charge. They were located in the nucleus.
22 Revisions to the Nuclear Model In 1913, Neils Bohr (who was working for Rutherford) believed Rutherford’s model needed improvement.
23 Bohr’s Atomic ModelBohr performed experiments with hydrogen atoms & light.He determined that electrons are in levels according to how much energy they have (energy levels) and that only certain energy amounts were allowed.
24 Bohr’s Atomic ModelBohr proposed that, within energy levels, electrons are found in specific circular paths, or orbits, around the nucleus.Bohr’s model came to be known as the planetary model.
25 The Bohr ModelThe orbit closest to the nucleus contains the lowest energy electrons.The first level can hold 2 electrons, then the next two levels can each hold 8 and then levels farther out can hold 18.
27 Use of the Bohr Model now We no longer believe electrons are in circular orbits.However, this is still a convenient way to show energy levels on 2-dimensional paper.
28 Modern Atomic TheoryBohr’s research lead the way for the study of quantum mechanics (the study of tiny particles) in the 1920’s.Quantum mechanics uses calculus equations to show how the electrons act as both particles and waves.These equations show the most probable location of electrons in the atom (known as atomic orbitals).