Chapter 4: Glow in the Dark

1. Chapter 4: Glow in the Dark

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 Greece • Various 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 (1766-1844), 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 atoms • Atoms of the same element have identical properties • Atoms of different elements have different properties • Atoms of different elements combine in whole-number ratios to form compounds • Chemical changes join, separate or rearrange atoms in compounds (p. 124 in text)

7. J. J. Thomson • Because 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 “cathode ray.” The cathode ray looked like a ray of light traveling through the tube, from the cathode plate to the anode plate. Cathode ray Metal plate (anode) to which stream travels Metal plate (cathode) releases stream

10. Cathode Ray Tubes & Charge Next Thomson put charged plates outside the tube. He found that the rays bent towards a positively charged plate and away from a negative one. - Negatively charged plate Positively 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 change • Thomson’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 Model • Eugene 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 portion The “chips” are stationary and don’t move within the “dough”

15. Gold Foil Experiment • In 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 Experiment • Geiger 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.

17. Gold Foil Experiment

18. Gold Foil Experiment • It 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 deflection • These particles did not run into anything; they traveled through empty space. • Some particles had slight deflections • These 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 space • There 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 Neutron • The 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 Model Bohr 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 Model • Bohr 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 Model • The 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.

26. Pictures of the Bohr Models Oxygen

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 Theory • Bohr’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).