1. Atomos: Not to Be Cut or “indivisible” The History of Atomic Theory

2. Objectives Define “atom” Define “atom” Explain Democritus’ model of the atom Explain Democritus’ model of the atom List the postulates of Dalton’s atomic theory List the postulates of Dalton’s atomic theory Explain what studies of cathode rays revealed about atoms Explain what studies of cathode rays revealed about atoms Explain the results of Rutherford’s gold foil experiment Explain the results of Rutherford’s gold foil experiment Evaluate the atomic theories presented by Democritus, Dalton, Thomson, and Rutherford Evaluate the atomic theories presented by Democritus, Dalton, Thomson, and Rutherford Construct a model based on each different theory Construct a model based on each different theory

3. Who are these men? In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

4. Democritus Greek philosopher Greek philosopher lived from 460 - 370 B.C. lived from 460 - 370 B.C. 400 BC

5. Atomos His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.” He named the smallest piece of matter “atomos,” meaning “not to be cut.”

6. Democritus’ Ideas Atoms are small hard particles. Atoms are small hard particles. Made of a single material that’s formed into different shapes and sizes. Made of a single material that’s formed into different shapes and sizes. They are always moving They are always moving They form different materials by joining together. They form different materials by joining together.

7. This theory was ignored and forgotten for more than 2000 years! This theory was ignored and forgotten for more than 2000 years!

8. Why? The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory. The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory. Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

10. What is an Atom? An atom is the smallest particle that an element can be divided and still be that element. For example the smallest particle of carbon is a single atom of carbon. If you divide it is no longer carbon anymore. For example the smallest particle of carbon is a single atom of carbon. If you divide it is no longer carbon anymore. PIECES OF CARBON CARBON ATOM Carbon atoms as seen through a scanning tunneling microscope

11. Dalton’s Model 1776-1844 In the early 1800s, the English Chemist and Schoolteacher John Dalton proposed a new atomic theory In the early 1800s, the English Chemist and Schoolteacher John Dalton proposed a new atomic theory His theory was based on experiments! His theory was based on experiments!

12. Dalton’s Theory 1. All elements are composed of atoms. 2. Atoms are indivisible and indestructible particles. 3. Atoms of the same element are exactly alike. 4. Atoms of different elements are different. 5. Compounds are formed by the joining of atoms of two or more elements.

13. Atoms and Electricity In 1839, English chemicst Michael Faraday suggested that the structure of atoms was related to electricity Experiments proved that he was right! Atoms contain particles that have electrical charge.

14. Atoms and Electricity The word electricity comes from the Greek word “elektron” which meant “amber.” The greeks knew that if you rubbed amber with a cloth, it would attract dust particles Today, this is known as static electricity Static electricity comes from electrical charges that are not in motion

15. Atoms and Electricity Benjamin Franklin derived several facts about electricity An object could have one of two kinds of charge, which he called (+) and (-). Like charges repel and opposite charges attract Some objects pick up excess negative charges and these charges may jump in a discharge of static electricity. (walk across a carpet and touch a doorknob) Lightning is static electricity on a larger scale Where do these charges come from?

16. Thomson’s Plum Pudding Model In 1897, the English scientist J.J. Thomson conducted a series of experiments with cathode ray tubes. Thomson discovered the electron

17. Thomson’s Cathode Ray Experiment Conclusions Cathode ray is composed of negative particles and these particles come from the cathode This meant that atoms were not indivisible, but had negative particles which he called electrons He determined the ratio of an electron’s charge to its mass

18. Thomson Model He proposed the “Plum Pudding” model. He proposed the “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

19. Rutherford’s Gold Foil Experiment 1908 1908 Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick) Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

20. Most of the positively charged “bullets” passed right through the gold foil Most of the positively charged “bullets” passed right through the gold foil Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges. Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

22. Rutherford’s Conclusions The gold atoms in the sheet were mostly empty space. Atoms were not a pudding filled with a positively charged material. The gold atoms in the sheet were mostly empty space. Atoms were not a pudding filled with a positively charged material. Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.” Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.” He called the center of the atom the “nucleus” He called the center of the atom the “nucleus” The nucleus is tiny compared to the atom as a whole. The nucleus is tiny compared to the atom as a whole.

23. Rutherford’s Model All of an atom’s positively charged particles were inside the nucleus. All of an atom’s positively charged particles were inside the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge. The negatively charged particles were scattered outside the nucleus around the atom’s edge.

24. Development of the Atomic Theory, Continued

25. Bohr Model 1913 1913 Niels Bohr placed each electron in a specific energy level. Niels Bohr placed each electron in a specific energy level.

26. Bohr Model electrons move in definite orbits around the nucleus, much like planets circle the sun. electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus. These orbits, or energy levels, are located at certain distances from the nucleus.

27. Schrodinger’s Wave Model

28. The Wave Model In today’s model, electrons do not move in a definite path, like the planets around the sun. In today’s model, electrons do not move in a definite path, like the planets around the sun.

29. The Wave Model It is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has. It is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has. According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral. According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

30. Electron Cloud: A space in which electrons are likely to be found. A space in which electrons are likely to be found. Electrons whirl about the nucleus billions of times in one second Electrons whirl about the nucleus billions of times in one second

31. Electron Cloud: Electrons with the lowest energy are found in the energy level closest to the nucleus Electrons with the lowest energy are found in the energy level closest to the nucleus Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus. Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

32. IndivisibleElectronNucleusOrbit Electron Cloud Greek X Dalton X Thomson X Rutherford X X Bohr X X X Wave X X X