1. Atomos: Not to Be Cut The History of Atomic Theory

2. Atomic Models This model of the atom may look familiar to you. This is the Bohr model. In this model, the nucleus is orbited by electrons, which are in different energy levels. This model of the atom may look familiar to you. This is the Bohr model. In this model, the nucleus is orbited by electrons, which are in different energy levels. A model uses familiar ideas to explain unfamiliar facts observed in nature. A model uses familiar ideas to explain unfamiliar facts observed in nature. A model can be changed as new information is collected. A model can be changed as new information is collected.

3. The Beginning…. The atomic model has changed throughout the centuries, starting in 400 BC, when it looked like a billiard ball → The atomic model has changed throughout the centuries, starting in 400 BC, when it looked like a billiard ball →

4. 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.

5. Democritus This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? 460-370 BC

6. 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.”

7. Atomos  To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.  Atoms were infinite in number, always moving and capable of joining together.

8. Long pause….. This theory was ignored and forgotten for more than 2000 years! This theory was ignored and forgotten for more than 2000 years!

9. 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.

11. Dalton’s Model In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms. In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

12. Dalton’s Theory He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles. He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles. Atoms of the same element are exactly alike. Atoms of the same element are exactly alike. Atoms of different elements are different. Atoms of different elements are different. Compounds are formed by the joining of atoms of two or more elements. Compounds are formed by the joining of atoms of two or more elements.

13. Dalton’s Theory All atoms of a given element are identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from those of any other element. All atoms of a given element are identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from those of any other element.

14. Dalton’s Theory In a chemical reaction, atoms are separated, combined, or rearranged. In a chemical reaction, atoms are separated, combined, or rearranged.

15. . This theory became one of the foundations of modern chemistry. This theory became one of the foundations of modern chemistry.

16. William Crookes’s discovery In 1879, English physicist Sir William Crookes accidentally discovers a cathode ray. In 1879, English physicist Sir William Crookes accidentally discovers a cathode ray. This discovery led to the invention of the television in the 20 th century This discovery led to the invention of the television in the 20 th century It also led to J.J. Thomson’s discovery of the electron It also led to J.J. Thomson’s discovery of the electron

17. Thomson’s Plum Pudding Model In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles. In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.

18. Thomson Model Thomson studied the passage of an electric current through a gas. Thomson studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles. As the current passed through the gas, it gave off rays of negatively charged particles.

19. Thomson Model This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from? This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from? Where did they come from?

20. Thomson concluded that the negative charges came from within the atom. A particle smaller than an atom had to exist. The atom was divisible! Thomson called the negatively charged “corpuscles,” today known as electrons. Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom. But he could never find them.

21. Thomson Model He proposed a model of the atom that is sometimes called the “Plum Pudding” model. He proposed a model of the atom that is sometimes called 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.

22. Robert Millikan (1868-1953) In 1909, American physicist Robert Millikan conducted his famous oil drop experiment In 1909, American physicist Robert Millikan conducted his famous oil drop experiment He determined the charge of the electron to be negative He determined the charge of the electron to be negative Mass of an electron = 9.1 x 10 -28 g Mass of an electron = 9.1 x 10 -28 g

23. Rutherford’s Gold Foil Experiment In 1911, the English physicist Ernest Rutherford was hard at work on an experiment that seemed to have little to do with unraveling the mysteries of the atomic structure. In 1911, the English physicist Ernest Rutherford was hard at work on an experiment that seemed to have little to do with unraveling the mysteries of the atomic structure.

24. 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) Gold foil experiment, 1911 Gold foil experiment, 1911

26. Gold Foil Experiment Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all. Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all. 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.

27. Animation http://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.html http://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.html http://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.html http://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.html http://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.html http://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.html http://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.html http://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.html http://www.mhhe.com/physsci/chemistry/e ssentialchemistry/flash/ruther14.swf http://www.mhhe.com/physsci/chemistry/e ssentialchemistry/flash/ruther14.swf http://www.mhhe.com/physsci/chemistry/e ssentialchemistry/flash/ruther14.swf http://www.mhhe.com/physsci/chemistry/e ssentialchemistry/flash/ruther14.swf

28. Results…. This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material. This could only mean that the gold atoms in the sheet were mostly open 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.

29. Rutherford Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge. Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge.

30. Rutherford’s contribution By 1920, eight years after his revolutionary gold foil experiment, Rutherford had refined the concept of the nucleus. By 1920, eight years after his revolutionary gold foil experiment, Rutherford had refined the concept of the nucleus. He concluded that the nucleus contained positively charge particles called protons. He concluded that the nucleus contained positively charge particles called protons. A proton is a subatomic particle carrying a charge equal to but opposite that of an electron. It has a charge of 1+. A proton is a subatomic particle carrying a charge equal to but opposite that of an electron. It has a charge of 1+.

31. Bohr Model In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level. In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level.

32. Bohr Model According to Bohr’s atomic model, 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. According to Bohr’s atomic model, 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.

33. Wave model or quantum mechanical model Bohr’s model was replaced by today’s modern quantum mechanical, or wave model, of the atom. Many contributed to this model: L. de Broglie L. de Broglie W. Heisenberg W. Heisenberg E. Schrodinger E. Schrodinger

34. Wave Model

35. Louis de Broglie, 1924 De Broglie, a young French graduate student proposed an idea that eventually accounted for the fixed energy levels of Bohr’s model. De Broglie, a young French graduate student proposed an idea that eventually accounted for the fixed energy levels of Bohr’s model. He discovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. Supported Einstein. He discovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. Supported Einstein. He developed an equation which predicts that all moving particles have wave characteristics. He developed an equation which predicts that all moving particles have wave characteristics.

36. Werner Heisenberg, 1927 Described atoms by means of formula connected to the frequencies of spectral lines. Described atoms by means of formula connected to the frequencies of spectral lines. Proposed the Heisenberg Uncertainty Principle - you cannot know both the position and velocity of a particle at the same time. Proposed the Heisenberg Uncertainty Principle - you cannot know both the position and velocity of a particle at the same time.

37. Erwin Schroedinger, 1930 Austrian physicist. Austrian physicist. Viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom. Viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom.

38. The Wave Model Today’s atomic model is based on the principles of wave mechanics. Today’s atomic model is based on the principles of wave mechanics. According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun. According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.

39. The Wave Model In fact, 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. In fact, 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.

40. 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 They are not moving around in random patterns. They are not moving around in random patterns. Location of electrons depends upon how much energy the electron has. Location of electrons depends upon how much energy the electron has.

41. Electron Cloud: Depending on their energy they are locked into a certain area in the cloud. Depending on their energy they are locked into a certain area in the 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.

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

43. James Chadwick, 1932 Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron.

44. Atoms in a crystal as seen through a Scanning Tunneling Microscope (STM)