Chapte r 4

1. Chapter 4 The Structure of the Atom

2. Early Theories of Matter • The concept of the atom developed over a few thousand years. • Consider that there were no controlled experiments and few tools for scientific exploration.

3. The power of the mind and intellectual thought were the primary ways to truth about the universe. • Curiosity drove discovery • Philosophers, scholarly thinkers, speculated about the nature of matter. • Ideas came from their own life experiences.

4. Early Greek philosophers thought that matter was made of Earth, Wind, Water, and Fire.

5. Commonly accepted that matter could be divided endlessly into smaller and smaller pieces….. • The Continuous Theory of Matter • No methods to test the validity of these ideas.

6. Democritus • a Greek Philosopher • the first to propose the idea that matter was not infinitely divisible. discontinuous theory • He proposed the idea that matter was made up of tiny individual particles called atomos. 

7. The English word for atom comes from this Greek word atomos. •  He believed that atoms could not be created, destroyed, or further divided (discontinuous theory).

8. His belief in atoms was amazingly accurate. • However, his ideas were met with much criticism from other philosophers. • The harshest criticism was from the influential Greek philosopher, Aristotle.

9. Aristotle • Aristotle rejected the theory of atoms because it did not agree with own ideas on nature. • He did not believe that atoms could move through empty space. • He did not believe that the “nothingness” of empty space could exist.

10. Democritus Rejected • Because Democritus had no way to answer the challenges to his ideas, his theory was eventually rejected. • It is important to realize that these ideas were just that—ideas and not science. • Incredibly, the ideas of Aristotle were so great and the science so primitive that his denial of the existence of the atom went unchallenged for two thousand years

11. John Dalton • In the 19th century John Dalton • an English School teacher • revived and revised the work of Democritus. • This time it was based on scientific research conducted by Dalton.

12. Dalton’s Atomic Theory • 1808 • marks the beginning of modern atomic theory.

13. Dalton was wrong about: • the atom being divisible (atoms are divisible into subatomic particles) • all atoms of an element having identical properties. (atoms of an element can have slightly different masses.)

14. Dalton’s Model of the Atom

15. Defining the Atom • An atom is the smallest part of an element that retains the characteristics of the element. • How small is a typical atom? • In the year 2000, the world population was about 6 billion people. • In comparison, a copper penny contain about 5 billion times as many atoms of copper. • World population 6 000 000 000 • Atoms in a Penny 29 000 000 000 000 000 000 000 • Diameter of a copper atom 1.28 x 10-10 m

16. Next Set of Questions • What is an atom like? • How are atoms shaped? • Is the atom composed of other particles?

17. The Discovery of the Electron • British scientist named J. J.Thompson • Used a glass tube, called a cathode-ray tube, connected to two electrodes.

18. Cathode Ray Tube

19. One electrode was attached to one end of the tube. • It was negative in charge and was thus called the cathode. • At the other end was attached a positive electrode, the anode.

21. Thomson’s Experiment • Thomson placed a paddle wheel inside the tube, between the cathode and the anode. • He passed an electric current through a variety of gases in the tube. The gases within the tube were changed with each experiment.

22. Thomson made the following observations • The current passing through the tube creates different colors in different gases. • The anode (positive end) glows where as the cathode (negative end) does not. •  An object placed within the cathode-ray casts a shadow. • The paddle-wheel spins in the direction of the anode.

23. Conclusions: • The different colors that were created by using different gases showed that atoms of different elements possessed different energies.   • The cast shadow was thought to be due to the beam of light created by the cathode-ray. However, the experiment made with the spinning paddle-wheel showed that the cathode-ray was composed of particles. • The fact that the anode glowed and the cathode did not shows that the cathode-ray travels from a negative potential to a positive potential. This was one of the most important findings of J. J. Thomson's experiment and was repeated using another apparatus.

24. To test the polarity of the cathode ray, Thompson replaced the paddle-wheel with another pair of electrodes. • When the current was passed through this new apparatus, Thompson found that the cathode-ray was "bent" towards the positive electrode and repelled by the negative electrode.   • Thompson attributed the "bending" of the cathode-ray to charge-charge repulsion of the second cathode and the negatively charged particles in the cathode-ray

26. Electrons • He named these negatively charged particles electrons. • Since Thompson could get these results regardless of what element he used within the tube, he concluded that all atoms have electrons.

27. Thomson’s Model of the Atom • The Plum Pudding Model

28. Oil Drop Experiment • In 1909, American scientist, Robert Millikan, determined the charge of an electron in his famous Oil Drop Experiment.

29. Discovery of the Nucleus • 1911 Ernst Rutherford • Rutherford’s Gold Foil Experiment • determined that nearly all of the mass of the atom consisted of a positively charged nucleus

30. Rutherford’s Gold Foil Experiment • He did this by measuring the deflection of heliumnuclei by gold atoms.

31. Predicted Results

32. While most atoms were not deflected much at all, a few were deflected by 180 degrees.

33. How was this possible? • Rutherford calculated that the only way this was possible was if the gold atoms consisted of a cloud of electrons surrounding the very dense, positively charged nucleus, for only then could the gold atom transfer enough momentum to the ions to turn them around.

34. Rutherford’s Conclusions • Atoms are mostly empty space and the nucleus is incredibly dense. • If the nucleus were a marble, the atom would be the size of a football field

35. Rutherford ‘s Nuclear Model  • By 1919 Rutherford refined his concept of the nucleus and concluded that it contained protons. • Protons have a charge of +1. • This model could not account for the mass of the atom. • 20 more years before this mystery was solved.

36. Discovery of the Neutron • 1932 • James Chadwick, an English Physicist • He showed that the nucleus also contained another particle, the neutron • The neutron is neutral, meaning it has no charge. • The neutron has a mass equal to the proton.

37. Composition of the Nucleus • Protons • Mass of the proton = 1.673 X 10-24grams • Positive Charge • Neutrons • Mass = 1.675 X 10-24grams • No charge 

38. Properties of Subatomic Particles

39. Nuclear Model of the Atom • An atom is an electrically neutral particle composed of protons, neutrons, and electrons. • Atoms are spherical in shape with a tiny, dense nucleus of positive charge surrounded by one or more negatively charged electrons • Nucleus contains 99.7 % of the mass of an atom.

40. Subatomic Particles • Most of the atom consists of fast-moving electrons traveling through the space around the nucleus. • The number of protons is equal to the number of electrons since atoms are neutral in charge. • The number of Protons in the nucleus determines the identity of an atom. (Atomic Number) • An Element consists of atoms with the same number of protons.

41. How Atoms Differ • Henry Moseley discovered that atoms of each element contain a unique positive charge in their nucleus. • Therefore, the proton number of an atom identifies the atom. • The number of protons in an atom is known as atomic number. • Example: All Hydrogen atoms have one proton

42. Atomic Number(Z): the number of protons in the nucleus of each atom ofthe element • Atomic Number = Protons • Elements are arranged in the periodic table from left to right in order of increasing atomic number. • Atomic number identifies the element.

43. Atoms are neutral, therefore: # of protons = # of electrons 

44. Isotopes and Mass Number • Naturally occurring elements are a mixture of atoms that have different numbers of neutrons. • Atoms with the same number of protons, but different number of neutrons are called Isotopes.

45. Hydrogen Has Three Isotopes: • Protium 1 proton 1 electron 0 neutrons • Deuterium 1 proton 1 electron 1 neutron • Tritium 1 proton 1 electron 2 neutrons

46. Isotopes and Mass Number • Isotopes: atoms of the same element that have different masses. • Different number of neutrons so mass is different. • Atoms of different isotopes have different masses so identity is given by name and mass. • Mass Number: the total number of protons and neutrons in the nucleus of an isotope

47. Isotope (Nuclear) Symbols: Consists of three parts • the symbol of the element • the atomic number of the element • the mass number of the specific isotope.

48. Nuclear Symbol

49. Isotope Names Element Name-Mass Number Examples: Helium-4 Potassium-39 Hydrogen-3

50. Neutrons = Mass Number – Atomic Number Atomic Number: _________ Mass Number: __________ # of Protons: _________ # of Electrons: _______ # of Neutrons: ________ Name of Isotope: ____________