1. Matter is Made up of Atoms Chapter 2

2. Atoms and Their Structure Section 2.1

4. HISTORY OF THE ATOM 460 BC Democritus develops the idea of atoms He pounded up materials in his mortar and pestle until he had reduced them to smaller and smaller particles which he called ATOMA (greek for indivisible)

5. HISTORY OF THE ATOM 460 BC Democritus develops the idea of atoms The idea that matter is made up of fundamental particles called atoms is known as the atomic theory of matter.

6. The well-known 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.

7. In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic Theory He observed that the mass of reactants in the container before a chemical reaction was equal to the mass of the products after the reaction.

8. Lavoisier’s conclusion became known as the law of conservation of mass. Lavoisier concluded that when a chemical reaction occurs, mass is neither created nor destroyed but only changed. Development of the Modern Atomic Theory

9. In 1799, another French chemist, Joseph Proust, observed that the composition of water is always 11 percent hydrogen and 89 percent oxygen by mass. Regardless of the source of the water, it always contains these same percentages of hydrogen and oxygen. Development of the Modern Atomic Theory

10. Proust studied many other compounds and observed that the elements that composed the compounds were always in a certain proportion by mass. This principle is now referred to as the law of definite proportions

11. John Dalton (1766- 1844), an English schoolteacher and chemist, studied the results of experiments by Lavoisier, Proust, and many other scientists. Dalton’s Atomic Theory

12. The following statements are the main points of Dalton’s atomic theory. Dalton’s Atomic Theory 1. All matter is made up of atoms. 2. Atoms are indestructible and cannot be divided into smaller particles (Atoms are indivisible). 3. All atoms of one element are exactly alike, but are different from atoms of other elements.

13. Dalton’s Atomic Theory Proposed his atomic theory of matter in 1803. Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that it has remained essentially intact up to the present time.

14. HISTORY OF THE ATOM 1897 Joseph John Thompson British physicist, J.J. Thomson, discovered that Dalton’s solid-ball model was not accurate. Thomson’s experiments used a vacuum tube.

15. A vacuum tube has had all gases pumped out of it. At each end of the tube is a metal piece called an electrode, which is connected through the glass to a metal terminal outside the tube. These electrodes become electrically charged when they are connected to a high-voltage electrical source. Cathode-Ray Tube

16. When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode. Cathode-Ray Tube Because these rays originate at the cathode, they are called cathode rays.

17. Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate. Cathode-Ray Tube Thomson concluded that cathode rays are made up of invisible, negatively charged particles referred to as electrons.

18. His idea was that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's charge like plums surrounded by pudding. 1904 PLUM PUDDING MODEL

19. From Thomson’s experiments, scientists had to conclude that atoms were not just neutral spheres, but somehow were composed of electrically charged particles. Cathode-Ray Tube If atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles.

20. In 1886, scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode. Years later, scientists determined that the rays were composed of positively charged subatomic particles called protons. Thomson was able to show that these rays had a positive electrical charge.

21. At this point, it seemed that atoms were made up of equal numbers of protons and electrons. However, in 1910, Thomson discovered that neon consisted of atoms of two different masses.

22. Today, chemists know that neon consists of three naturally occurring isotopes. Atoms of an element that are chemically alike but differ in mass are called isotopes of the element.

23. Because of the discovery of isotopes, scientists hypothesized that atoms contained still a third type of particle that explained these differences in mass. The existence of this neutral particle, called a neutron, was confirmed in the early 1930s.

24. Rutherford’s Gold Foil Experiment In 1909, 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 cookie-dough model of the atom.

25. Rutherford’s Gold Foil Experiment Conducted an experiment to see if alpha particles would be deflected as they passed through a thin foil of gold. Rutherford was aware of Thomson’s “plum pudding” model and expected only minor deflections of alpha particles.

27. Rutherford’s Model Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space. To explain the results of the experiment, Rutherford’s team proposed a new model of the atom.

28. Rutherford’s Model He suggested that the positive charge was all in the central nucleus. With this holding the electrons in place by electrical attraction However, this was not the end of the story…

29. HISTORY OF THE ATOM 1913 Niels Bohr Studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's idea by adding that the electrons were in orbits. Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons.

30. Bohr’s Atom electrons in orbits nucleus

31. ATOMIC STRUCTURE Particle proton neutron electron Charge + charge - charge No charge 1 1 0 Mass

32. It is the number of protons that determines the identity of an element, as well as many of its chemical and physical properties. The atomic number of an element is the number of protons in the nucleus of an atom of that element. Atomic Numbers

33. Therefore, the atomic number of an element also tells the number of electrons in a neutral atom of that element. Because atoms have no overall electrical charge, an atom must have as many electrons as there are protons in its nucleus. Atomic Numbers

34. The sum of the protons and neutrons in the nucleus is the mass number of that particular atom. The mass of a neutron is almost the same as the mass of a proton. Masses

35. Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number. Masses

36. Atomic Mass In order to have a simpler way of comparing the masses of individual atoms, chemists have devised a different unit of mass called an atomic mass unit, which is given the symbol u. An atom of the carbon-12 isotope contains six protons and six neutrons and has a mass number of 12.

37. Atomic Mass Chemists have defined the carbon-12 atom as having a mass of 12 atomic mass units. Therefore, 1 u = 1/12 the mass of a carbon- 12 atom. 1 u is approximately the mass of a single proton or neutron.

38. Information in the Periodic Table The number at the bottom of each box is the average atomic mass of that element. This number is the weighted average mass of all the naturally occurring isotopes of that element.

39. 3545358035Br 1822184018Ar 20 4020Ca e–e– n0n0 p+p+ MassAtomic