1. Atoms: The Building Blocks of Matter

2. Foundations of Atomic Theory • Nearly all chemists in late 1700s accepted the definition of an element as a substance that cannot be broken down further • Knew about chemical reactions • Great disagreement as to whether elements always combine in the same ratio when forming a specific compound

3. Law of Conservation of Mass • With the help of improved balances, investigators could accurately measure the masses of the elements and compounds they were studying • This lead to discovery of several basic laws • Law of conservation of mass states that mass is neither destroyed nor created during ordinary chemical reactions or physical changes

4. Law of Definite Proportions • law of definite proportions  A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound Each of the salt crystals shown here contains exactly 39.34% sodium and 60.66% chlorine by mass.

5. Law of Multiple Proportions • Two elements sometimes combine to form more than one compound • For example, the elements carbon and oxygen form two compounds, carbon dioxide and carbon monoxide • Consider samples of each of these compounds, each containing 1.0 g of carbon

6. In carbon dioxide, 2.66 g of oxygen combine with 1.0 g of carbon • In carbon monoxide, 1.33 g of oxygen combine with 1.0 g of carbon

7. The ratio of the masses of oxygen in these two compounds is exactly 2.66 to 1.33, or 2 to 1

8. 1808 John Dalton • Proposed an explanation for the law of conservation of mass, the law of definite proportions, and the law of multiple proportions

9. He reasoned that elements were composed of atoms and that only whole numbers of atoms can combine to form compounds

10. His theory can be summed up by the following statements

11. Dalton’s Atomic Theory • All matter is composed of extremely small particles called atoms. • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. • Atoms cannot be divided, created or destroyed. • Atoms of different elements combine in simple whole-number ratios to form chemical compounds. • In chemical reactions, atoms are combined, separated, or rearranged.

12. Modern Atomic Theory • Dalton turned Democritus’sidea into a scientific theory which was testable • Not all parts of his theory have been proven correct

13. Ex. We know atoms are divisible into even smaller particles • We know an element can have atoms with different masses

14. Structure of the Atom

15. All atoms consist of two regions • Nucleus very small region located near the center of an atom • In the nucleus there is at least one positively charged particle called the proton • Usually at least one neutral particle called the neutron • Surrounding the nucleus is a region occupied by negatively charged particles called electrons

16. Discovery of the Electron • Resulted from investigations into the relationship between electricity and matter • Late 1800s, many experiments were performed  electric current was passed through different gases at low pressures

17. These experiments were carried out in glass tubes known as cathode-ray tubes

18. Cathode Rays and Electrons • Investigators noticed that when current was passed through a cathode-ray tube, the opposite end of the tube glowed

19. Hypothesized that the glow was caused by a stream of particles, which they called a cathode ray • The ray traveled from the cathode to the anode when current was passed through the tube

20. Observations • 1. cathode rays deflected by magnetic field in same way as wire carrying electric current (known to have negative charge) • 2. rays deflected away from negatively charged object

21. Observations led to the hypothesis that the particles that compose cathode rays are negatively charged • Strongly supported by a series of experiments carried out in 1897 by the English physicist Joseph John Thomson

22. He was able to measure the ratio of the charge of cathode-ray particles to their mass • He found that this ratio was always the same, regardless of the metal used to make the cathode or the nature of the gas inside the cathode-ray tube

23. Thomson concluded that all cathode rays are composed of identical negatively charged particles, which were later named electrons

24. Charge and Mass of the Electron • Confirmed that the electron carries a negative electric charge • Because cathode rays have identical properties regardless of the element used to produce them, it was concluded that electrons are present in atoms of all elements

25. Cathode-ray experiments provided evidence that atoms are divisible and that one of the atom’s basic constituents is the negatively charged electron

26. Thomson’s experiment revealed that the electron has a very large charge for its tiny mass • Mass of the electron is about one two-thousandth the mass of the simplest type of hydrogen atom (the smallest atom known) • Since then found that the electron has a mass of 9.109 × 10−31 kg, or 1/1837 the mass of the hydrogen atom

27. Based on information about electrons, two inferences made about atomic structure • b/c atoms are neutral they must have positive charge to balance negative electrons • b/c electrons have very little mass, atoms must have some other particles that make up most of the mass

28. Thomson’s Atom • Plum pudding model (based on English dessert) • Negative electrons spread evenly through positive charge of the rest of the atom • Like seeds in a watermelon

29. Discovery of Atomic Nucleus • 1911 by New Zealander Ernest Rutherford and his associates Hans Geiger and Ernest Marsden • Bombarded a thin, gold foil with fast-moving alpha particles (positively charged particles with about four times the mass of a hydrogen atom)

30. Assume mass and charge were uniformly distributed throughout atoms of gold foil (from Thomson’s model of the atom) • Expected alpha particles to pass through with only slight deflection

31. What Really Happened… • Most particles passed with only slight deflection • However, 1/8,000 were found to have a wide deflection

32. Rutherford explained later it was “as if you have fired a 15-inch artillery shell at a piece of tissue paper and it came back and hit you.”

34. Explanation • After 2 years, Rutherford finally came up with an explanation • The rebounded alpha particles must have experienced some powerful force within the atom

35. The source of this force must occupy a very small amount of space because so few of the total number of alpha particles had been affected by it

36. The force must be caused by a very densely packed bundle of matter with a positive electric charge • Rutherford called this positive bundle of matter the nucleus

37. Rutherford had discovered that the volume of a nucleus was very small compared with the total volume of an atom • If the nucleus were the size of a marble, then the size of the atom would be about the size of a football field • But where were the electrons?

38. Rutherford suggested that the electrons surrounded the positively charged nucleus like planets around the sun • He could not explain, however, what kept the electrons in motion around the nucleus

39. Rutherford’s Atom

40. Composition of Atomic Nucleus • Except hydrogen, all atomic nuclei made of two kinds of particles • Protons • Neutrons

41. Protons = positive • Neutrons = neutral • Electrons = negative

42. Atoms are electrically neutral, so number of protons and electrons IS ALWAYS THE SAME

43. The nuclei of atoms of different elements differ in the number of protons they contain and therefore in the amount of positive charge they possess • So the number of protons in an atom’s nucleus determines that atom’s identity

44. Forces in the Nucleus • Usually, particles that have the same electric charge repel one another • Would expect a nucleus with more than one proton to be unstable

45. When two protons are extremely close to each other, there is a strong attraction between them

46. Nuclear forces short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together

47. The Sizes of atoms • Area occupied by electrons is electron cloud – cloud of negative charge • Radius of atom is distance from center of nucleus to outer portion of cloud

48. Unit – picometer (10-12 m) • Atomic radii range from 40-270 pm • Very high densities – 2 x 108 tons/cm3

49. Counting Atoms

50. Atomic Number • Atoms of different elements have different numbers of protons • Atomic number (Z) number of protons in the nucleus of each atom of that element • Shown on periodic table • Atomic number identifies an element 3 Li Lithium 6.941 [He]2s1