Introduction to Atomic Theory: Dalton and the Basics of Atoms

1. AHS Chemistry Unit 2Atomic Structure The Basics of Atoms and the Periodic Chart

2. Democritus Nothing exists except atoms and empty space; everything else is opinion Born: about 460 BC in Abdera, Thrace, GreeceDied: about 370 BC atomic theory Atomic theory is the theory that matter and space are not infinitely divisible.

3. Atomists hold that the absolute existents are the ultimate parts or components of visible bodies, though these absolute existents can exist separately from these visible bodies. These existents are completely full or solid; they contain no gaps, no holes, no empty places. They cannot be subdivided, cut, cracked, split, or penetrated; for this would imply that nonbeing has gained a foothold in them. These absolute existents are called atoms, from the Greek word atomon, which means "that which cannot be divided." Atoms do not come into being or pass away; for if they come into being (or pass away), then they would have to partially exist and partially not exist; but partial existence makes no sense for ultimate things

4. John Dalton 1803 John Dalton proposes the atomic theory of matter.

5. 1780's, Lavoisier ushered in a new chemical era by making careful quantitative measurements which allowed the compositions of compounds to be determined with accuracy By 1799 enough data had been accumulated for Proust to establish the Law of Constant Composition ( also called the Law of Definite Proportions). In 1803 Dalton noted that oxygen and carbon combined to make two compounds. Of course, each had its own particular weight ratio of oxygen to carbon (1.33:1 and 2.66:1), but also, for the same amount of carbon, one had exactly twice as much oxygen as the other. This led him to propose the Law of Simple Multiple Proportions, which was later verified by the Swedish chemist Berzelius.

6. In an attempt to explain how and why elements would combine with one another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated his atomic theory. Assumption One: His atomic theory, stated that elements consisted of tiny particles called atoms Assumption Two: He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass Assumption Three: He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses

7. Assumption Four: Atoms of different elements may have nearly identical masses Assumption Five: Atoms are not subdivided in a chemical reaction On Compounds: He also said that compounds consisted of atoms of different elements combined together Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms.

8. Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were.

9. Atoms & Molecules Atoms • can exist alone or enter into chemical combination • the smallest indivisible particle of an element Molecules • a combination of atoms that has its own characteristic set of properties

10. Law of Conservation of Matter In an ordinary chemical reaction matter is neither created nor destroyed. The sum of the masses of the reactants equals the sum of the masses of the products.

11. Law of Constant Composition A chemical compound always contains the same elements in the same proportions by mass. Law of Multiple Proportions compounds can be formed by the same elements combining in different ratios. For Example CO2 and CO or H2O and H2O2 Movie

12. The particles in the beam are negative, what will happen as they pass through the electrically charged plates?

13. The cathode ray is Negative, describe what is happening here?

14. Another look at the Cathode Ray set up.

15. Describe what is happening in each situation a, b, and c.

16. Watch the Movie Thomson discovered the ratio of charge over mass of an electron to be 1.7588196 x 10^11 C kg^-1. Later, the concept of the electron was furthered by the American physicist Robert Millikan, who defined e, the charge of the electron, as 1.6021773 x 10^-19 Coulombs. Knowing e and the ration e/me, the mass of the electron was found to be 9.109390 x 10^-31kg Watch the Movie

17. Animated Model Movie

18. Animated Model

19. Rutherford’s Model of the Atom • atom is composed mainly of vacant space • all the positive charge and most of the mass is in a small area called the nucleus • electrons are in the electron cloud surrounding the nucleus

20. Structure of the AtomComposed of: • protons • neutrons • electrons

21. Composed of: protons neutrons electrons protons found in nucleus relative charge of +1 relative mass of 1.0073 amu Structure of the Atom

22. Composed of: protons neutrons electrons neutrons found in nucleus neutral charge relative mass of 1.0087 amu Structure of the Atom

23. Composed of: protons neutrons electrons electrons found in electron cloud relative charge of -1 relative mass of 0.00055 amu Structure of the Atom

24. Animated Model

25. Radioactivity A movie about radioactive particles!

26. Alpha Radiation • composed of 2 protons and 2 neutrons • thus, helium-4 nucleus • +2 charge • mass of 4 amu • creates element with atomic number 2 lower

27. Beta Radiation • composed of a high energy electron which was ejected from the nucleus • “neutron” converted to “proton” • very little mass • -1 charge • creates element with atomic number 1 higher

28. Gamma Radiation • nucleus has energy levels • energy released from nucleus as the nucleus changes from higher to lower energy levels • no mass • no charge

29. Ions • charged single atom (monatomic ions) • charged cluster of atoms (polyatomic ions) Atoms become ions when they gain or lose an electron. When Na gives up an electron it has one more protons than electrons thus you get Na+1 When Cl gains an electron you have one more electron than protons thus you get Cl-1

30. Ions • cations • positive ions • anions • negative ions • ionic compounds • combination of cations and anions • zero net charge

31. Atomic number, Z • the number of protons in the nucleus • the number of electrons in a neutral atom • the integer on the periodic table for each element

32. Isotopes • atoms of the same element which differ in the number of neutrons in the nucleus • designated by mass number

33. Mass Number, A • integer representing the approximate mass of an atom • equal to the sum of the number of protons and neutrons in the nucleus

34. Masses of Atoms Carbon-12 Scale

35. Isotopes of Hydrogen H-1, 1H, protium • 1 proton and no neutrons in nucleus • only isotope of any element containing no neutrons in the nucleus • most common isotope of hydrogen

36. Isotopes of Hydrogen H-2 or D, 2H, deuterium • 1 proton and 1 neutron in nucleus

37. Isotopes of Hydrogen H-3 or T, 3H, tritium • 1 proton and 2 neutrons in nucleus

39. Isotopes of Oxygen O-16 • 8 protons, 8 neutrons, & 8 electrons O-17 • 8 protons, 9 neutrons, & 8 electrons O-18 • 8 protons, 10 neutrons, & 8 electrons

40. The radioactive isotope 14C has how many neutrons? 6, 8, other

41. The identity of an element is determined by the number of which particle? protons, neutrons, electrons

42. The mass spectrometer measures the relative atomic mass of atoms

43. The percentage of atoms at certain masses are collected by the mass spectrometer and the abundance of each isotope is then known by the frequency of the “hits” on the collector plate at different mass numbers

44. Measurement of Atomic Masses Mass Spectrometer a simulation is available at http://www.colby.edu/chemistry/ OChem/DEMOS/MassSpec.html

45. Atomic Masses andIsotopic Abundances natural atomic masses = sum[(atomic mass of isotope) *(fractional isotopic abundance)]

46. Amadeo Avagadro Amadeo Avagadro was born on June 9, 1776 in Turin - the capital city of the independent nation of Piedmont. He began his career as a lawyer, earning his doctorate of law in 1796. With Gay-Lussac's discovery in 1802 that all gases expand at the same rate with a change in temperature, Avogadro gave up law and turned to the world of science. In 1811, he showed that Dalton's Atomic theory and Gay-Lussac's Gas Law could exist together if there was, at a constant temperature and pressure, an equal number of particles in a given volume of a gas. This became known as Avogadro's Hypothesis. In developing this idea, he stated that the particles did not need to be separate atoms but could exist as a combination of atoms, which he called a molecule. He also proposed that certain gases (hydrogen and oxygen) existed as diatomic molecules.

47. Unfortunately, Avogadro's theories were not readily accepted due to the new terminology he used and the concepts developed by Berzelius, which were the accepted norm at the time. Not until Stanislao Cannizzaro presented Avogadro's theories at a convention of chemist in 1860 did the world begin to accept Avogardro's Hypothesis. Unfortunately Avogadro did not live to receive this acclaim, dying in obscurity on July 9, 1856 in his hometown of Turin.

48. The Mole • a unit of measurement, quantity of matter present • Avogadro’s Number 6.022 x 1023 particles • Latin for “pile”