History of the Modern Atomic Theory

1. History of the Modern Atomic Theory Chapter 3 - Sections 3-1 and 3-2

2. Greek Philosophers • Democritus (400 B.C. ) – All matter is made of basic particles which are indivisible and are called “atomos” . • Aristotle (400 B.C.) – All matter is made of earth, fire, wind, and water. • Aristotle’s theory of earth, wind, fire, and water was believed until the early 1800’s.

4. Dalton’s Atomic Theory - 1808 1. All matter is composed of extremely small particles called atoms. 2. Atoms of a given element are identical in size, mass, and other properties. • Atoms cannot be subdivided, created, or destroyed. 4. Atoms of different elements can combine in simple, whole-number ratios to form chemical compounds. 5. Chemical reactions change the arrangement of atoms, but do not change atoms of one element into atoms of another element.

5. Dalton’s Atomic Theory Not all of the points of Dalton’s theory are correct: 2 – Dalton did not know about isotopes. 3 – Dalton did not know about subatomic particles.

7. Discovery of the Electron • 1897 J.J. Thomson (British) is credited with the discovery of the electron Used cathode ray tube and magnets • 1907 R.A. Millikan (University of Chicago) Mass of electron = 1/1837 mass of proton

9. Prediction of Proton • 1913 - J.J. Thompson found a negative particle and predicted the discovery of the proton a positive particle to make the atom neutral • JJ Thompson- Plum pudding model • Protons and electrons are arranged randomly in the atom.

10. Thompson’s Model of the Atom

12. Discovery of the Nucleus • 1911 • Lord Ernst Rutherford • Gold foil experiment • Found that most of the atom was empty space but there was a dense, positive area in the middle that he called the nucleus.

13. Rutherford shot alpha particles at a piece of gold foil. Most of the particles passed through the gold foil without deflection and only a small percentage bounced off the gold foil at a very large angle

15. Discovery of the Proton • 1919 • Ernst Rutherford is credited with the discovery • Found he could make one element into another by hitting the nucleus with a helium nucleus. A hydrogen nucleus was emitted by the process and this was found to be only a proton

16. Discovery of the Neutron • 1932 • Sir James Chadwick • Student of Ernst Rutherford • The atom was heavier than the total mass of the proton and the electron. The charge of the atom was neutral. • There must be a neutral particle about the size of the proton.

18. Bohr Model • Danish physicist Niels Bohr, 1913 • A student of Rutherford, developed a new atomic model. • Added new discoveries about how the energy of an atom changes when the atom absorbs or emits light. • Proposed that an electron is found only in specific circular paths, or orbits, around the nucleus • Discovered a Hydrogen atom absorbed light when an electron is excited from a low energy orbit into a higher energy orbit. Atoms can give off light when an electron drops from the high to the low energy orbit.

19. Bohr Hydrogen Experiment

20. Quantum Mechanical Model- • Current model of the atom • In the Bohr model of the atom, electrons move in circular orbits around the nucleus. • Scientists now know electrons DO NOT travel in orbits around the nucleus. • However, in our currently accepted model of the atom, we only know the probability of finding an electron in an electron cloud, which is a region of space around the nucleus.

21. Aristotle- 400BC Said all matter is made earth, fire, wind, and water 1897- JJ Thomson discovered electron using cathode ray tube 1911- Rutherford discovered the dense (+) charged nucleus- Gold Foil Experiment 1922- Niels Bohr Model of atom Democritus- 400BC Said all matter is made of “atomos” John Dalton- 1808 Came up with 5 postulates to the atomic theory. 1900- JJ Thomson’s plum pudding model. Protons and electrons are arranged randomly 1919- Rutherford discovered the proton 1940’s– Quantum Mechanic Model of atom 1932 Chadwick discovered the neutron

22. EXIT TICKET List the following scientists in order of their contributions to the structure of an atom. ____ Niels Bohr ____ James Chadwick ____ John Dalton ____ Democritus ____ Ernst Rutherford ____ J. J. Thompson

23. Day 2

24. The Atom • Nucleus • Proton- positively charged • Neutron- neutral • Outside the nucleus- Clouds • Electron- negatively charged

25. Subatomic particles

27. Important Definitions • Atomic Number (Z) – The number of protons in the nucleus • Mass #= protons + neutrons • Electrons= atomic # • Neutrons= mass # - atomic # Ions- Loss or gain of electrons Cation= + charged particle, lost an e- Anion= - charged particle, gained an e-

29. Important Definitions • Atomic Mass – The weighted average of the mass numbers of all of the naturally occurring isotopes of an element.

30. Important Definitions • Mass Number – The total number of protons and neutrons in the nucleus of an isotope. Isotope # p+ # e- # n Mass number

31. Important Definitions • Isotope – Two or more atoms of the same element that have the same number of protons and electrons but a different number of neutrons. • Nuclide – A general term for an isotope of any element. • Every element has at least one isotope and some elements have many isotopes

32. General Format of Isotopes • Hydrogen -3 • Oxygen-17 • Lithim-5 • Carbon-12

33. Hyphen Notation

34. Write the general and hyphen notation

36. Which of the following pairs are isotopes? A. C. B. D.

37. Isotopes of chlorine Chorine-35 accounts for about 75 percent of the naturally occurring chlorine atoms; chlorine-37 accounts for only about 25 percent.

38. Natural percent abundance Natural Percent Abundance of Stable Isotopes of Some Elements

39. Atomic Mass Practice • Calculate the average atomic mass of nitrogen. • Nitrogen-14 has a mass of 14.003 amu and a natural % abundance of 99.63 • Nitrogen-15 has a mass of 15.00 amu and a natural % abundance of 0.37 ( 14.003 x .9993) + (15 x .0037)

40. Chromium has 4 isotopes. Fill in the remainder of the chart and calculate the average atomic mass of chromium