Completing the Model of the Atom

1. Completing the Model of the Atom

2. ATOMIC THEORY In 460 B.C., a Greek philosopher, Democritus, develop the idea of atoms. He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.

3. Democritus (460-370 BC) • Proposed that matter is made of tiny particles and empty space • atoms = smallest part of matter • different types of atom for each type of matter

4. People considered Aristotle's opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also. For more than 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter.

5. Lavoisier (1743-1794) • Made measurements of a chemical change in a sealed container • law of conservation of matter

6. Proust (1799) • Observed that water is 11% H and 89% O. • Law of definite proportions Photo not available. definitions

7. Law of definite proportions • The principle that the elements that comprise a compound are always in a certain proportion by mass.

8. In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms). http://antoine.fsu.umd.edu/chem/senese/101/atoms/dalton.shtml

9. Postulates 1. All matter consists of tiny particles. 2.Atoms are indestructible and unchangeable. 3.Elements are characterized by the mass of their atoms. 4.When elements react, their atoms combine in simple, whole-number ratios.

11. J. J. Thomson(1897) • Discovered the electron and proton • Vacuum Tube Experiment

12. cathode - • The beam starts at the cathode. • Cathode ray tube. • CRT anode + power source

13. cathode - • The beam is bent away from a negatively charged plate. • Like charges repel. • The beam must be negative particles. anode + power source

14. cathode - • Positive particles were deflected from the anode. anode + power source

15. THOMSON’S MODEL In 1897, the English physicist J.J. Thomson discovered the electron and proposed a model for the structure of the atom. Thomson knew that electrons had a negative charge and thought that matter must have a positive charge. His model looked like raisins stuck on the surface of a lump of pudding.

16. The Gold Foil Experiment

17. Moments in History “You, lab tech, just stay here and count the flashes of light. I’m going out to have a drink with the other professors” - Rutherford to Geiger

18. “I say! It was like firing a gun at a tissue and having the bullet bounce back at you!

19. Moments in History “You’re wrong. Do you hear me? You’re wrong! I say!” -Rutherford to Thomson

20. Experiment Conclusion • The atom is mostly empty space. • The atom contains a small, dense positive core. • Nucleus • If the nucleus was the size of a golf ball the nearest electron would be one mile away. • model

21. RUTHERFORD’S MODEL Rutherford knew that atoms consist of a compact positively charged nucleus, around which circulate negative electrons at a relatively large distance. The nucleus occupies less than one thousand million millionth of the atomic volume, but contains almost all of the atom's mass. If an atom had the size of the earth, the nucleus would have the size of a football stadium.

22. Nucleus • The small, dense, positively charged central core of an atom.

23. SUBATOMIC PARTICLES negative smallest ELECTRONS positive PROTONS same mass neutral NEUTRONS

24. Comparing Subatomic Particles

25. Atomic Number 17 Cl Chlorine 35.45 • Equals the number of protons • defines the element • all chlorine atoms have 17 protons

26. Mass number 17 Cl Chlorine 35.45 • Protons + neutrons • Isotope have different masses • because they have a different # of neutrons • Chart mass is an average of the isotopes

27. Mass number 17 Cl Chlorine 35.45 • What is the unit for mass? • Atomic mass unit • symbol = u • based on carbon-12 • 1p+= 1n0 = 1u

28. Why aren’t the electrons attracted to the nucleus?

29. BOHR’S MODEL In 1912 a Danish physicist, Niels Bohr came up with a theory that said the electrons do not spiral into the nucleus and with some rules for what does happen. RULE 1: Electrons can orbit only at certain allowed distances from the nucleus. RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit.

30. BOHR-SOMMERFIELD’S MODEL According to the Bohr-Sommerfeld model, not only do electrons travel in certain orbits but the orbits have different shapes and the orbits could tilt in the presence of a magnetic field. Orbits can appear circular or elliptical, and they can even swing back and forth through the nucleus in a straight line.

31. QUANTUM MECHANICAL MODEL The visual concept of the atom now appeared as an electron "cloud" which surrounds a nucleus. The cloud consists of a probability distribution map which determines the most probable location of an electron.

32. ATOM divisions NUCLEUS ELECTRON CLOUD particles particles ELECTRONS PROTONS NEUTRONS charge charge NEGATIVE NEUTRAL POSITIVE Relative mass Relative mass is 1/1900 1 If atom is neutral is ATOMIC MASS (A) ATOMIC NUMBER (Z)

33. N. Bohr(1885-1962) • Proposed that the electrons orbited the nucleus . • The further away the more energy was needed. • Electrons only occupy orbits of certain energy.

34. WAVES • Carry energy not matter • Wavelength is the distance between corresponding points on the wave. • Wavelengths per second = frequency • 1 wavelength/second = 1 Hertz (Hz)

35. Red light has • low frequency • long wavelength • less energy The Modern Model • Visible light can be split into colored light. • Each color has its own • frequency • wavelength • energy • Blue light has • high frequency • short wavelength • more energy

36. Electromagnetic Spectrum

37. Electromagnetic Spectrum Gamma Rays….....Pass through most substances. X rays…....Pass through soft tissue, but not bone. UV light…….....causes sunburn, stopped by ozone Light……………………………………………………..visible Infrared………………………………………..radiant heat Radiowaves…..Carry the sounds of radio stations.

38. The Electromagnetic Spectrum

39. Electrons & Light • When energy is put into an atom, electrons absorb the energy and become “excited”. • Electrons can absorb only fixed amounts of energy. • When excited, electrons move into a higher energy level (state).

40. Electrons & Light • The electrons return to their original ground state. • The electrons must give off energy in the form of light. • This light has a specific frequency (and color.) • This frequency shows up as a line when seen through a prism.

41. Flame Test

42. The Modern ModelBright Line Spectra When atoms absorb energy, from fire or electricity, the electrons give off light. • When viewed through a prism a bright line spectra appears. • This is unique for every element. H Hg Ne

43. Energy levels and sublevels hydrogen The big gaps between spectral lines represent electron transitions from one energy level to another.

44. Energy levels and sublevels helium The groups of fine lines indicate that electrons are jumping from energy levels that are close in energy.

45. Illustration for Hydrogen Lines 7 6 INFRARED ENERGY LEVEL 5 ULTRAVIOLET 4 3 2 1

46. Quantum Theory Model(Electron Cloud Model) • Heisenberg Uncertainty Principle • You can never know exactly where an electron is if you know how fast it is moving. • If you know its exact location, you can’t know how fast it’s moving.