Modern Atomic Theory and the Periodic Table

1. Modern Atomic Theoryand the Periodic Table

2. Outline A Brief HistoryElectromagnetic Radiation and the Electromagnetic SpectrumThe Bohr AtomEnergy Levels of ElectronsAtomic Structure of the First 18 ElementsElectron Structures and the Periodic Table

3. A Brief History

5. ElectromagneticRadiation

6. Electromagnetic Radiation travels at the speed of light: 3 X 108 m/s carries energy does not have mass exhibits behavior of both waves and particles has electrical and magnetic components Sound waves are NOT electromagnetic radiation and travel much slower: 344m/s in dry air.

7. Examples of Electromagnetic Radiation light, all the colors of the rainbow radio and TV waves microwaves X-rays radiant heat

8. Characteristics of a wave: wavelength (measured from trough to trough) wavelength (measured from peak to peak)

9. Frequency is the number of wavelengths that pass a particular point per second.

10. Speed is how fast a wave moves through space.

11. EMR also exhibits the properties of a particle. EMR particles are called photons.Both the wave model and the particle model are used to explain the properties of EMR.

12. The ElectromagneticSpectrum

14. Equations ν=c/λ Tells us the higher the frequency the shorter the wavelength. E=hv Tells us the higher the frequency the higher the energy.

15. Which color of light has more energy? Blue Orange The answer is blue.

16. The Bohr Atom

17. At high temperatures or voltages, elements in the gaseous state emit light of different colors. When the light is passed through a prism or diffraction grating a line spectrum results.

18. These colored lines indicate that light is being emitted only at certain wavelengths. Line spectrum of hydrogen. Each line corresponds to the wavelength of the energy emitted when the electron of a hydrogen atom, which has absorbed energy falls back to a lower principal energy level.

19. Niels Bohr, a Danish physicist, in 1912-1913 carried out researchon the hydrogen atom.

20. Electrons revolve around the nucleus in orbits that are located at fixed distances from the nucleus. An electron has a discrete energy when it occupies an orbit.

21. When an electron falls from a higher energy level to a lower energy level a quantum of energy in the form of light is emitted by the atom. The color of the light emitted corresponds to one of the lines of the hydrogen spectrum.

22. Light is not emitted continuously. It is emitted in discrete packets called quanta.

23. E1 E2 E3 An electron can have one of several possible energies depending on its orbit.

24. Bohr’s calculations succeeded very well in correlating the experimentally observed spectral lines with electron energy levels for the hydrogen atom. Bohr’s methods did not succeed for heavier atoms. More theoretical work on atomic structure was needed.

25. In 1924 Louis De Broglie suggested that all objects have wave properties. • De Broglie showed that the wavelength of ordinary sized objects, such as a baseball, are too small to be observed. • For objects the size of an electron the wavelength can be detected.

26. Schröedinger’s work led to a new branch of physics called wave or quantum mechanics. • Using Schröedinger’s wave mechanics, the probability of finding an electron in a certain region around the atom can be determined. • The actual location of an electron within an atom cannot be determined. In 1926 Erwin Schröedinger created a mathematical model that showed electrons as waves.

27. Based on wave mechanics it is clear that electrons are not revolving around the nucleus in orbits. Instead of being located in orbits, the electrons are located in orbitals. An orbital is a region around the nucleus where there is a high probability of finding an electron.

28. Energy Levels of Electrons

29. The wave-mechanical model of the atom predicts discrete principal energylevels within the atom

30. The first four principal energy levels of the hydrogen atom. As n increases, the energy of the electron increases. Each level is assigned a principal quantum number n.

31. Each principal energy level is subdivided into sublevels.

32. Within sublevels the electrons are found in orbitals. An s orbital is spherical in shape. The spherical surface encloses a space where there is a 90% probability that the electron may be found.

33. An atomic orbital can hold a maximum of two electrons. An electron can spin in one of two possible directions represented by ↑ or ↓. The two electrons that occupy an atomic orbital must have opposite spins. This is known as the Pauli Exclusion Principal.

34. A p sublevel is made up of three orbitals. Each p orbital has two lobes. Each p orbital can hold a maximum of two electrons. A p sublevel can hold a maximum of 6 electrons.

35. pz The three p orbitals share a common center. The three p orbitals point in different directions. px py

36. A d sublevel is made up of five orbitals. The five d orbitals all point in different directions. Each d orbital can hold a maximum of two electrons. A d sublevel can hold a maximum of 10 electrons.

37. Number of Orbitals in a Sublevel

38. Distribution of Subshells by Principal Energy Level

39. The Hydrogen Atom • The diameter of hydrogen’s electron cloud is about 100,000 times greater than the diameter of its nucleus. The diameter of hydrogen’s nucleus is about 10-13 cm. The diameter of hydrogen’s electron cloud is about 10-8 cm. In the ground state hydrogen’s single electron lies in the 1s orbital. Hydrogen can absorb energy and the electron will move to excited states.

40. Atomic Structure of the First 18 Elements

41. To determine the electronic structures of atoms, the following guidelines are used.

42. No more than two electrons can occupy one orbital

43. 2 s orbital 1 s orbital • Electrons occupy the lowest energy orbitals available. They enter a higher energy orbital only after the lower orbitals are filled. • For the atoms beyond hydrogen, orbital energies vary as s<p<d<f for a given value of n.

44. Each orbital in a sublevel is occupied by a single electron before a second electron enters. For example, all three p orbitals must contain one electron before a second electron enters a p orbital.

45. number of protons and neutrons in the nucleus number of electronsin each sublevel Nuclear makeup and electronic structure of each principal energy level of an atom.

46. Arrangement of electrons within their respective sublevels. 2p6 Electron Configuration Number of electrons insublevel orbitals Type of orbital Principal energy level

47. Orbital Filling

48. In the following diagrams boxes represent orbitals. • Electrons are indicated by arrows: ↑ or ↓. • Each arrow direction represents one of the two possible electron spin states.

49. Filling the 1s Sublevel

50. He Helium has two electrons. Both helium electrons occupy the 1s orbital with opposite spins. ↑ H 1s1 Hydrogen has 1 electron. It will occupy the orbital of lowest energy which is the 1s. ↑ ↓ 1s2