Looking back at Electrons in Atoms

1. Looking back at Electrons in Atoms Chemistry documented materials to restore health (pharmacy). Atoms and elements were recognized during 16th to 18th centuries. The discovery of electrons in 1897 showed that there were more fundamental particles present in the (Dalton) atoms. Fourteen years later, Rutherford discovered that most of the mass of an atom resides in a tiny nucleus whose radius is only 1/100000 of that of an atom. In the mean time, Max Planck (1858-1947) theorized that light beams were made of photons that are equivalent to particles of wave motion. Explain waves and particles Electrons in Atoms

2. Announcement Please enroll Chem123 and related physics using Quest by doing the following: First screen - add LAB class No. - DO NOT PRESS CONTINUE! Press INSERT CLASS (Again) - add LECTURE class No. - then press CONTINUE You will see two boxes. Update your attributes (add your tutorials where applicable) and then SUBMIT. This should allow you to enrol in the lecture and lab, which are co-requisites. Electrons in Atoms

3. Discovery of Electrons J.J. Thomson (1856-1940) determined the charge to mass ratio for electrons in 1897. Robert Millikan’s oil-drop experiment determined the charge of electrons. Thus both the charge and mass are known Review Chapter 2 Electrons in Atoms

4. 1-, 2-, & 3-dimensional Waves Demonstrate a single wave movement Explain continuous set of 1-dimensional waves Water wave and drum-skin movement as 2-dimensional waves 3-dimensional waves: sound waves seismic waves Electrons in Atoms Explain wave motions

5. Wavelength, Frequency, and Speed Electromagnetic waves are due to the oscillations of electro- and magnetic-fields. Wavelength (l): the diagram shows one whole wave, and note the wavelength (in m, cm, nm, pm). Frequency (n) is the number of waves passing a single point per unit time (s–1 or Hz) . Speed of light Be able to apply:n = c / l; l = c / n c = l n = 3.0e8 m s–1 Electrons in Atoms

6. Frequency, Wavelength & Wave-numbers A typical red light has a wavelength of 690 nm. What is its frequency? Solution: c 3e8 m s–1n = ----- = ------------------ = __________s–1l 690e–9 m By the way, the wave_number is the number of waves per unit length. 1wave_number = --- = __________ m–1l Electrons in Atoms

7. Momentum of Photons For a particle with restmass mo, its relative mass m when moving with a velocity of u relative to the speed of light c ismom = -----------------[1 – (u/c)2] Light particles, photons, have zero rest mass, travel at the speed c. From this relationship, the momentum of the photon, p, (Text p309) ishp = -------where h is the Planck’s constant, and is the wavelength. Momentum in any collision is conserved. Electrons in Atoms

8. Superposition of Waves A sine function y1 = sin x is a typical wave function. Plot y1 vs. x Plot y2 = – sin x. When the two waves combine, what happens?y1 – y2y1 + y2 Electrons in Atoms

9. Interference Combination or superposition of waves is called interference. Electrons in Atoms

10. Radiation spectra There are three types of spectra Continuous spectrum (generated by hot solid) Line spectrum (generated by hot gas, atoms) Absorption spectrum (continuous spectrum with black lines) Give the name of the spectrum from a known source Electrons in Atoms

11. The Electromagnetic Spectrum Electrons in Atoms

12. Hydrogen Emission Spectrum The visible spectra of H consists of red (656.3 nm) green, (486.1 nm), blue, (343.0 nm), indigo (410.1 nm), and violet (396.9 nm) lines. Variation of wavelength follow this formula 1 1 1 --- = RH ( ---- – ----) (RH =10973731.534m-1l 22 n2 in wave_number)This is the Balmer series c 1 1n = ---- = R’H ( ---- – ----) (R’H = 3.2881e15s-1, in frequency)l 22 n2 Electrons in Atoms

13. Atomic Spectroscopy The study of light emitted by or absorbed by atoms is called atomic spectroscopy (AA). It offers qualitative and quantitative analysis of samples, because each element has a unique set of lines. The simplest form is identify elements by flame color. Atomic spectroscopy can be dividedinto emission and absorptionspectroscopy, (AES and AAS). Electrons in Atoms

14. Max Planck’s Photon Max Planck (1858 – 1947) proposed that light consists of little quanta of energy, and he called them photons. The energy of the photon E, is proportional to its frequency n. E = h n The proportional constant h is now known as Planck constant, h = 6.62606876e-34 J s–1, a universal constant. His proposal or assumption was made while studying the radiation from hot (black) body. Know relationships among frequency, n, wavelength, l, wave number, speed, c, and energy E. Given one, be able to calculate the others. Electrons in Atoms

15. Wavelength Frequency, Wave-number and Energy of Photon The red line in Balmer series of hydrogen has a wavelength l = 656.3 nm. Calculate the frequency, wave number, and energy of the photon. Solution: frequency n = = = 4.57e14 s–1energy E = hn = 6.626e –34 J s–1 * 4.57e14 s–1 = _______ wave number = = _______ E = h c / l cl 3e8 m s–1656.3e-9 m 1l See slide 12 and complete the calculation Electrons in Atoms

16. The Photoelectric Effect In 1888, Hertz discovered that electrons are emitted when light strikes a metal surface – photoelectric effect. In 1905, Einstein observed and explained* electrons are emitted only when light frequency exceeds a particular value no, he called these values threshold* number of electrons emitted is proportional to the intensity of light* kinetic energies of emitted electrons depend on the frequency, n, not the intensity of light See science.uwaterloo.ca/~cchieh/cact/c120/quantum.html for photoelectric effect Electrons in Atoms

17. Einstein’s Experiment Kinetic energy of electron (½ m u 2) is measured by retarding potential Vs. ½ m u 2 = eVs Vs is proportional to the light frequency, but unrelated to light intensity. The frequency n must be greater than certain threshold no, which is metal dependent. Vs = k (n – no ) = kn – k no = kn – Vo (substitute Vo for k no) e Vs = e kn – e Vo = ½ m u2 = hn – eVo (h = e k, the Planck constant) hn = e (Vs + Vo) Electrons in Atoms

18. Graph Einstein’s Result Vs = ½ m u2 kinetic energy of electron hn = e (Vo + Vs) e Vs = e kn – e Vo n0 threshold n Electrons in Atoms

19. A typical problem Radiation with wavelength of 200 nm causes electron to be ejected from the surface of a metal. If the maximum kinetic energy of electrons is 1.5e-19 J, what is the lowest frequency of radiation that can be used to dislodge electrons from the surface of nickel? Solution: Energy of the photon E = h c / l = 6.6262e-34 J s * 3e8 m / 200e-9 m = _E1_ J Threshold energyEo = E1 – 1.5e-19 J Threshold photon wavelength, lo = h c / Eo = __please calculate __ m Electrons in Atoms

20. Significance of Einstein’s Result Max Planck’s assumption is true, a proof. Light indeed consists of photons (quanta of light, not continuous) Quantity of energy in photon E=hn (energy of photon) Photochemical reactions O2 + hn O + O O2 + O  O3 (formation of ozone) Be able to calculate energy of photons, E, threshold, no, and kinetic energy of electrons, in photoelectric experiment. Electrons in Atoms

21. The Bohr Atom Bohr tried to interpret the hydrogen spectrum by applying Planck’s quantum hypothesis and Rutherford’s atom, and he postulated: The e revolves around the nucleus (Rutherford’s atom) The electron has a set of allowed orbits (angular momentum = nh/2p, where n is an integer) that are stable. Electron changes from one state to another by absorbing or emitting a photon. From Newton’s physics, he showed the energy level of the electron to be E n = – R H n2 Electrons in Atoms

22. Bohr’s Energy Levels of Electrons in H 1/ R H = 0 – 1/36 R H – 1/25 R H R H n 2 – 1/16 R H E n = – – 1/9R H – 1/4R H R H = 2.179e-18 J State transitions – R H Given RH and state of transition, nf, ni, be able to calculate E, n, & l, of transition. Electrons in Atoms

23. H-spectra & Energy Levels Ionization energy Electrons in Atoms

24. Excitation and Ionization of H 1/ R H = 0 – 1/36 R H – 1/25 R H R H n 2 – 1/16 R H E n = – – 1/9R H – 1/4R H R H = 2.179e-18 J Excitation and ionization of an atom differs from those of a molecule Absorption of a photon with energy equal or greater than RH results in ionization of H atom Absorption of a suitable hnexcites an atom – R H Electrons in Atoms

25. A typical problem The electron in a hydrogen atom undergoes a transition from 4s to one of the 5p orbitals when the atom absorbs a single photon. What is the frequency of the absorbed photon? Solution: DEni – nf = Ei – Ef = RH (---- – ----) RH = 2.179e-18 J l = h c / DEni – nfh = 6.6262ee-34 J s c = 3e8 m s-1 1 1ni2 nf2 Electrons in Atoms

26. Photons & Transition Electrons in Atoms

27. Wave-Particle Duality h = lp = l m v E = hn = m c 2 = m c = p = hl hnc E – energy of photon and particleh – Planck constanth/l, m v, m u, or p – momentumc, u, v – velocity of photon or particlen – wavelength of photon or particle A particle with momentum p = m vis a wave with wave length l Louis de Broglie (1892-1987)Nobel laureate 1929 When in 1920 I resumed my studies ... what attracted me ... to theoretical physics was ... the mystery in which the structure of matter and of radiation was becoming more and more enveloped as the strange concept of the quantum, introduced by Planck in 1900 in his researches into black-body radiation, daily penetrated further into the whole of physics. Electrons in Atoms

28. Wavelength of Electrons Estimate the velocity and wavelength of electrons with kinetic energy of 100 eV. Solution: (data look up and background information required)Mass of e – me = 9.1e-31 kg; h = 6.626e-34 J s E = ½ m v 21 J = 1 N m = 1 kg m2 s–2 1 eV = 1.6e-19 J; 100 eV = 1.6e-17 J v = (2 E / m)½ = (2 * 1.6e-17 kg m2 s–2 / 9.1e-31 kg)½ = 5.9e6 m s–1 m v = 9.1e-31 kg * 5.9e6 m s–1 = 5.4e-24 kg m s–1 l = h / p = 6.626e-34 1 kg m2 s–1 / 5.4e-24 kg m s –1 = 2.23e-10 m (approximately the diameter of atoms) Be able to calculate momentum and wavelength of particle when its speed is given. Estimate p & l when an electron travels at 50% c Electrons in Atoms

29. Validity of Particle-Wave Duality Electrons are usually considered particles. In 1927, a Davisson and Germer observed electron diffraction by Ni surface. Low energy electron diffraction (LEED) uses a beam of 30-to-300 eV electrons to bombard a sample; a diffraction pattern is shown. Example of a LEED pattern from the Si(111)7×7 surface. Electrons in Atoms

30. The Heisenberg Uncertainty Principle When electrons are considered particles, we should be able to measure their positions (x) and momenta (p) accurately, but Heisenberg showed that is not the case. The arguments seem complex, but the result is simple. The uncertainty of position Dx and uncertainty in momentum Dp has this relationship: DxDp> h4p The implications: the position (location) is fussy if we know the energy accurately. We are concerned with the energy more than we are with location probability of finding the electron correlates to orbital (not orbit) Read the arguments for the uncertainty principle. Heisenberg at 22 Electrons in Atoms

31. Quantities and the Uncertainty Principle If the uncertainty of an electron is 1e-10 m (100 pm), what is the uncertainty of the momentum? Dp = h / (4p * Dx) = = 5.3e-25 kg m s–1 6.63e–34 kg m2 s–14*3.1416*1e-10 m Rest mass of electron = 9.1e-31 kg Speed of e with p = 5.3 e-25 kg m s–1 Dve = 5.3e-25 kg m s–1 / 9.1e-31 kg = 5.8e5 m s–1 DEnergy = ½ * 9.1e-31 kg * (5.8e5 m s–1)2 = 1.5e-19 J (recall 1 eV= 1.6e-19 J) Ionization energy of H is –13.6 eV; DEnergy = 0.9 / 13.6 = 7% If ionization energy of H is 7% accurate, the e– is some-where within 100 pm, size of H atom. More accuracy will result in an electron in larger volume. Discuss physical meaning of results. Electrons in Atoms

32. Announcement International Exchange Information NightNov 5, 2003, 5:30-6:30pm in DC 1301 (Fishbowl) Pizza and Beverages will be served. This information session is geared mainly toward students in their1st or 2nd year who are interested in International Academic Exchanges.Criteria to be accepted for an International Exchange Program: Completed two years of University and maintained an overall accumulative average of 70% Proficient in the language of desired country of exchange (ie. must be proficient in French to go to France) Electrons in Atoms

33. Standing Waves A traveling wave can be of any wavelength. The boundaries restrict a standing wave to some integer times the half-wavelength as illustrated by the 1-dimensional diagram. Note that the points at the boundary are fixed. Electrons in Atoms www2.biglobe.ne.jp/~norimari/science/JavaEd/e-wave4.html

34. Wavelength in Standing Waves For standing waves with both ends fixed in a length L, the wavelength l is limited to (where n is an integer) l = ; n = 1, 2, 3, …. In quantum mechanics, electrons in atoms are treated as standing waves confined by the electric field due to the atomic nuclei. Thus, the electrons are represented by wave functions. The wave, energy etc are called state of the electron, and with spin, each state accommodate 2 electrons. A state is called an orbital (not orbit) 2 Ln Electrons in Atoms

35. Particle in a 1-Dimensional Box of Length L The wave representing a particle in a box of length L (variable x) can be represented by (x) =  ( ) sin ( ), n = 1, 2, 3, … 2L n p xL Please work out the following values for n = 1 and n = 2 yourself n = 1n = 2 (0) = ___0_ ___0_ (boundary) (L/4) = _____ _____ (L/2) = _____ _____ (3L/4) = _____ _____ (L) = ___0_ ___0_ (boundary) Electrons in Atoms

36. An electron viewed as a wave in an atom, imagination goes a long way Animation by Naoki Watanabe Electrons in Atoms

37. Energy of Waves Kinetic energy of a particle with speed u Ek = ½ m u 2 = (m u)2 / 2 m = p 2 / 2m de Broglie’s relationship p = h / l, 2 Ln h22 m l2 p22 m Recall l = ----- Ek = ---------- = ------------ = -------------------------- = -------------- h2 2 m (2 L / n)2 n 2h 28 mL2 The lowest energy of a (wave) particle is when n = 1, the zero point energy Electrons in Atoms

38. Energy Level of a Particle in a Box n 2h 28 mL2 En = -------------- n = 3 In a 3-dimensional spaceE(nx, ny, nz) = ------- ( ----- + ------ + ----- ) n = 2 nx 2Lx ny 2Ly nz 2Lz h 28 m n = 1 What is the expression for E(nx, ny, nz) in a 3-D cube? Electrons in Atoms

39. Wavefunction of H atom The wave function of hydrogen atom satisfies the Schrodinger equation: – ---------- ( ------- + ------- + ------- ) – --------- = EjSolutions of this equation is beyond the scope of this course, but a few points can be made. This second order DE has many solutions and by implying the boundary conditions in here, these solutions are characteristic of three quantum numbers n, l and mn – the principle q.n. (dominate energy)l – the orbital angular momentum q.n. (orbital momentum)m – the magnetic q.n. h 28 p2m 2jx2 2jy2 2jz2 Ze2jr2 Electrons in Atoms

40. Properties of Quantum Numbers Restrictions of quantum numbers are due to physical and mathematical reasons. Thus,n = 1, 2, 3, 4, … (integer)l = 0, 1, 2, 3, … (n – 1)m = - l, - (l –1), - - (l – 2) … 0 … (l –2), - (l –1), l The consequency: Subshells are named according to value of ll = 0 (s-subshell) (1 state, m = 0)l = 1 (p-subshell) (3 states, m = -1, 0, 1)l = 2 (d-subshell) (5 states, m = -2, -1. 0, 1, 2)l = 3 (f-subshell) (7 states, m = -3, -2, -1. 0, 1, 2, 3) … Electrons in Atoms

41. Wave (electron density) of Some Orbitals Know the shape of 1s, 2s, 2p, 3s, 3p, & 3d orbitals – for your bonding lessons in the future. Know the sign of the orbitals in various regions. Explain the significance of j vs. r plots, j2 vs. r plots, r2j2 vs. r plots etc. Explain nodes, know how numbers of nodes related to n and l. Animations are used during the lecture to illustrate all the above points, and these are subjects of tests and final exams. Electrons in Atoms

42. Energy Levels of H-atoms Solutions to the Schrodinger equation results in expressions for the energy, which is essentially the same as the one derived by Bohr, En = – ---------------- = – 13.6 eV ------- The Zeff is the effective atomic number (modified atomic number). Zeff 2me e48e0h 2n 2 4s –– 4p –– –– –– 4d – – – – – 4f - - 3s –– 3p –– –– –– 3d –– –– –– –– –– Zeff2n2 2s –– 2p –– –– –– 1s –– Energy levels of H orbitals Electrons in Atoms

43. Energy Levels of Many-electron atoms 4f –– For many electron atoms, the energy levels of subshells change slightly due to Zeff En = – ---------------- = – 13.6 eV ------- 4d –– –– –– –– –– 4p –– –– –– 3d –– –– –– –– –– 4s –– 2p –– –– –– Zeff 2me e48e0h 2n 2 3s –– 2s –– Zeff2n2 1s –– Energy levels of many electron atoms Electrons in Atoms

44. Electron Spins Stern-gerlach experiment In a magnetic field, a beam of electrons splits into two beams, and a beam of atoms are also splits into two beams. The interpretation of this observation came after some years is due to the spin of electrons, thus a fourth quantum number s. For an applet animation of this experiment seewww.if.ufrgs.br/~betz/quantum/SGPeng.htm Electrons in Atoms

45. Electronic Configurations of Atoms Electrons go to the lowest possible energy levels (minimize the energy of the atom). Pauli’s exclusion principle: No two electrons in an atom may have all four quantum numbers alike Hund’s rule: electrons occupy singly in orbitals of identical energy (degenerate orbitals: p – 3, d – 5, f – 7 etc.) The aufbau (build-up) process: illustrate this process during lecture and urge students to do it. Understand the energy level is the key. Electrons in Atoms

46. The Modern Periodic Table Work out the filling order of orbitals and the electronic configurations from the periodic table 1s1 2s1-2 3s1-2 4s1-2 5s1-2 6s1-2 7s1-2 1s2 2p1 – 2p6 3p1 – 3p6 4p1 – 4p6 5p1 – 5p6 6p1 – 6p6 7p1 – 7p6 3d1 – – – 3d10 4d1 – – – 4d10 5d1 – – – 5d10 6d1 – – – 6d10 4f1 – – – – – 4f14 Th Pa U – – – 5f14 Quantum mechanical theory led to the modern periodic table, which correlates chemical properties of elements nicely! Electrons in Atoms

47. Writing Electronic Configuration Z= 2 10 18 36 54 86 1s2 2s22p6 3s23p6 4s23d104p6 5s24d105p6 6s24f145d106p6 He Ne Ar Kr Xe Rn Electrons in Atoms

48. The Wavefunctions 1s & 2s 1s 2s URL: optoele.ele.tottori-u.ac.jp/~abe/hyd/ Electrons in Atoms

49. The Wavefunctions, 2ps 2pz 2px 2py Electrons in Atoms

50. Atomic orbitals The Orbitron is a British website that gives wonderful views of the atomic orbital, and its URL iswww . shef.ac.uk/chemistry/orbitron/ See Table 9.1 of your Text for A table of wavefunctions of atomic orbitals of the hydrogen atom. Please identify the wave function for the following orbitals: 1s 2s, 2px, 2py, 2pz 3s, 3px, 3py, 3pz 3dxy, 3dyz, 3dx2 – y2, 3dz2 Electrons in Atoms