Chapter 6 Electronic Structure of Atoms KEY QUESTIONS THE BIG PICTURE What is the structure of an atom What are the properties of atoms Remember structure affects function Where are the electrons located What is the energy of an electron By the end of Lecture 6 you should know wavelength and frequency of light dual nature of light electronic transitions and line spectra Bohr model Matter acts as waves Heisenberg Uncertainty Principle Electronic transitions of orbitals Quantum numbers n m ms Orbitals their shapes and energies Orbitals and atoms with many electrons Spin quantum number ms Pauli Exclusion Principle and Hunds Rule Electron Configurations using the periodic table CH110 FA10 SAS 1 Concept Check Understanding electromagnetic energy led to quantum theory it explains what we observe which led to atomic structure and predicts electron configurations and explains periodic trends of the elements CH110 FA10 SAS 2 Review Light Behaves Like a Wave or Does it Light and other electromagnetic radiation behaves as a c Speed of light In a vacuum Light and other electromagnetic radiation behaves as a with kinetic energy E Kinetic Energy Light behaves like a particle with a wavelength called a E CH110 FA10 SAS 3 Practice Problem What is the total energy of one mole of green photons wavelength equal to 523 nm A B C D E 3 80 x 10 19 kJ 6 32 x 10 43 kJ 2 29 x 105 kJ 229 kJ 632 kJ CH110 FA10 SAS 4 Atomic Structure 1 Electron Ejection Albert Einstein The Photoelectric Effect direct observation of quantum effects Albert Einstein 1921 Nobel Prize in Physics Light in Electrons out metal Observation Electrons are emitted by metal only if light has frequency greater than certain minimum value When electrons are emitted the emitted is proportional to the The energy of the light follows this rule Ek KE of emitted photoelectron NO e will be emitted if CH110 FA10 SAS 5 Atomic Structure 2 Line Spectra of Atoms What if h isn t enough to eject an e Absorption energy of photon causes energy of electron to Emission electron energy by giving off a photon Observation Only certain of light are absorbed emitted Conclusion the electrons in atoms can only have certain Energy E3 E2 excited states energies E1 ground state energy CH110 FA10 SAS 6 The Bohr Model of the H Atom 1913 Neils Bohr 1922 Nobel Prize in Physics really not such a bad guy Two Postulates 1 Energy levels are quantized e can only have specific energies in an atom 2 Transitions are quantized only certain discrete energy changes are possible Allowed Energy Levels Orbits n principal quantum n 1 2 En RH 1 n2 RH Rydberg constant 2 180 x 10 18 J Note Orbit energy in the Bohr model is negative so it must correspond to energy needed to put electron into the orbit CH110 FA10 SAS 7 The Bohr Model of the H Atom 1913 Predicts Line Spectra etc n 1 n 3 n 2 n 4 Transitions are accompanied by absorption or emission of light E Efinal Einitial E RH 1 1 ni2 nf2 h Predicts the H atom spectrum EXACTLY If nf ni then E is photon If nf ni then E is photon CH110 FA10 SAS 8 The Bohr Model of the H Atom 1913 Predicts Electron Ejection Energy n n 3 n 2 n E RH n 4 E RH n 3 E RH n 2 E RH n 1 E RH n 1 Coulomb s Law E h RH 1 1 ni2 nf2 CH110 FA10 SAS 9 Practice Problem Which of the following transitions in an H atom results in the emission of the highest energy photon 1 2 3 4 5 n 1 n 6 n 6 n 3 n 3 n 6 n 1 n 4 n 6 n 1 CH110 FA10 SAS 10 Lecture Problem For the H atom what is the wavelength of light emitted for the ni 2 nf 1 electronic transition 1 2 3 4 5 574 nm 472 nm 219 nm 122 nm 101 nm CH110 FA10 SAS 11 Wave Nature of Matter The Bohr model explained some experimental data for the hydrogen atom but it failed for other atoms Dual nature of light Wave properties c Particle properties E h de Broglie 1924 asked the following If light has a duality why not matter too h m v m v is Because 1 m the effects are only observable for extremely masses De Broglie wavelength Wave properties of electrons are confirmed experimentally Electron microscope CH110 FA10 SAS 12 Consequences of the Wave Nature of Electrons in Atoms Electrons have wave properties The mass of the electron is well known Uncertainty in knowing the exact position or of an electron Heisenberg Uncertainty Principle Can calculate the of knowing the location of an electron and know it s Not Orbits but Orbitals Erwin Schr dinger 1933 Nobel Prize in Physics Werner Heisenberg 1932 Nobel Prize in Physics CH110 FA10 SAS 13 Orbitals and Quantum Numbers Overview 1 Principal quantum number n 2 Azimuthal quantum number 3 Magnetic quantum number m 4 Spin quantum number ms Bohr again CH110 FA10 SAS 14 Quantum Numbers are like Train Tickets Figure from Moore Stanitski Jurs 2005 Chemistry The Molecular Science Thomson Brooks Cole CH110 FA10 SAS 15 What Should You Know Properties of Electromagnetic Radiation 1 What is spectroscopy A continuous spectrum A line spectrum 2 What is the relationship of wavelength to frequency Of E to Of E to 3 What are the types of EMR The relative order of wavelength The relative order of colors and range of wavelength in the visible spectrum 4 What are the conversion rules for the various units we have discussed 5 What wavelengths have higher frequency Higher energy Particle Behavior of Light 1 Calculate the energy of a photon or mole of photons 2 Can you calculate Eb of an electron 3 What is a quantum CH110 FA10 SAS 16 What Should You Know Bohr Model of the Hydrogen Atom 1 How does Bohr describe an atom 2 Calculate the energy of the electron in an orbit 3 Is the atom absorbing or emitting light when an electron changes orbit 4 Calculate the energy needed to cause a transition in the Bohr atom E 5 What is the sign of E for emission Absorption 6 Know the definition of energy of an electron E given off when an electron is put into an orbit Matter Waves 1 Find the wavelength of any object given its velocity and mass CH110 FA10 SAS 17