CHEM 251 1st Edition Lecture 4 Outline of Last Lecture I. Clicker QuestionII. Quantum NumbersIII. Orbital StructureIV. Electron ConfigurationOutline of Current Lecture V. Clicker QuestionVI. Effective Nuclear ChargeVII. Rationalizing Periodic TrendsVIII. Nuclear PowerCurrent LectureV. Clicker Questiona. 2 outer shell electrons do not shield each other strongly => feel higher 4+ charge more stronglyVI. Effective Nuclear Chargea. Zeff = measure of how much of the positive charge of the nucleus is felt by each electronb. Between the nucleus and outer shell electrons, inner shell electrons shield the nuclear chargec. Periodic trends: increases across a period, decreases down a groupd. Slater’s Rulesi. Zeff = Z – σii.σ = “screening constant,” calculated according to Slater’s RulesThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.1. Each electron in orbitals of greater principle quantum number (in shells beyond the shell of interest) contributes 0.2. (a) If e– of interest is in a s- or p-orbital, each e– with the same principle quantum number contributes 0.35.(b) If e– of interest is in a d or f orbital, the d or f e–’s contribute 0.35 but the s and p e–’s (of same quantum number) contribute 1.0.3. (a) If e– of interest is in a s or p orbital, each e– in the (n–1) quantum level contributes 0.85(b) If e– of interest is in a d or f orbital, then they contribute 1.0.4. Each e– in an orbital of smaller principle quantum number contributes 1.0.iii. Example: Zeff for 2p electron in F atom = 5.20VII. Rationalizing Periodic Trendsa. Atomic Radiusi. Across a period: Zeff increases, pulling electrons closer, smaller radiusii. Down a group: Zeff decreases, less pull on electrons, larger radiusb. Ionization Energyi. Example: Lithium (1s22s1)1. Why is second ionization energy larger than first?a. Electrons in Li+ receive no shielding & strongly attracted to nucleusVIII. Nuclear Powera. Takes advantage of unstable heavy atom isotopesb. Nuclear Decay Pathwaysi. Alpha emission of He nuclei (2 neutrons/2 protons)ii. Beta emission of electroniii. Gamma emission of high energy photonsiv. Spontaneous fission-unusual, heavy atom splitting into two lighter atomsc. Harnessing Nuclear Chemistryi.238U is most stable, 235U is most commonii. Steps 1. Extraction: starts w/ mining, mixture of U 235&238 to form gaseous UF6, which is then enriched, converted to UO2 2. Power generation: bombard UO2 pellets w/ neutrons, which creates chain reaction of fissiona. Released energy heats water into steam which drives turbine3. Avoid meltdown: low level of enrichment to control reactiona. Control rods made of neutron-absorbing materialb. Moderator (H2O or D2O) slows down neutrons, conducts
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