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# MidTerm Study Guide 10/26 Chem H2A

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This 11 page Study Guide was uploaded by Dana Roach on Wednesday October 21, 2015. The Study Guide belongs to Chem H2A at University of California - Irvine taught by SHAKA, A. in Fall 2015. Since its upload, it has received 81 views. For similar materials see Honors General Chemistry in Chemistry at University of California - Irvine.

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Date Created: 10/21/15

OO Honors General Chem Shaka A wave is any repeating pattern in space or time or both The basic wave is a sine or cosine Fourier any wave can be made up from a sum of sines or cosines O The more terms we add the squarer the waves become Rutherford then used alpha particles to discover the structure of the nucleus with the gold foil experiment in 1910 Alpha particles are very energetic like ri e bullets to fire them into gold foil and thought they would ricochet off which he could track Thought they would go right through the foil but some scattered straight back to the particle source The something the alpha particles hit needed to be much more massive to ricochet them back Thompson s proposal of plum pudding didn t fit with Rutherford s trackings of the ricochets What bounced the electrons back had to be positive to be strong enough to bounce them back UNITS 0 Use mathematics as a tool to help us solve problems In chemistry there are usually units involved ALWAYS WRITE DOWN AND CANCEL OUT UNITS Answers without units are WORTHLESS EX Mars rover crash Never throw away digits by rounding during a calculation Carry them all through until the end of the calculation The answer may well have more or fewer sig figs than the answer 0 If we do not use scientific notation then the number of digits that we should use is not really clear ALWAYS USE SIG FIGS 0 When in doubt quote 2 or 3 digits 0 In scientific problems use scientific notation 0 Initial errors can either blow up in your face or pretty much disappear O X 096 plus or minus 0005 How many digits quote for y Just put in 955 and 965 and find that y changes so much we cant put 25 plus or minus 05 for y OOOOOO O Interval of uncertainty is no even symmetric around 25 We would have to write y2536 28 to capture the real situation Therefore be careful about using simpleminded rules 0 If you round numbers during a calculation you will make things much much worse 0 0 Dimensional Analysis 0 We can analyze equations without knowing anything except the units involved 0 Never quote answers without units 0 When in doubt use MKS and make sure that the units of your results make sense 0 Beware of constants like gas constant R which can by L atm mol lKl or J OULES 0 Light 0 Newton believe that light was corpuscular in nature because it traveled in straight lines Through narrow slits revealed it was a wave like water Huygens work made Newton give up his theory 0 Waves can have positive or negative phases and therefore can add or subtract unlike particles Hence the interference 0 Cell service is sketchy because it is bouncing off two sides of a canyon and cancelling out 90 degree phase difference the constructive interference is reduced 150 degree phase difference we get some destructive interference 180 cancels to O Waves with difference frequency they will interfere at different points in time leading to a beading frequency wowwowwow wow 0 Newton s Rings 0 White light has a mic of different wavelengths O Convex surface is brought near a at surface there is a difference of refractive index the condition for constructive interference depends on the wavelength CO 0000 O Creates a rainbow in the oil and repeats the rainbow just like the beading frequencing 0 Blackbody Radiation 0 19th century accepted that light was an electromagnetic wave 0 Maxwell s equations were fabulously successful at unifying electricity and magnetism predicting the speed of light and explaining diffraction re ection refraction how lenses work and SO on O 2 crucial experiments shoed that this description of light was incomplete Blackbody Radiation light from a black body lamp black when heated in a vacuum gave a characteristic spectrum that depended only on the absolute temperature Rayleigh and Jeans calculated the spectrum of wavelike light and found that it had more and more high frequency radiation because each frequency would be equally likely The octave is twice as much twice the frequency More frequencies at high octaves than at low This predicted the ultraviolet catastrophe that e would be killed by X rays if we opened the kitchen oven Lord Rayleigh actually did the calculation Max Planck found the correct solution that light was quantized one atom of hydrogen one photon of light You can have one or zero but nothing in between The minimum amount of light that could exist once the frequency was chosen the energy of light was Ehv Clarification nuv The frequency v could be continuous but once chose there was a minimum amount and for higher frequencies the minimum amount of energy was higher This cuts out the x Rays 0 Quantization O Rayleigh and Jeans Law supposed that all frequencies were equally likely But there are many more modes at high frequencies O Planck s includes there law but added his own section afterwards that includes Planck s constant K which is Boltzmann s constant and T which is temperature in Kelvin Suppose hvkT is less than 1 Easier to have blackbody at microwave frequencies Planck s limits the higher radiation energy preventing it from reaching crazy X ray levels 0 Physical meaning 0 O 0 Don t notice the digital nature of light in day to day observation because of relative size of the constants Fact or 10 to the 11th power means there are many many low frequency photos around at room temperature However at very high frequencies we start to notice that hv has a minimum value 0 Photoelectric Effect 0 Another experiment was at odds with the wave description of light the photoelectric effect Shining light on a clean metal surface in a vacuum caused the ejection of electrons so called photoelectrons from the metal Classically any wavelength of light should be able to eject electrons eventually The longer the wavelength the longer it should take The number of photoelectrons coming out should depend on the amplitude of the light Twice the intensity of the waves 2 photoelectrons Electron kinetic energy depend on INTENSITY of light BUT INSTEAD 1 photoelectrons ejected instantaneously 2 below a threshold frequency there were no photoelectrons 3 turning up the intensity of lowfrequency light made no difference This led Einstein to interpret this experiment in term of PHOTONS namely that particles of light were kicking out the electrons and that each particle had an energy in accordance with Planck s formula One photo hits and and electron and if its energy is insufficient it bounces off Shaka Lecture 9 1014 0 Atomic Structre and Spectroscopy and Tuffy Review and Trends 0 O O O Angular momentum is conserved the photon has one unit of angular momentum Thus transitions to the ls ground state must be from 2p3p4petc but not from s or d Transitions to the 2p state can be from 35 or 3d and so on But not from 3p4p or 4f In general delta L l for electric dipole allowed transitions These are the strongest ones There are weaker magnetic dipole transitions and electric quadrupole transitions in some systems We needn t worry about them now This gives up the following selection rules for electric dipole allowed transitions Delta n anything including 0 sometimes Delta L l for electric dipole allowed transitions Delta ms 0 mag Spin untouched by electric eld We can use the rules to gure out whether a transition is allowed or not Practice problem Suppose a H atom has its electron in 3p orbital What transition do you predict 25 or ls 3p ls will happen much faster than 25 o the more energetic the electron the faster it will come out o If you go to 25 you re stuck Stuck in metastable state Not an electric dipole transition It is a forbidden transition Remember that the photo takes off like a corkscrew twisting with one unit of angular momentum l for electric dipole allowed transitions o This any p can go to any lower s or d but cannot go to p or to an f orbital 0 Integrating Wavefunction Recall the probability of nding an electron given volume element dVdxdydz is psixyzpsixyzdxdydz absolute value of psi8xyzquot2dxdydz But for hydrogenlike orbitals we do not use Cartesian coordinates because the equations are much easier to solve in spherical polar coordinates The difference is that a volume element changes size as a function of either theta or r and so we have extra factors in there 0 Spherical Polar Coordinates Problem 8 The total probability of being within an voume is just the integral of the probability density over volume We need an integral because the probability density changes from point to point in space 0 lonic Radii Note that H anion is larger than F Negative ions get bigger Positive ions get smaller Not going to ask any numbers lonic sizes affect salt structure greatly Shaka Chem Lecture 1012 You are not meant to look up somebody else s answers on the internet for Tuffys Orbitals Atoms and Atomic Structure The allowed values of n are the positive integers Allowed values of L are from zero to n1 LO is called s L1 is p L2 is d L3 iff L4 is g Helium atom nucleus is very stable The allowed values of m sub L from L to L There are two values of m sub s electron itself or 12 Using these orbitals we can understand and build up the periodic table The list of lled orbitals is called the electron con guration in a problem Practice Problem 1 H 1s1 We begin with nO O m sub I O orbital and then ll them up Without a magnetic eld we do not distinguish between the three p orbitals of the 5 d orbitals they have the same energy Na 1s22s22p63s1 Sodium atomic number is 11 with 11 e The 3s electron is outside a closed shell Bromide has an extra electron compared to neutral bromine atom It has con guration of 1s22s22p63s23d104s24p6 This has a closed shell with the same electron con guration as krypton When we have other electrons occupying orbitals the energies get jumbled a bit 0 4s is lower than 3d Thorium midly radioactive 1s22s22p63p64s23d104p65s24d105p66s24f145d106p6 7s26d2 The radius of an atom contracts as you go from left to right in a row on the periodic table And expands when a new shell is added Bigger down the column Transition metals can be subtle whether they get bigger or smaller Geometric Picture For each value of n we can establish a way to keep track of the orbitals by drawing an nn square comprised of boxes Each box in the gure can hold two electrons with opposite spinds Each principal quatum number n there are 2n2 electrons allowed ln Hydrogen the energy only depends on n EEn in other atoms it depends on both n and l EEn The two phenomena that in uence the order are penetration and shielding S electrons have nonzero probability of being found very near the nucleus This is true even if they are 2s and 3s electrons On the other hand p has 0 probability of being found near the nucleus Energy Ordering by L Suppose we have an atom like potassium the electron outside the closed shell could go into either 4s or 3d Why does it choose 4s Because a 4s orbital electron can penetrate the other inner shells and quotseesquot the 19 positive charged nucleus and is attracted to it 3d orbital electron never sees the nucleus as there are two nodal places thus 4s wins even though n is bigger Radial and Angular Parts the wavefunction for an electron in a hydrogen orbital can be expressed as a product of a radial part that depends only on r and an angular part that depends only on theta phi if we plot the radial part Rr for different orbitals we can see the penetration effect for sorbitals but not so much for p orbitals Shielding the constant a sub 0 Is called Bohr radius is the nominal radium of the H atom which is about half an Angstrom shielding is the ip side of penetration the s orbital hide the positive charge more effectively and so keep electrons farther out The Aufbau Principle we can build up electron con gurations by slotting electrons in boxes each box holds at most two electrons if boxes are the same energy electrons spread out among them before pairing up This is due to electronelectron repulsion for the most part Spherical Atoms Which of the following atoms are spherical o H 0 He 0 Li 0 B o N H should be spherical We ll up the orbitals and see if we expect spherical symmetry based on which orbitals are occupied Closed shells will always be spherical A double headed arrow always means a PAIR OF ELECTRONS if there is only one then you should draw half an arrow He s orbitals are always spherical yes spherical Li we don t bother with the lled 1s because it is spherical there is a single electron in an sorbital so Li is sphencal B boron with 5 electrons and 3 in valence shell we have it with electrons in the p orbital making it nonspherical it looks like an egg o N with 7 electrons and 5 n the vakence shell we have one in each p orbital making it spherical Closed shells and half lled shells are especially stable because they are always spherical atoms Atomic Spectroscopy The energies of atoms can be worked out bu looking at the discrete wavelengths of light that they can emit We can rip electrons off atoms with a high voltage a discharge 0 We can also heat the material up using a very hot ame Electroons are excited by collision with other electrons in a discharge or ame They then emit light and drop back down to a lower state 0 Light is a photon and photons have one unit of angular momentum bar h 0 Has to change by one unit P can go to s or d photon but cannot go to p or f o The photon can either rotate clockwise or counter clockwise so they atom must change its orbital angular momentum by one unit of bar h 0 And the frequency of light must obey energy conservation change in atom energy energy of photohnu Don t use bleach because it destroys dye molecules Atomic Spectroscopy H atom Various series of transition 0 10000 per cell per day DNA xes

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