Class Note for CHM 218 with Professor Berger at IPFW
Class Note for CHM 218 with Professor Berger at IPFW
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Date Created: 02/06/15
Chapter 2 An Overview of the Periodic Table The Periodic Table Period In 1869 Dmitri Mendeleev a Russian chemist and J Lothar Meyer a German chemist independently discovered that when elements were arranged in horizontal rows in order of increasing atomic weight the rows could be stacked in such a way that elements in the same column had similar chemical properties 21 With a few exceptions this is the same order as in the modem periodic table The modem periodic table is ordered by increasing atomic number rather than increasing atomic weight MainGroup Elements MainGroup Elements 1 Atomic number 13 39A Symbol Atomic weight 1 2 HA 2 2 2 L SE Transition Metals 2222 2222222 A M 22 9 3 Na Mg 3 4 5 6 7 B VIIIB 10 11 12 22222722 222222 22222222 2 IIIB IVB VB VIB VIB IB IIB 1 22 22 22 22 22 22 22 22 27 22 22 22 22 4 K Ca SE Ti V Cr Mn FE Cu Ni Cu Zn Ga 22 2222 22 272 22222222 27 22 22 2222 22 2222 2222222 22 222 22 22222 22 2222 22 222 22 22 22 722 27 22 22 22 22 22 22 22 22 22 27 22 22 5 Rb Sr V Zl Nb MEI TE Ru Rh Pd Ag Cd in 222272 2722 2222222 22222 2222222 2222 a 22227 22222222 22222 2272222 222222 222222 222722 22 22 27 72 72 72 72 72 77 72 72 22 22 22 6 05 Ba La Hf Ta W RE 05 lr PL Au Hg Tl Pb 22222222 227227 2222222 27222 2222272 22222 22227 22222 222227 22222 22222222 22222 2222222 2272 22222227 222 27 22 22 222 222 222 227 222 222 222 222 222 7 Fr Ra AB Rf Db SQ Eb HS Nil 222 222 m 222 222 22 2222 2222 222 222 272 277 InnerTransition Metals Metal 22 22 22 22 22 22 22 22 22 27 22 22 72 72 Lamhanides CE Pr Nd Pm Sm Eu Gd Tb Dy HEI Er Tm Vb LU 222222 22222722 22222 222 22222 222222 22722 22222222 22222 22222222 22722 22222222 27222 272227 Metallold 22 22 22 22 22 22 22 27 22 22 222 222 222 222 Adinides Tn Pa U Np Pu Am Cm BK or E5 Fm Md N2 Lr 222 2222 222 22222 222 2222 2227 222 222 227 227 222 222 2227 222 222 222 Nonmetal A period is a horizontal row in the periodic table while a group or family consists of the elements in a given column 22 Groups 12 and 1318 the A groups are referred to as main group or representative elements while groups 311 are the transition metals Transition metals are those elements that have a partially lled d subshell in the neutral atom or any common oxidation state Your text omits group 3 because the chemistry of these elements more closely resembles that of the lanthanoids The two rows at the very bottom of the chart lanthanoids and actinoids are referred to as the inner transition metals Elements in group 12 are not classi ed as transitions metals either Some families are given speci c names The elements in group 1 with the exception of hydrogen are the allmli metals Elements in group 2 are the alkaline earths Elements in group 15 are the pnictogens 16 are the chalcogens 17 are the halogens and those in group 18 are the noble gases Stability of the Elements From Coulomb s Law we know that like charges repel each other quite strongly at short distances In addition to this repulsion however there is an extremely short range attractive force between nucleons This attractive force is called the nuclear force and is much stronger than the Coulombic repulsions but acts only at very short distances 103915 m Beyond distances corresponding to nuclear dimensions the nuclear force is negligible According to the shell model of the nucleus the protons and neutrons in the nucleus occupy energy levels analogous to the energy levels occupied by electrons outside the nucleus of an atom Just as certain closed shell electron con gurations are associated with the special stability of the noble gases nuclei with certain numbers of neutrons and protons are especially stable These numbers are referred to as magic numbers and correspond to the number of protons or neutrons in a completed shell For protons magic numbers are 2 8 20 28 50 82 and possibly 114 Magic numbers for neutrons are 2 8 20 28 50 82 and 126 There is also evidence that pairs of protons or neutrons impart stability to a given nucleus As indicated in the following table the majority of stable nuclei have an even number of protons and neutrons while a smaller number have either an even number of protons or neutrons but not both A very small number of stable nuclei have an odd number of protons and neutrons Number of Stable Isotopes 157 52 50 5 Number of protons even even odd odd Number of neutrons even odd even odd Another important factor in determining whether a nucleus is stable or not in the neutron to proton ratio For stable nuclei of low atomic number up to 20 the neutron to proton ratio is approximately 1 to l 1 However as the atomic number increases so does the neutron to proton ratio for stable nuclei For nuclei with higher Z more neutrons are required to offset the increasing repulsive interactions among the protons When Z becomes very large no stable nuclei exist Note that no stable nuclei with Z gt 83 exist and all elements with Z 83 with the exception of Tc Z 43 and Pm Z 61 have at least one stable isotope Thorium Th Z 90 and Uranium U Z 92 have no stable isotopes however they are very common because of the extremely long halflives 108 to 109 years of some of their isotopes Number of neutrons N 23 130 Alpha emission 120 110 100 90 80 70 60 Electron capture and 0 1 positron emission 50 40 30 20 10 0 102030 405060 70 80 90100 Number of protons Z A plot of the number of neutrons N vs atomic number Z for stable nuclei shows that these stable nuclei fall in a narrow band referred to as the band of stability Classi cation of the Elements The elements can be classi ed with respect to a number of diiTerent characteristics 1 Phase at Standard Ambient Temperature and Pressure SATP 25 C and 100 kPa Under these conditions two elements are liquid 11 are gases and the rest are solids 2 MetalNonmetal We must carefully de ne our metalnonmetal criteria Metals typically have a characteristic luster 7 However some nonmetals such as iodine and metalloids such as silicon also have lustrous surfaces Metals tend to be good conductors of heat7 However diamond an allotrope of the nonmetal carbon has an extremely high thermal conductivity 24 Metals are usually malleable anal ductile 7 However some of the transition metals are very brittle High three dimensional electrical conductivity at SATP is likely the best criterion of a metal We specify 3D because graphite another allotrope of carbon has a high electrical conductivity in two dimensions due to its layered structure We specify 25 C because below 18 C tin exists in the gray tin form which is a semiconductor and 100 kPa because at elevated pressure iodine becomes a conductor Furthermore the conductivity of a metal decreases with increasing temperature whereas the conductivity of nonmetals increases under these conditions The metalloids or semimetals B Si Ge As Te exhibit behavior which is intermediate between metals and nonmetals Periodic Properties Many properties of the elements can be understood in terms of the electron con gurations of their atoms As we mentioned previously atoms of elements in the same group or family have the same valence shell electron con guration Since valence shell con guration in uences chemical reactivity elements in the same family will exhibit similar chemical properties Periodic Law states that when the elements are arranged in order of increasing atomic number their chemical and physical properties vary periodically These properties are therefore called periodic properties and we will consider three of them atomic radius ionization energy and electron affmity The latter two are of particular importance in the subject of chemical bonding 1 25 Alum Raduus Aenme mdms us a snmewhat arnbxgunus pmperty K r smee amms du nnthave abmptbnundanes Quantum mechamcs umy auuws us m duseuss me pmbabxhnes nf ndmg elemurrs at vannus ddsLances Sam the nucleus Fur Example the pm m the ugu shnws me alsmmn ddsmbunnn m an aran atnm The pmbabxhty nf ndmg an elemn deereases gadually thh mtreasmg ddsLance Sam the nucleus but dnesnntabmpdy reach zem As a result amme raddus rs de ned m a mure ur1ess aruraary mamer and seuead measures nf amme mdrus Exist mar m m A Cuvaleruraduus Halftheddnancebetweenthenudanf Wm xdmncal aeums when Jnmed by a 2 l39cuv smglebnnd Ynu shnuld reeau that an angska A rs aumt nf measure equal m 10 mm 100 pm rm 2128A064A F2 01128A B VanderWazlsmtlms Halftheddsmncebetweenthenudanftwnatnmsnfathacent mnlemles e2 l39vdwa C Metalle raduus Half the dxsmce between the nude ufwm nagdburmg atnms m the snhd metal 26 Fairly reliable values of covalent radii are tabulated for most of the elements but these are experimental values so there may be variations from one set of measurements to another Two generalizations can be made regarding the variation in covalent radius with position in the periodic chart 1 Covalent radii tend to decrease from le to right across a given period 2 Covalent radii tend to increase from top to bottom in a given family 250 i39 200 150 Atomic radius pm 100 50 Atomic number 27 M m HlA w VA WA VHA m H He mm a lt1 mm o a 05 5 1 a d a e 0 0630909 Few00090999 tevaGGooooo weeeaaoew These trends ean be Explamed on thebasrs ofthe factors thatrn uenee the srze othe outermost orbrtal of an atom In general there are two sueh factors whteh rntIluenee the slze of the outermost orbttal and m turn determrnes the srze ofthe atom l The etfeeave nuelear charge aeang on the eleetrons m the orbrtal The etfeeave nuelear chargels the posruve eharge aetang on agvm eleetronm an atom In geneal the oute eleetrons expmence a posruve charge that ls somewhat smaller than the aetual nuelear eharge Thrs ls the result of smemmgquot of nuelear eharge by mner eleetrons whreh on avarage are between the nueleus and the eleetron of mteest The etreetave nuelear charge zd ls equal to the aetual nuelear eharge Z mmus some sereemng faetor 0 to whreh all the mta39vmmg eleetrons eontnbute Thtsmeluoles eleetrons m the same shell z ze 0 2 The pnnclpal quantum number n ofthe orbrtal Thelargerthevalue ofn thelargerthe orbrtal Thlsls apparemlfyou eonsrder the eleetron dtstnbutaon for the argon atom shown above 28 The first factor is of more importance in determining the trend within a given period where 11 remains constant As each successive electron is added to the valence shell on moving from one element to the next a proton is also added to the nucleus Electrons within the same shell are not perfectly effective in screening or shielding and so the effective nuclear charge increases across a period Li He 2s1 134 A Be He 2s2 091 A B He 2s2 2p1 082 A C He 2s2 2p2 077 A N He 2s2 2p3 074 A 0 He 2s2 2p4 070 A F He 2s2 2p5 068 A The second factor in more important in determining the radius trend in a given family because the effective nuclear charge remains essentially constant in a given family 1139 2s1 134 A Na 3s1 154 A K 481 196 Rb 581 216 Cs 6s1 235 A 29 Slater39s Rules Wnte me elector stxucmre ofthe atommu m are followmg goupmgs 15 25v 21gt 3539 301 4549 46 40 5559 To calculate ofor angm e39 e39 m groups to me n31 do not sheldthose to me le for us andnp e39 bah 1 e39 m are same us up goup conmbute 0 351o 0 except m 1s where 0 3015 used e39 m are rr 1 goup conmbute 0 85 e39 m are rr2 andlower groups conmbute 1 00 1 e they shxeld completely Dwmnc 1nmu hm gt 1 rs 211 m renarnn m for nd or rrfe39 2131 n meth e39 m the same goup conmbute 0 3510 0 arms e39 m groups to the m conmbute 1 00 Consrder a4d e39 m zr 152 252 21gt 352 3p 301 452 4p 4012 4 552 5pquot 0035361003635 z z 040 36 353 65 Consrder a 5s e39 m zr 00 35100 852810036 85 Z1 0403685315 210 Removal of e39 from an atom reduces the effects of shielding making the ion more Hlike Principle quantum number becomes a more dominant effect in determining energy Ionization Energy Ionization energy is de ned as the minimum energy required to remove the highest energy electron from an isolated neutral gas phase atom For Li the ionization energy is the energy associated with the process Li a L e39 IE 520 k Two generalizations can be made regarding the variation in ionization energy IE with position in the periodic chart 1 IE generally decreases from the top of a group to the bottom As r1 increases the electron to be removed is farther away from the nucleus and less tightly bound As a result IE decreases 2 IE generally increases from le to right across a given period although this is not a smooth trend Just as it is responsible for the decrease in atomic radius going across a given period the increasing effective nuclear charge is also responsible for the general increase in IE These trends are illustrated in the accompanying plot of IE vs atomic number As you can see there are some glitches in the variation in IE across a period Ionization energy kJmol Li Be Ne 10 18 36 LE kJmol 520 899 801 1086 1402 1314 1681 2081 54 Atomic number H 25 H 25 H H 25 211 86 1 3 2 2p L 1 L 2p 1 W 2p 212 You will note that in the second period the IE of B is actually smaller than that of Be and that in period three the IE of Al is smaller than that of Mg B and Al have a ns2 np1 valence shell con guration while Be and Mg have a ns2 valence shell con guration In general binding energies for p orbitals are generally smaller than for s orbitals because of the slight shielding effect s e39 have on p e39 Less energy is require therefore to remove the np electron from B or Al than an s electron from Be or Mg In the same two periods 0 and S have smaller IEs than N and P respectively This is due to the fact that O and S have ns2 np4 con gurations in which two electrons occupy the same p orbital whereas N and P have ns2 np3 con gurations and each electron resides in its own orbital The electronelectron repulsion of the two electrons in the same orbital in O and S makes it easier to remove one of them than it would be if they occupied separate orbitals IEs of transition metals gradually increase due to the relatively ineffective shielding of s e39 by d e39 Electrons can be removed successively from atom so atoms have a second ionization energy a third ionization energy etc As each electron is removed an ion with a greater positive charge remains In addition the orbitals containing the remaining electrons contract As a result the value of each successive ionization energy becomes larger Consider the atoms Li Be and B 1131 1132 113 1134 kJmol Li 520 7298 11815 Be 899 1757 14848 21006 B 801 2427 3 660 2 5025 The removal of electrons from a core shell requires large amounts of energies The second ionization energy of Li corresponds to the removal of one of the ls electrons and is more than a factor of 10 greater than the first ionization energy Large increases can also be seen in the third and fourth ionization energies of Be and B respectively 2 l 3 Electron Attachment Enthalpy AHEA The property which your text refers to as electron affinity of an atom is actually the electron attachment enthalpy The electron attachment enthalpy is the enthalpy change for the process of adding an electron to a neutral atom in the gas phase to form a negative ion F g e39 a F39 g AHEA 328 ldmol IfAHEA lt 0 energy is released in the process and the formation of a negative ion is energetically favorable If AHEA gt 0 the formation of a negative ion is not energetically favorable The periodic variation in AHEA is somewhat more complicated than atomic radius or ionization energy However AHEA tends to become more negative as you move from le to right across a period AHEA kJmol Li 60 Be 240 1 if 7 if e39 goes into 2p subshell 2s 2p B 27 C l22 N 0 i e39hastopairup 28 p O l 41 F 328 Ne 30 i 1 JL i e39 goes into n 3 lell 2s 2p Note that the formation of Li39 is exothennic while the formation of L is endothermic Sequential electron attachment enthalpies 0 g e39 a 0 g AHEA 141 ldmol 039 g e39 a 0239 g AHEA 744 ldmol This may seem surprising and points out that the oxide ion is stable only in the presence of some other driving force such as the formation of a crystal lattice where the additional energy required can be made up for
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