Following the completed shells and subshells in krypton, the subsequent elements first start filling shell O, subshell 5s, in alkali metal Group I element 37, rubidium Rb, then with two
Trang 1Platinum Group Elements
AJ Appleby,Texas A&M University, College Station, TX, USA
& 2009 Elsevier B.V All rights reserved.
Introduction
Electronic Structure
The platinum group metals occupy the second and third
rows of periodic table Group VIII, the first row of which
consists of iron (Fe), cobalt (Co), and nickel (Ni) The
second-row elements ruthenium (Ru), rhodium (Rh), and
palladium (Pd) all have the krypton (Kr, inert gas, atomic
number 36) core of completed electron shells and
sub-shells (see Chemistry, Electrochemistry, and
Electro-chemical Applications: Oxygen) With n as the principal
quantum number, these are (n ¼ 1): (shell K, subshell 1s2);
(n ¼ 2): (shell L, subshells 2s2, 2p6), (n ¼ 3): (shell M,
subshells 3s2, 3p6, 3d10), (n ¼ 4): (shell N, subshells, 4s2,
4p6) It should be noted that subshells s, p, d, f, y
rep-resent those with angular or azimuthal quantum number
l ¼ 0, 1, 2, 3, 4, y Following the completed shells and
subshells in krypton, the subsequent elements first start
filling shell O, subshell 5s, in alkali metal Group I
element 37, rubidium Rb, then with two in alkaline earth
Group IIA element 38, strontium Sr Subshell 5s can only
accommodate two electrons, so the previously
un-occupied shell N, subshell 4d, the next highest in energy,
is filled next It should be noted that shell N, subshell 4f,
which is of still higher energy, is as yet unoccupied
compared with shell O, subshell 5s Then shell N,
sub-shell 4d, is filled by one electron at a time from element
39, yttrium, Y, to the next element zirconium, 40, Zr
Following this, electrons are exchanged between shell N,
subshell 4d, and shell O, subshell 5s because of similar
energies Thus element 41, niobium, Nb, has four
elec-trons in subshell 4d, and only one in subshell 5s This
continues until element 43, technecium, Tc, which has
six subshell 4d electrons and again one in subshell 5s We
now arrive at the first-row platinum metal, element 37,
Ru, with seven electrons in subshell 4d, and one in
subshell 5s; element 38, Rh, with eight in 4d and again
one in 5s; and finally element 39, Pd with ten in 4d and
none in 5s This is followed by element 40, silver, Ag
(from the Latin word argentum), in Group IB, which has
the inner electronic structure of Pd with one 5s electron
Following this, 5s and 5p are filled until the next inert
gas electronic configuration with fully completed shells,
element 54, xenon, Xe, is reached It has completed s and
p shells with 2 and 6 electrons, respectively Group IA
element 55, caesium, Cs, then follows by having one
electron in shell P, subshell 6s Group IIA element 56,
barium, Ba, follows with two electrons to complete this
subshell, the arrangement corresponding to that for Rband Sr discussed above
Having filled shell P, subshell 6s, the rare earth elementlanthanum, La, adds an electron to complete shell O,subshell 5d Successive one-electron additions to shell N,subshell 4f, which has previously been empty, then suc-cessively occur until rare earth element 71 lutecium, Lu, isreached, to give its maximum occupancy of 14 electrons.This results in very large progressive downward change inatomic volume (Ba, 36.34 cm3g-atom1; La, 22.6; element
76, osmium, Os, 8.49) This is known as the lanthanidecontraction In all the metal of the rare earth series, sub-shell 5d contains only one electron We now arrive at theelectron core of element 72, hafnium, Hf, which has thelutecium arrangement but with two 5d electrons Thehafnium core to the 5p subshell provides the basis of theinner electronic structure of the third-row Group VIIIplatinum metals After Hf, tantalum, Ta (element 73) adds
a further 5d electron to give a total of 3 They are furtheradded through tungsten, W (element 74), and rhenium, Re,
75 Os (element 76), the first third-row Group VIIIelement has 6 However, in element 77, iridium, Ir, a lowerenergy state results from combining its two 6s electronswith its seven 6d electrons to give a total of nine 6delectrons Platinum, Pt (element 78) has the same 9 elec-tron 6d configuration, but with an additional 6s electron
Chemical Properties Resulting from Electron Configurations
First Row: Iron, Cobalt, and NickelThe first-row Group VIII metals Fe, Co, and Ni aretypical transition metals Their atomic weights are 55.85,58.93, 58.69; their room temperature specific gravities are7.86, 8.90, 8.90; and their MPs (MPs) are 1535, 1495, and
1455 1C, respectively Their atomic volumes (cm3atom1) are 7.43, 7.11, and 6.76, respectively They are allface-centered cubic They all oxidize in water close to thereversible hydrogen potential For the metals, the po-tentials given below are standard states at 25 1C, in water
g-in the absence of complexg-ing agents, taken from M.Pourbaix’s Atlas of Electrochemical Equilibria in Aqueous So-lutions (at 25 1C) For example, the Fe–FeO (Fe2þ) tran-sition is at 0.047 V vs HE (the reversible hydrogenelectrode in the same solution) This is followed by for-mation of the mixed valence oxide FeO Fe2O3 (mag-netite, Fe3O4) at 0.085 V vs HE Fe2O3(Fe3þ) forms at
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