Transition elements ionic radii

Element Ionic radii Transition elements Ionic radii Absorption (color) Emission Rare earths Ionic radii Emission... 

A transition from tris-glycolates->tris-glycolate dihydrate or hydroxyacetate-oxyacetate may be followed by a decrease in van der Waals repulsion, and the decrease is large for elements with small ionic radius. Thus the observations that the lanthanides with smaller ionic radius form the tris-glycolate dihydrates or hydroxyacetate-oxyacetate complexes 150,151) and have lower CN can be rationalized. A decrease in stability due to short oxygen-oxygen contacts is not an unique feature for the anhydrous tris-glycolates, but may be. an important property. 

Symbol La atomic number 57 atomic weight 138.91 a rare-earth transition metal, precursor to a series of 14 inner-transition elements known as the lanthanide series electron configuration [XejSdiGs oxidation state -i-3 atomic radius 1.879A ionic radius (LaS+) 1.061A electronegativity 1.17 two natural isotopes are La-139 (99.911%) and La-138 (0.089%). 

Symbol Lu atomic number 71 atomic weight 174.97 a lanthanide series element an /-block inner-transition metal electron configuration [Xe]4/i45di6s2 valence -1-3 atomic radius (coordination number 12) 1.7349A ionic radius (Lu3+) 0.85A two naturally-occurring isotopes Lu-176 (97.1%) and Lu-175(2.59%) Lu-172 is radioactive with a half-life of 4xl0i° years (beta-emission) several artificial isotopes known, that have mass numbers 155, 156, 167—174, 177—180. 

Symbol Nd atomic number 60 atomic weight 144.24 a rare earth lanthanide element a hght rare earth metal of cerium group an inner transition metal characterized by partially filled 4/ subshell electron configuration [Xe]4/35di6s2 most common valence state -i-3 other oxidation state +2 standard electrode potential, Nd + -i- 3e -2.323 V atomic radius 1.821 A (for CN 12) ionic radius, Nd + 0.995A atomic volume 20.60 cc/mol ionization potential 6.31 eV seven stable isotopes Nd-142 (27.13%), Nd-143 (12.20%), Nd-144 (23.87%), Nd-145 (8.29%), Nd-146 (17.18%), Nd-148 (5.72%), Nd-150 (5.60%) twenty-three radioisotopes are known in the mass range 127-141, 147, 149, 151-156. 

Symbol Ni atomic number 28 atomic weight 58.693 a transition metal element in the first triad of Group VIll(Group 10) after iron and cobalt electron configuration [Ar]3d 4s2 valence states 0, -i-l, +2, and -f-3 most common oxidation state +2 the standard electrode potential, NF+ -1- 2e Ni -0.237 V atomic radius 1.24A ionic radius (NF+) 0.70A five natural isotopes Ni-58 (68.08%), Ni-60 (26.22%), Ni-61 (1.14%), Ni-62 (3.63%), Ni-64 (0.93%) nineteen radioactive isotopes are known in the mass range 51-57, 59, 63, 65-74 the longest-lived radioisotope Ni-59 has a half-life 7.6x10 years. 

The variations in ionic radii of the transition elements of the 4th period serve to exemplify the arguments needed to rationalize similar variations in the other transition series. Figure 7.3 includes a plot of the radii of the 2 + ions of those transition elements of the 4th period that form them. The plot includes the radius of the Ca2+ ion, which represents the beginning of the series but has no 3d electrons, as also has the Zn2+ and Ga3+ ions (both 3d10) at the end of the series. The radii are those of octahedrally coordinated ions as they are found in crystalline compounds, the counter-ions (i.e. the ions of opposite charge) being situated at the vertices of an octahedron, as shown in Figure 7.4. 

Although there is no space to develop a detailed discussion of the solubilities of compounds of the transition elements, the general insolubility of their + 2 and + 3 hydroxides is important. The rationale underlying their insolubility can be summarized (i) the hydroxide ion is relatively small (152 pm ionic radius) and the ions of the +2 and +3 transition metals assume a similar size if their radii are increased by 60-80 pm, and (ii) the enthalpy of hydration of the hydroxide ion (—519 kJ mol ) is sufficiently negative to represent a reasonable degree of competition with the metal ions for the available water molecules, thus preventing the metal ions from becoming fully hydrated. Such effects combine to allow the lattice enthalpies of the hydroxides to become dominant. 

One property of a transition metal ion that is particularly sensitive to crystal field interactions is the ionic radius and its influence on interatomic distances in a crystal structure. Within a row of elements in the periodic table in which cations possess completely filled or efficiently screened inner orbitals, there should be a decrease of interatomic distances with increasing atomic number for cations possessing the same valence. The ionic radii of trivalent cations of the lanthanide series for example, plotted in fig. 6.1, show a relatively smooth contraction from lanthanum to lutecium. Such a trend is determined by the... 

Cations of the first transition series do not conform to the smooth pattern for the lanthanide elements shown in fig. 6.1. This is illustrated in fig. 6.2a by the radii of divalent cations in oxides containing transition metal ions in high-spin states. There is an overall decrease of octahedral ionic radius from Ca2+to Zn2+, but values first decrease to V2+, then rise to Mn2+, decrease to Ni2+, and rise again to Zn2+. The characteristic double-humped curve shown in fig. 6.2a has... 

Ionic radius. The wide variation of metal-oxygen distances within individual coordination sites and between different sites in crystal structures of silicate minerals warns against too literal use of the radius of a cation, derived from interatomic distances in simple structures. Relationships between cation radius and phenocryst/glass distribution coefficients for trace elements are often anomalous for transition metal ions (Cr3+, V3+, Ni2+), which may be attributed to the influence of crystal field stabilization energies. 

The covalent radii of transition elements are subject to two additional effects that influence the values of ionic radii also. A large covalent radius for a given atom is favored by both a low oxidation number and a high coordination number. These two effects are independent neither of each other nor of bond order effects however, an adequate unified treatment of the interrelationships between bond number, coordination number, oxidation number, and bond distances for compounds of the transition metals is best postponed to a more advanced text. 

A preferential uptake of the Ba " ion (due to having an ionic radius close to that of the ammonium ion) among the alkaline earth metals also confirms this hypothesis. At the same time, the affinity for other divalent elements is at least one order of magnitude higher than that for alkali or alkaline earth metal cations. The Kd values for most transition metals and lead are in the range from 20000 to more than 100000. However, the absolute values of their uptake in 0.1 M metal nitrate solutions are relatively low. The lEC values are higher than 1.0 meq g" for only two ions, Cu and Hg. ... 

The atomic and ionic properties of an element, particularly IE, ionic radius and electronegativity, underly its chemical behaviour and determine the types of compound it can form. The simplest type of compound an element can form is a binary compound, one in which it is combined with only one other element. The transition elements form binary compounds with a wide variety of non-metals, and the stoichiometries of these compounds will depend upon the thermodynamics of the compound-forming process. Binary oxides, fluorides and chlorides of the transition elements reveal the oxidation states available to them and, to some extent, reflect trends in IE values. However, the lEs of the transition elements are by no means the only contributors to the thermodynamics of compound formation. Other factors such as lattice enthalpy and the extent of covalency in bonding are important. In this chapter some examples of binary transition element compounds will be used to reveal the factors which determine the stoichiometry of compounds. 

The following ionic radii (in angstroms) are estimated for the +2 ions of selected elements of the first transition-metal series, based on the structures of their oxides Ca (0.99), TP+(0.71), V +(0.64), Mn +(0.80), Fe +(0.75), Co +(0.72), Ni (0.69), Cu (0.71), Zn (0.74). Draw a graph of ionic radius versus atomic number in this series, and account for its shape. The oxides take the rock salt structure. Are these solids better described as high- or low-spin transition-metal complexes ... 

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