20.18 Locate on the periodic table below the transition elements with the following electron configurations. Identify each element. (a) [Ar] 3d 7 4s2 (c) [Kr] 4d 2 5s2 (b) [Ar] 3d 5 4s1 (d) [Xe] 4f 3 6s2 20.19 What is the general trend in the following properties from left to right across the first transition series (Sc to Zn)? Explain each trend. (a) Atomic radius (b) Density (c) Ionization energy (d) Standard oxidation potential 20.20 Classify the following ligands as monodentate, bidentate, or tridentate. Which can form chelate rings? 20.21 Draw the structure of the following complexes. What are the oxidation state, coordination number, and coordination geometry of the metal in each? (a) Na[Au(CN)2] (b) [Co(NH3)5Br]SO4 (c) Pt(en)CL2 (d) (NH4)2[PtCL2(C2O4)2 20.22 Consider the following isomers of [Cr(NH3)2CL4] : (a) Label the isomers as cis or trans. (b) Which isomers are identical, and which are different? (c) Do any of these isomers exist as enantiomers? Explain. 20.23 Consider the following ethylenediamine complexes: (a) Which complexes are chiral, and which are achiral? (b) Draw the enantiomer of each chiral complex. (c) Which, if any, of the chiral complexes are enantiomers of one another? 20.24 Predict the crystal field energy-level diagram for a square pyramidal ML5 complex that has two ligands along the +x and +y axes but only one ligand along the z axis. Your diagram should be intermediate between those for an octahedral ML6 complex and a square planar ML4 complex. 20.25 Imagine two complexes, one tetrahedral and one square planar, in which the central atom is bonded to four different ligands (shown here in four different colors). Is either complex chiral? Explain.