Tantalum ionization energy

On the basis of your knowledge of periodicity, place each of the following sets of elements in order of decreasing ionization energy. Explain your choices, (a) Selenium, oxygen, tellurium (b) gold, tantalum, osmium (c) lead, barium, cesium. 

Owing to lanthanide contraction, niobium and tantalum have virtually identical atomic rad (1.47 A) and close ionization energies (Nb6.67, Ta7.3eV), and usually display very similt chemical behavior. Some definite differences can however be noted these can usually be trace to the lower sensitivity of tantalum to reduction and to its higher affinity for dioxygen. lb tantalum-element multiple bonds are usually stabler, while MfiXg arrangements are so ft known only for niobium. 

Fig. 3. The ionization energy of the 4f shell in oxides (empty cirdes), fluorides (squares) and other compounds (filled circles) of lanthanides, hafnium and tantalum. The effects of spin-pairing energy are compared with values of I derived from the calculated uncorrected optical electronegativities (116) for M(IV) and given as full curves appropriate for the ionization of M III). The dashed lines connect the iso-electronic 4f and 4f systems with differing oxidation state.

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