Tellurium is a period 5 chalcogen. Selenium is a period 4 chalcogen. If the only

Question

Tellurium is a period 5 chalcogen. Selenium is a period 4 chalcogen. If the only factor affecting ionization energies was the nuclear charge, then electrons would be easier to remove (ionize) from Se than Te. Experimentally the opposite is true. It takes 941.0 kJ/mol of energy to ionize the outermost electron from Se while it only takes 869.3 kJ/mol to ionize from Te. A good model should account for this. Quantum mechanical calculations do predict this but require access to sophisticated software, large amounts of computing power and technical expertise. Slater suggested that some simple empirical rules that take into account electron electron repulsion (or shielding) could give a good estimate of the effective nuclear charge (Zeff). The Zeff for the outermost electron in Te is ________. The Zeff for the outermost electrons in Se is _______. According to Slater's calculation of effective nuclear charge _______ (does or does not) predict the correct ordering of ionization energies for Se and Te. A better means of rationalizing ionization energies is to include the atomic _______ as follows: Z subscript e f f end subscript over r squared . For Te, r = 136 pm and for Se r = 117 pm. This new model _______ (does or does not) predict the correct ordering of first ionization energies for Se and Te.

Explanation / Answer

Calculating the Zeff for Se and Te, first let's write the complete electron configuration for both atoms:

ZSe = 34; ZTe = 52

[Se] = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4  

arranging: (1s)2 (2s,2p)8 (3s,3p)8 (3d)10 (4s,4p)6

[Te] =  1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p4

arranging:  (1s)2 (2s,2p)8 (3s,3p)8 (3d)10 (4s,4p)8 (4d)10 (5s,5p)6

To do the Zeff:

Zeff = Z - s

to get S to both of them for the outermost electron:

sSe = 5*0.35 + 8*0.85 + 10*1 + 10*1 = 28.55

sTe = 5*0.35 + 8*0.85 + 10*1 + 28*1 = 46.55

Zeff Se = 34 - 28.55 = 5.45

Zeff Te = 52 - 45.05 = 5.45

According to Slater's calculation of effective nuclear charge does not predict the correct ordering of ionization energies for Se and Te. A better means of rationalizing ionization energies is to include the atomic radius as follows: Z subscript e f f end subscript over r squared . For Te, r = 136 pm and for Se r = 117 pm. This new model does predict the correct ordering of first ionization energies for Se and Te.

Hope this helps

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