b) Write the complete electron configuration (e.g.,1s 2 2s 2 …etc.) for a seleni

Question

b) Write the complete electron configuration (e.g.,1s22s2…etc.) for a selenium atom in theground state. Indicate the number of unpaired electrons inthe ground-state atom, and explain your reasoning.

c) In terms of atomic structure, explain why the firstionization energy of selenium is

           (i) less than that of bromine (atomic number 35), and

           (ii) greater than that of tellurium (atomic number 52).

Explanation / Answer

a) all isotopes have the same number of protons and electrons. Theydiffer in the number of neutrons. so all isotopes have the same atomic number and possible number ofoxidation states They differ in mass numbers (protons + neutrons ) b) 1s22s22p63s23p64s23d104p4 c) Bromine has one more proton and one more electron. The electron isstill in the same period, so it is added to the same principalelectron shell. Effectively, the nuclear charge density hasincreased, but the new outermost electron is not much farther awayfrom the nucleus as in selenium E ~ Z2/r, where Z is the atomic numver Z_Br > Z_Se.   But r_Br ~ r_Se Tellerium has a lower ionization energy, because a new electron isadded to the 5p shell, a principal shell higher than the that ofselenium's valence electron. r_Te > r_Se, despite Z_Te > Z_Se. It so happens that r_Te is large enough to offset the gains innuclear charge. So E_Te is lower General trends to look for: ionization energy increasesacross a period, but decreases down a column (group)

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