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conantee
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« on: December 31, 2009, 02:23:25 PM »

30. The configuration of the potassium atom in its ground state is 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1. Which of the following statements about potassium is true?
(A) Its n =3 shell is completely filled.
(B) Its 4s subshell is completely filled.
(C) Its least tightly bound electron has l = 4
(D) Its atomic number is 17
(E) Its electron charge distribution is spherically symmetrical.
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conantee
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« Reply #1 on: December 31, 2009, 02:33:03 PM »

It may be helpful to write electron configuration as the diagram as follows:
1s^2
2s^2     2p^6
3s^2     3p^6   
4s^1   

Now,  n=3 level is not completely filled since d subshell is not filled yet (you may remember s, p, d for n=3) So, (A) is wrong.
(B) is also wrong since, as shown in previous s subshell, one more electron can be filled.
(C) is also wrong since among s and p shell. The quantum number l does not reach number 4 yet.
(D) Counting all electrons, we have 2+2+6+2+6+1 = 19 not 17.

The only answer left is (E). One can see the symmetry since the s subshell (outer most one) is symmetric.

The correct answer is (E). 35 of 100 people got the right answer.
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