17 must be in group 7 and have 7 electrons in the outside shell. So that makes it a halogen. It's one element short of Ar(which you should know is Ar and that makes it sure bet to be Cl. So I would write [Ne] (which is 10) 3s2 3p5.
The others are done the same way.
20 must be two after Ar and in group 2 and that would make it [Ar]4s2. Element 23 must be in the 3d transition series so it is just 3 more than Ca. Where would those extra three electrons go?
Without referring to a periodic table, pick the electron configuration of elements with the following atomic numbers and classify the elements. (Type your answer in noble gas notation using the format [Ar] 4s2 3d10 4p2 for [Ar]4s23d104p2.)
(a) 17
(b) 20
(c) 23
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