Given that A={1,2,3,4,5} and B={3,4,5,6,7} find AUB
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b) [Ar]4s²3d1¹

Explanation:

Chromium, Cr, is the atom with atomic number 24 (That is, 24 protons).

A Cr³⁺ contains 3 electrons less than number of protons. That is 21electrons.

The Argon, Ar, contains 18 electrons. That means the electronic configuration of Cr³⁺ contains 3 electrons more than Ar.

Following the Hund's rule, these three electrons will occupy the 4S orbital (2 electrons) and the 2d orbital (1 electron).  

Thus, right solution is: