Question

Why is the electronic configuration of Cu is [Ar] 3d10 4s1 and not 4s2 3d9. Why does 3d comes first​

Answer 1

Answer:

well u can't argue upon this it's just a rule . and that's how I also learned it . I'm telling u trick u can also use to remember

just remember that

1. we see principal quantum no.

2. azimuthal

now first see n in 3d and 4s. 3d is small hence came first

but suppose u have now 4s and 4p see l s come first

so in configuration 3d then 4s then 4p

hope u get it now

also Cu is exception its configuration is 3d10 4s1 and not 3d9 4s2 because of half and fully filled orbital are extra stable .

Cu is becoming more stable with 3d10 orbital hence its configuration is like that.

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Answer 2

Cu has electronic configuration [Ar]3d¹⁰4s¹ but not 4s²3d⁹ because it's d- subshell is completely filled and more stable than incomplete subshell.

Hope this helps.