The ns and (n-1)d subshells are so close in energy that

Once d elecrons are added,  the (n-1)d subshell becomes sligtly

lower in energy than the ns subshell.

As a result, the ns electrons are at higher energy and

Are always removed before (n-1)d electrons when transition metals

form cation.

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여기서만 20분째 고민중인데 도와주실분  계신가요?