An element has the electron configuration [Ar] 3d^7 4s^2. Which of the following statements about this element is correct?
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Correct answer
B. It is a transition element with 7 electrons in the 3d subshell.
Principle or equation
Elements with partially filled d subshells are transition metals; the 4s is filled before 3d, and the outermost shell is the highest principal quantum number.
Why this answer is correct
The configuration [Ar] 3d^7 4s^2 has partially filled 3d, so it is a transition metal. The outermost shell is n=4, containing 2 electrons in 4s, not 10. It is in period 4 and group 9 (not group 2).
Example
For [Ar] 3d^7 4s^2, the element is cobalt (Co), a transition metal.
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