An element has the electron configuration [Ar] 3d⁷ 4s². Which block of the periodic table does it belong to, and what is its position in the periodic table?
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Correct answer
C. d-block, period 4, group 9
Principle or equation
The periodic table is divided into blocks according to the highest-energy subshell that is occupied. The period number is the highest principal quantum number (n) of an occupied orbital. For d-block elements, the group number is often the sum of the electrons in the (n-1)d and ns subshells (for groups 3-12).
Why this answer is correct
The configuration [Ar] 3d⁷ 4s² shows that the last electrons are in the 3d subshell, so the element is in the d-block. The highest principal quantum number is 4, so it is in period 4. The total number of d and s electrons is 7 + 2 = 9, so it is in group 9 (cobalt, Co).
Example
For [Ar] 3d⁵ 4s¹, the element is in the d-block, period 4, group 6 (chromium).
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