CBSE Class 12 · 2002 · All India Compartment · d and f Block ElementsVery Short Answer

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Write the atomic number of the element in which the filling of the 3d subshell just starts.

Answer: 21 (scandium).

Solution

The 3d subshell begins to fill after 4s is complete (at Ca, Z = 20).
The first element with a 3d electron is scandium: Z = 21, [Ar]3d4s.

CBSE Class 12 Board · 2002 · All India Compartment · Subjective

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