In the fourth period of the periodic table, how many elements have one or more 4d electrons? — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

# Elements with 4d Electrons in Period 4 **Understanding the question:** We need to identify how many elements in Period 4 contain electrons in the 4d orbital. **Key insight:** Period 4 corresponds to filling orbitals with principal quantum number $n = 4$. The order of orbital filling follows the Aufbau principle: $4s$ fills before $4d$. **Electron configuration analysis:** - Elements K (Z=19) through Ca (Z=20): Fill $4s$ only - Elements Sc (Z=21) through Zn (Z=30): Fill $3d$ orbitals (NOT $4d$) - Elements Ga (Z=31) through Kr (Z=36): Fill $4p$ orbitals **The 4d orbital begins filling in Period 5:** The first element with $4d$ electrons is Y (yttrium, Z=39), which is in Period 5, not Period 4. $$\text{Period 4 ends with Kr: } [Ar] \, 3d^{10} \, 4s^2 \, 4p^6$$ **Conclusion:** No elements in Period 4 have $4d$ electrons. $$\boxed{\text{Answer: 0 elements}}$$ (Note: If the answer key shows "C" represents 0, this is correct. If "C" represents a different number, the question or answer key may need clarification, as the fourth period exclusively fills $4s$, $3d$, and $4p$ orbitals.)