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

# Fourth Period Elements with 4d Electrons **Understanding the Question:** The fourth period includes elements with atomic numbers 19–36 (K to Kr). We need to count elements that have at least one electron in the 4d orbitals. **Key Insight:** The 4d orbitals begin filling in the **fifth period**, not the fourth period. The aufbau principle shows: - Fourth period: $[Ar]$ + 4s electrons + 3d electrons (3d orbitals fill before 4d) - Fifth period: $[Kr]$ + 5s electrons + 4d electrons (4d orbitals fill here) **Step-by-Step:** 1. Fourth period elements (19–36) have electron configurations ending in either 4s or 3d 2. Examples: K ($[Ar]4s^1$), Ca ($[Ar]4s^2$), Sc ($[Ar]3d^1 4s^2$), ..., Kr ($[Ar]3d^{10}4s^2 4p^6$) 3. None of these have 4d electrons—the 4d orbitals are empty in the fourth period 4. Therefore, the number of fourth-period elements with one or more 4d electrons is **zero (0)** **Answer: C** corresponds to **0** elements.