Without looking at the periodic table, select the elements of IIIA group of the periodic table (atom — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

# Solution: Identifying Group IIIA Elements **Group IIIA (Group 13) Configuration:** Elements in Group IIIA have **3 valence electrons** in their outermost shell with the general electron configuration: $ns^2np^1$ **Method to identify without periodic table:** 1. **Identify the period** (row) from atomic number using shell capacity: - Period 1: 1-2 (capacity: 2) - Period 2: 3-10 (capacity: 8) - Period 3: 11-18 (capacity: 8) - Period 4: 19-36 (capacity: 18) 2. **Determine group position** within that period by counting valence electrons: - IIIA elements have **3 valence electrons** - They appear as the 3rd element in the p-block of each period 3. **Known IIIA elements** and their atomic numbers: - Boron (B): Z = 5 → [He] 2s² 2p¹ - Aluminum (Al): Z = 13 → [Ne] 3s² 3p¹ - Gallium (Ga): Z = 31 → [Ar] 3d¹⁰ 4s² 4p¹ - Indium (In): Z = 49 → [Kr] 4d¹⁰ 5s² 5p¹ - Thallium (Tl): Z = 81 → [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ **Why D is correct:** Option D contains the atomic numbers corresponding to the known Group IIIA elements (5, 13, 31, 49, 81), each showing exactly 3 valence electrons in the outermost shell.