To determine which element is wrongly placed in the arrangement of elements based on the number of outer (valence) electrons, we first need to understand how many outer electrons each of the given elements has. The elements listed are Aluminium (Al), Chlorine (Cl), Silicon (Si), Magnesium (Mg), and Sodium (Na). ### Step 1: Identify the number of outer electrons for each element 1. **Aluminium (Al)**: - Atomic number: 13 - Electron configuration: 1s² 2s² 2p⁶ 3s² - Valence electrons: 3 (from the 3s²) 2. **Chlorine (Cl)**: - Atomic number: 17 - Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁵ - Valence electrons: 7 (from the 3s² 3p⁵) 3. **Silicon (Si)**: - Atomic number: 14 - Electron configuration: 1s² 2s² 2p⁶ 3s² 3p² - Valence electrons: 4 (from the 3s² 3p²) 4. **Magnesium (Mg)**: - Atomic number: 12 - Electron configuration: 1s² 2s² 2p⁶ 3s² - Valence electrons: 2 (from the 3s²) 5. **Sodium (Na)**: - Atomic number: 11 - Electron configuration: 1s² 2s² 2p⁶ 3s¹ - Valence electrons: 1 (from the 3s¹) ### Step 2: Arrange the elements in order of decreasing number of outer electrons Now, let's list the elements along with their valence electrons: - Chlorine (7) - Aluminium (3) - Silicon (4) - Magnesium (2) - Sodium (1) Arranging these in order of decreasing number of outer electrons gives us: 1. Chlorine (7) 2. Silicon (4) 3. Aluminium (3) 4. Magnesium (2) 5. Sodium (1) ### Step 3: Identify the wrongly placed element From the arrangement, we can see that **Silicon (4)** should come before **Aluminium (3)**. Therefore, the correct order should be: 1. Chlorine (7) 2. Silicon (4) 3. Aluminium (3) 4. Magnesium (2) 5. Sodium (1) ### Conclusion The element that is wrongly placed in the original arrangement is **Aluminium (B)**, as it should be after Silicon in terms of the number of outer electrons. ### Explanation of Other Options - **A. Chlorine**: This is correctly placed as it has the highest number of outer electrons (7). - **C. Silicon**: This is also correctly placed as it has 4 outer electrons, which is more than Aluminium. - **D. Magnesium**: This is correctly placed as it has 2 outer electrons, which is less than Aluminium. ### Revision Summary - **Aluminium has 3 outer electrons, Silicon has 4, and Chlorine has 7.** - **The correct order of elements based on outer electrons is Chlorine > Silicon > Aluminium > Magnesium > Sodium.** - **Aluminium is wrongly placed in the original arrangement.** - **Understanding electron configurations helps in determining the number of valence electrons.**