Two elements X and Y have atomic numbers 6 and 17 respectively. The sum of their period numbers is

Explanation

To find the sum of the period numbers for elements X and Y with atomic numbers 6 and 17 respectively, we can follow these steps:

### Step 1: Identify the Elements
- **Element X** has an atomic number of 6. Referring to the periodic table, this corresponds to **Carbon (C)**.
- **Element Y** has an atomic number of 17. This corresponds to **Chlorine (Cl)**.

### Step 2: Determine the Electronic Configurations
- For **Carbon (C)** with atomic number 6:
- The electronic configuration is:
- 2 electrons in the first shell (K shell)
- 4 electrons in the second shell (L shell)
- Therefore, the electronic configuration is **1s² 2s² 2p²**.

- For **Chlorine (Cl)** with atomic number 17:
- The electronic configuration is:
- 2 electrons in the first shell (K shell)
- 8 electrons in the second shell (L shell)
- 7 electrons in the third shell (M shell)
- Therefore, the electronic configuration is **1s² 2s² 2p⁶ 3s² 3p⁵**.

### Step 3: Determine the Period Numbers
- The period number corresponds to the highest principal quantum number (n) in the electronic configuration:
- For **Carbon (C)**, the highest principal quantum number is 2 (from the L shell), so it is in **Period 2**.
- For **Chlorine (Cl)**, the highest principal quantum number is 3 (from the M shell), so it is in **Period 3**.

### Step 4: Calculate the Sum of the Period Numbers
- Now, we add the period numbers:
- Period number of Carbon (C) = 2
- Period number of Chlorine (Cl) = 3
- Sum = 2 + 3 = **5**.

### Final Answer
The sum of the period numbers of elements X and Y is **5**.

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