The number of elements and no. of shells in third period is respectively.

Options

A. 18,3
B. 8,3
C. 18,4
D. 8,2

Answer

To solve the question regarding the number of elements and the number of shells in the third period of the periodic table, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Period**: The third period of the periodic table includes elements from sodium (Na) to argon (Ar).

2. **Count the Elements**:
- The elements in the third period are:
- Sodium (Na)
- Magnesium (Mg)
- Aluminum (Al)
- Silicon (Si)
- Phosphorus (P)
- Sulfur (S)
- Chlorine (Cl)
- Argon (Ar)
- Counting these elements gives us a total of 8 elements in the third period.

3. **Determine the Number of Shells**:
- The number of shells corresponds to the principal quantum number (n).
- For the third period, the principal quantum number is n = 3, which means there are 3 shells.
- The electron configuration for the elements in the third period shows that they all have electrons filling up to the third shell.

### Final Answer:
- The number of elements in the third period is **8**.
- The number of shells in the third period is **3**.