The fourth period of modern periodic table includes:

Options

A. 8 elements
B. 18 elements
C. 32 elements
D. unknown elements

Explanation

To determine how many elements are included in the fourth period of the modern periodic table, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Period Number**:
The fourth period of the periodic table corresponds to period number \( n = 4 \).

2. **Determine if the Period Number is Even or Odd**:
Since \( n = 4 \) is an even number, we will use the formula for even periods.

3. **Use the Formula for the Number of Elements**:
The formula for the number of elements in an even period is:
\[ \text{Number of elements} = \left( \frac{n + 2}{2} \right)^2 \times 2 \]

4. **Substitute the Value of \( n \)**:
Substitute \( n = 4 \) into the formula:
\[ \text{Number of elements} = \left( \frac{4 + 2}{2} \right)^2 \times 2 \]

5. **Calculate the Expression**:
- First, calculate \( 4 + 2 = 6 \).
- Then, divide by 2: \( \frac{6}{2} = 3 \).
- Next, square the result: \( 3^2 = 9 \).
- Finally, multiply by 2: \( 9 \times 2 = 18 \).

6. **Conclusion**:
Therefore, the fourth period of the modern periodic table includes **18 elements**.

### Final Answer:
The fourth period of the modern periodic table includes **18 elements** (Option B).

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