Which of the following pairs of elements do not follow octave rule

Options

A. Na, K
B. Ca, Sr
C. F, Cl
D. O, S

Answer

To determine which pairs of elements do not follow the octave rule, we will analyze the elements based on Newland's Octave Rule, which states that when elements are arranged in order of increasing atomic mass, every eighth element has similar properties.

### Step-by-Step Solution:

1. **Understanding the Octave Rule**:
- Newland's Octave Rule suggests that elements can be arranged in a way that every eighth element has similar properties. This was based on the arrangement of elements up to calcium.

2. **Identifying Elements**:
- We need to consider the pairs of elements provided in the question and check their positions in the periodic table.

3. **Analyzing Each Pair**:
- **Sodium and Potassium**: Sodium (Na) is in the 3rd period, and Potassium (K) is in the 4th period. They are one after the other in the alkali metals group, thus they follow the octave rule.
- **Calcium and Strontium**: Calcium (Ca) is in the 4th period, and Strontium (Sr) is in the 5th period. Newland's arrangement did not extend beyond calcium, so this pair does not follow the octave rule.
- **Fluorine and Chlorine**: Fluorine (F) is in the 2nd period, and Chlorine (Cl) is in the 3rd period. They are both halogens and follow the octave rule.
- **Oxygen and Sulphur**: Oxygen (O) is in the 2nd period, and Sulphur (S) is in the 3rd period. They are both chalcogens and follow the octave rule.

4. **Conclusion**:
- The pair that does not follow the octave rule is **Calcium and Strontium**.

### Final Answer:
The pair of elements that do not follow the octave rule is **Calcium and Strontium**.