The line spectrum of two elements is not identical because :

Options

A. (1) They do not have same number of neutrons
B. (2) They have dissimilar mass number
C. (3) They have different energy level schemes
D. (4) They have different number of valence electron

Answer

### Step-by-Step Solution:

1. **Understanding Line Spectra**:
- Line spectra, also known as atomic spectra, are unique patterns of light emitted or absorbed by atoms. Each element has a distinct line spectrum that serves as its "fingerprint."

2. **Identifying the Reason for Differences**:
- The question asks why the line spectra of two different elements are not identical. To answer this, we need to consider the factors that influence line spectra.

3. **Analyzing the Options**:
- **Option 1**: "They do not have the same number of neutrons."
- This is incorrect because neutrons do not affect the electronic transitions that produce line spectra.
- **Option 2**: "They have dissimilar mass numbers."
- This is also incorrect as mass number does not influence the electronic structure or transitions.
- **Option 3**: "They have different energy level schemes."
- This is correct. Different elements have different arrangements of electrons and energy levels, leading to unique spectra.
- **Option 4**: "They have different numbers of valence electrons."
- This is misleading because while valence electrons do play a role in chemical properties, they do not solely determine the line spectrum.

4. **Conclusion**:
- The line spectra of two elements are not identical primarily because they have different energy level schemes. Each element's unique arrangement of electrons leads to distinct energy transitions, resulting in unique wavelengths of emitted or absorbed light.

5. **Final Answer**:
- The correct reason for the non-identical line spectra of two elements is **Option 3: They have different energy level schemes**.