Spectroscopy

Step 1: Use the Rydberg formula.
The wavelength of the Kα X-ray line is related to the atomic number by the following formula: where is the Rydberg constant, and is the principal quantum number (for the Kα line, and ).
Step 2: Rearrange the formula.
Since the given wavelength corresponds to , we can substitute the values and solve for . Solving the equation for , we get:
Step 3: Calculate the atomic number .
Substitute the known values:
Step 4: Conclusion.
Thus, the atomic number of the element is 18.