The number of unpaired electrons in a paramagnetic diatomic molecule of an element with atomic number 16 is:

1. Identify the Element: The element with atomic number 16 is Sulphur (S).
2. Identify the Molecule: The question refers to its paramagnetic diatomic form, which is $S_2$ (typically found in the vapour phase at high temperatures).
3. Determine Valence Electrons: Sulphur belongs to Group 16 (like Oxygen). Electronic configuration: $[Ne] 3s^2 3p^4$. Each sulphur atom contributes 6 valence electrons. Total valence electrons in $S_2 = 6 + 6 = 12$.
4. Apply Molecular Orbital Theory (MOT): The filling order for valence electrons (analogous to $O_2$) is: $\sigma_{3s}^2 < \sigma_{3s}^{*2} < \sigma_{3p_z}^2 < (\pi_{3p_x}^2 = \pi_{3p_y}^2) < (\pi_{3p_x}^{*1} = \pi_{3p_y}^{*1})$
5. Count Unpaired Electrons: The last 2 electrons occupy the two degenerate antibonding $\pi^*$ orbitals singly (following Hund's Rule). Thus, there are 2 unpaired electrons.

(Note: While the specific properties of $S_2$ are covered in the p-Block elements chapter, the principles of paramagnetism and unpaired electrons are detailed in the provided Class 12 Chemistry source, Unit 4 and 5, and the methodology derives from general Chemical Bonding.)