All three compounds share essentially the same quantitative physicochemical profile except for the expected increase in mass when the halogen is changed. For example, all three have:

• A topological polar surface area (TPSA) of 12.9  
• One hydrogen‐bond acceptor and no donors  
• Seven heavy atoms and zero rotatable bonds  
• A similar lipophilicity (xlogP of about 1.2–1.4)

Their only systematic quantitative difference is their molecular (exact) weight – 2‑chloropyridine is about 113.5, 2‑bromopyridine about 158.0, and 2‑iodopyridine about 205.0—which is exactly what one would expect when substituting Cl with the heavier Br or I. In other words, aside from the simple, quantifiable increase caused by the different halogen atoms, all the measured properties are virtually the same.

Because the classification based on these databases is driven by specific quantitative measures and here all the key parameters fall into one narrow range (with predictable, linear increases in weight), it is justified to place all three candidate substances into one class.