Below is one rational, data‐driven way to classify these three candidates based solely on measurable physicochemical properties from a database:

• All three compounds share many values in common—they have identical numbers of rotatable bonds (1), the same topological polar surface area (TPSA 9.2), one hydrogen‐bond acceptor and no donors, and very similar calculated xlogP values (2.8–2.9). In other words, aside from the identity of the halogen substituent, they are nearly identical.

• The most quantitative difference is in their molecular (monoisotopic) masses:
  – 1‑chloro‑4‑methoxybenzene: molecular weight ~142.58 (monoisotopic mass 142.02)
  – 1‑bromo‑4‑methoxybenzene: molecular weight ~187.03 (monoisotopic mass 185.97)
  – 1‑iodo‑4‑methoxybenzene: molecular weight ~234.03 (monoisotopic mass 233.95)

Using this measurable difference, one may classify the substances into three groups based on increasing molecular weight (and thus, by quantitative substitution of the halogen):

  Group 1 (“Low Mass”): 1‑chloro‑4‑methoxybenzene  
  Group 2 (“Intermediate Mass”): 1‑bromo‑4‑methoxybenzene  
  Group 3 (“High Mass”): 1‑iodo‑4‑methoxybenzene

Each group is justified by its distinct, directly reported molecular weight and monoisotopic mass value from PubChem. This classification relies solely on quantitative data (molecular mass differences) that are typically available in comprehensive physicochemical property databases.