We can use quantitative, PubChem‐derived values to “bin” these three compounds by the identity of their halo‐substituent. For example, note that all three are nonpolar (tPSA = 0, no H‐bond donors/acceptors) with the same core (para‑substituted ethylbenzene) and one rotatable bond. Their only measurable differences are in their molecular mass and calculated lipophilicity (xlogP):

• 1‑chloro‑4‑ethylbenzene has a monoisotopic mass of about 140.04 (molecular weight ≈140.61) and an xlogP of 3.6.  
• 1‑bromo‑4‑ethylbenzene has a monoisotopic mass of about 183.99 (weight ≈185.06) and an xlogP of 3.6.  
• 1‑ethyl‑4‑iodobenzene has a monoisotopic mass of about 231.97 (weight ≈232.06) and an xlogP of 4.3.

Because these differences are entirely quantitative and arise from the nature of the halogen substituent, we can classify these compounds into three groups (one per halogen):

Group 1 (Chloro‑substituted):  
 • 1‑chloro‑4‑ethylbenzene  
  – Molecular weight ≈140.61 (lowest)  
  – xlogP = 3.6

Group 2 (Bromo‑substituted):  
 • 1‑bromo‑4‑ethylbenzene  
  – Molecular weight ≈185.06 (intermediate)  
  – xlogP = 3.6

Group 3 (Iodo‑substituted):  
 • 1‑ethyl‑4‑iodobenzene  
  – Molecular weight ≈232.06 (highest)  
  – xlogP = 4.3

This grouping is justified by the quantitative values: the halogen’s identity directly changes the weight and, in the case of iodine, also increases the lipophilicity (as seen by the higher xlogP), while all other properties remain effectively comparable.