From Chemguide:

Chlorine to iodine

Considering the halogens from chlorine to iodine, it is the lattice enthalpy which has fallen most. It falls by 87 kJ mol-1. By contrast, the heat needed to remove the electron has only fallen by 54 kJ mol-1.

Shouldn't it be "has fallen the most"?

There are several characteristics, and one has fallen by the highest amount compared with the others.

  • 2
    Either is fine. ............ New housing starts are up in Q2, but of all the economic strength indicators, the new jobs number has risen (the) most. Apr 12 '16 at 18:43
  • 3
    As an adverbal use of most, it's fine without a definite article, for example in the sentence "That was one of things she liked most about him".
    – JavaLatte
    Apr 12 '16 at 19:25
  • P.S. "Practical English Usage, Unit 356.5" (Damkerng) Apr 14 '16 at 4:01
  • 5
    Possible duplicate of most versus the most. Also of interest is Are “most” and “the most” the same?
    – choster
    Apr 27 '16 at 19:05

You can use either of the following:

it is the lattice enthalpy which has fallen most


it is lattice enthalpy which has fallen the most

A repetitive use of "the" makes it somewhat unnatural, although not grammatically incorrect.

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