'Isomorphic' isn't the right term, but in first order classical logic, and many others, there is a simple relationship between a valid sentence and a valid argument form.
For any given argument, you can form the corresponding conditional, which is single sentence whose main connective is a material conditional, with the conjunction of the premises as its antecedent, and its conclusion as the consequent. For example, suppose you have an argument with two premises:
A
B
---
C
Then the corresponding conditional is a single sentence:
(A ∧ B) → C
Where ∧ is conjunction and → is the material conditional.
In classical logic, the argument form is valid if and only if the corresponding conditional is a valid sentence. This is guaranteed by the introduction and elimination rules for the material conditional.
There are non-classical logics, and conditionals other than the material conditional, in which this relation does not hold.
As PW_246 correctly points out, there is a difference between a valid argument form and a proof. A proof requires a specific proof system, so an argument may be valid but may fail to instantiate a correct proof in a given system.