The electron named in 1894(wrong)
stoney named the electron in 1894(correct)
the electron was named in 1894(correct)
ENAFEX gave you a right answer. Let me explain it more in details.
Let's break first sentence: "The electron" + "named in 1894"
The electron --> is the subject and it has a past participle phrase attached to it i.e. named in 1894. Therefore, you don't have a sentence here. All that you have is a noun phrase i.e. the electron named in 1894.
named is a transitive verb. A transitive verb has both a subject and an object. Subject performs the action and the object has the action performed on it. The verb glues the subject and the verb together.
So in the first sentence, you only have a noun, making it a fragment.
As I said earlier, a transitive verb required both a subject and a verb. In the second example, the person "stoney" performs an action. The action is naming the electron. Therefore, the sentence is a complete sentence.
this is a passive sentence. Here the person or the agency performing the action has been hidden. So someone named the electron, but it's hidden. Though in passive voice, the sentence is complete.
Active voice:Stoney named the electron in 1894.
Passive voice: the electron was named by stoney in 1894.
Passive voice after removing the performer of the action: The electron was named in 1894.
I hope this makes things clear. In case not, let me know, and I can elaborate.