free electron vs electron

free electron and electron both are nouns.

Word	Noun	Adjective	Verb	Adverb
free electron	Yes	No	No	No
electron	Yes	No	No	No

As nouns, electron is a hypernym of free electron; that is, electron is a word with a broader meaning than free electron:

free electron: electron that is not attached to an atom or ion or molecule but is free to move under the influence of an electric field
electron: an elementary particle with negative charge

Other hypernyms of free electron include negatron.

free electron (noun)	electron (noun)
electron that is not attached to an atom or ion or molecule but is free to move under the influence of an electric field	an elementary particle with negative charge