Rendaku in 一本気 but not in 本気


As I'm encountering more words with rendaku, I thought I started to understand
it, but I came across an example that does not make sense to me.

Why is 気 voiced き in 元気 and 本気, but is voiced ぎ in 一本気 ? In other words, why
is there no rendaku in 本気 ?

From other questions, I see that there is some unpredictability about rendaku…
but those words are really close to one another.




Rendaku occurs when two independent words come together to form a compound. I
think 一本気 is a compound, while 本気 and 元気 are not. (Just because they are
analyzable as 本+気 and 元+気 doesn't mean they are compounds. 元 and 本 are not
independent words, unless you mean "a book" by 本.) Also, Sino-Japanese words
don't normally participate in rendaku. In that sense, 一本気 is exceptional.




Rendaku occurs when two independent words come together to form a compound. I
think 一本気 is a compound, while 本気 and 元気 are not. (Just because they are
analyzable as 本+気 and 元+気 doesn't mean they are compounds. 元 and 本 are not
independent words, unless you mean "a book" by 本.) Also, Sino-Japanese words
don't normally participate in rendaku. In that sense, 一本気 is exceptional.

