What is the oxidation number for ethyne?

Acetylene is a quite reduced form of carbon; the carbons each have a ##-I## oxidation state.

Note that acetylene is neutral and while we can speak of the of its atoms, we cannot speak of the molecule's oxidation state.

If we break up the ##C-H## bonds we get ##2xxH^+##, and ##{C-=C}^(2-)## (carbon is more electronegative than hydrogen, so when you (for the purposes of assigning oxidation number) break this bond you put a formal ##+1## charge on hydrogen, and a formal ##-1## charge of carbon.

In fact, the acetlyide unit ##{C-=C}^(2-)## occurs as calcium carbide, ##CaC_2##, which is an important industrial feedstock.

More reduced forms of carbon include ethylene, ##H_2C=CH_2##, ##C^(-II)##, and the methylene unit of a carbon , ##-CH_2##, ##C^(-II)##. Oxidation state assignments are of course formalisms; they do not have real significance other than what we assign for them. When we break a ##C-C## bond in such a process, we conceive we get ##2xxC*##, i.e. neutral carbon radicals.