Anyway, which is a spectator ion involved in the reaction of k2cro4(aq) and ba(no3)2(aq)?

Nitrate ions and potassium ions .

Potassium dichromate undergoes a double-decomposition reaction with barium nitrate to produce barium dichromate and potassium nitrate. The reaction is possible due to the low solubility of barium dichromate that precipitate out of the solution readily after its production.

Start with the balanced chemical equation for this process:

Rewrite the chemical equation as an ionic one; express all soluble salts- those with state symbol (aq)- as their constituting ions while leaving the insoluble (s) intact.

and are found on both sides of the equation by the same quantity. The two ions thus took no part in the net reaction and act as spectator ions.

you need to have a picture of the atom that way we can answer it

answer carbon and three oxygen atoms;