( ( An ( ionic compound ) ) ( ( is ( named ( by ( ( its cation ) ( followed ( by ( its anion ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT An) (JJ ionic) (NN compound)) (VP (VBZ is) (VP (VBN named) (PP (IN by) (NP (NP (PRP$ its) (NN cation)) (VP (VBN followed) (PP (IN by) (NP (PRP$ its) (NN anion)))))))) (. .)))
An ionic compound is named by its cation followed by its anion.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (VP (VBG naming) (NP (DT an) (JJ ionic) (NN compound))))) (, ,) (NP (DT the) (NN cation)) (VP (VBZ is) (VP (VBN written) (ADVP (RB first)))) (. .)))
When naming an ionic compound, the cation is written first.