( ( ( ( ( ( When ( ( ( the atoms ) ( of ( two ( different elements ) ) ) ) ( unite ( into ( a molecule ) ) ) ) ) ( , ( they ( form ( a compound ) ) ) ) ) , ) and ) ( ( their ( chemical symbols ) ) ( are ( combined ( to ( provide ( ( a ( molecular formula ) ) ( for ( that compound ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (SBAR (WHADVP (WRB When)) (S (NP (NP (DT the) (NNS atoms)) (PP (IN of) (NP (CD two) (JJ different) (NNS elements)))) (VP (VB unite) (PP (IN into) (NP (DT a) (NN molecule)))))) (, ,) (NP (PRP they)) (VP (VBP form) (NP (DT a) (NN compound)))) (, ,) (CC and) (S (NP (PRP$ their) (NN chemical) (NNS symbols)) (VP (VBP are) (VP (VBN combined) (S (VP (TO to) (VP (VB provide) (NP (NP (DT a) (JJ molecular) (NN formula)) (PP (IN for) (NP (DT that) (NN compound)))))))))) (. .)))
When the atoms of two different elements unite into a molecule, they form a compound, and their chemical symbols are combined to provide a molecular formula for that compound.
(ROOT (S (NP (DT A) (JJ binary) (JJ molecular) (NN compound)) (VP (VBZ is) (VP (VBN made) (PRT (RP up)) (PP (IN of) (NP (CD two) (NNS elements))))) (. .)))