( ( As ( ( the atoms ) ( form ( new bonds ) ) ) ) ( , ( they ( ( gain ( thermal energy ) ) . ) ) ) )
(ROOT (S (SBAR (IN As) (S (NP (DT the) (NNS atoms)) (VP (VBP form) (NP (JJ new) (NNS bonds))))) (, ,) (NP (PRP they)) (VP (VBP gain) (NP (JJ thermal) (NN energy))) (. .)))
As the atoms form new bonds, they gain thermal energy.
(ROOT (S (NP (NN Bond) (NN energy)) (VP (VBZ holds) (NP (NNS atoms)) (ADVP (RB together))) (. .)))
Bond energy holds atoms together.