( ( ( The energy ) ( needed ( to ( pull ( the ( two atoms ) ) ) ) ) ) ( apart ( ( is ( called ( the ( bond energy ) ) ) ) . ) ) )
(ROOT (S (NP (NP (DT The) (NN energy)) (VP (VBN needed) (S (VP (TO to) (VP (VB pull) (NP (DT the) (CD two) (NNS atoms))))))) (ADVP (RB apart)) (VP (VBZ is) (VP (VBN called) (NP (DT the) (NN bond) (NN energy)))) (. .)))
The energy needed to pull the two atoms apart is called the bond energy.
(ROOT (S (NP (NN Bond) (NN energy)) (VP (VBZ holds) (NP (NNS atoms)) (ADVP (RB together))) (. .)))
Bond energy holds atoms together.