( ( ( The mechanism ) ( that ( holds ( a ( metallic bond ) ) ) ) ) ( together ( ( is ( ( the attraction ) ( between ( ( ( the ( positive kernels ) ) and ) ( the ( negative ( electron sea ) ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (NP (DT The) (NN mechanism)) (SBAR (WHNP (WDT that)) (S (VP (VBZ holds) (NP (DT a) (JJ metallic) (NN bond)))))) (ADVP (RB together)) (VP (VBZ is) (NP (NP (DT the) (NN attraction)) (PP (IN between) (NP (NP (DT the) (JJ positive) (NNS kernels)) (CC and) (NP (DT the) (JJ negative) (NN electron) (NN sea)))))) (. .)))
The mechanism that holds a metallic bond together is the attraction between the positive kernels and the negative electron sea.
(ROOT (S (NP (JJ Metallic) (NNS bonds)) (VP (VBP hold) (PRT (RP together)) (NP (NP (JJ positive) (NN metal) (NNS ions)) (CC and) (NP (PRP$ their) (JJ valence) (NNS electrons)))) (. .)))