( ( When ( ( a ( hydrogen atom ) ) ( is ( bonded ( to ( ( a ( small ( , ( electronegative atom ) ) ) ) ( such ( as ( oxygen ( , ( ( nitrogen or ) fluorine ) ) ) ) ) ) ) ) ) ) ) ( , ( ( the ( ( highly electronegative ) atom ) ) ( ( ( ( ( draws off ) ( most ( of ( ( the ( electron density ) ) ( around ( the hydrogen ) ) ) ) ) ) , ) ( leaving ( a ( ( partially bare ) proton ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT a) (NN hydrogen) (NN atom)) (VP (VBZ is) (ADJP (JJ bonded) (PP (TO to) (NP (NP (DT a) (JJ small) (, ,) (JJ electronegative) (NN atom)) (PP (JJ such) (IN as) (NP (NN oxygen) (, ,) (NN nitrogen) (CC or) (NN fluorine))))))))) (, ,) (NP (DT the) (ADJP (RB highly) (JJ electronegative)) (NN atom)) (VP (VBZ draws) (PRT (RP off)) (NP (NP (JJS most)) (PP (IN of) (NP (NP (DT the) (NN electron) (NN density)) (PP (IN around) (NP (DT the) (NN hydrogen)))))) (, ,) (S (VP (VBG leaving) (NP (DT a) (ADJP (RB partially) (JJ bare)) (NN proton))))) (. .)))
When a hydrogen atom is bonded to a small, electronegative atom such as oxygen, nitrogen or fluorine, the highly electronegative atom draws off most of the electron density around the hydrogen, leaving a partially bare proton.
(ROOT (S (NP (NNP Oxygen)) (VP (VBZ is) (NP (NP (DT the) (NN name)) (PP (IN of) (NP (DT the) (JJ second) (ADJP (RBS most) (JJ electronegative)) (NN element))))) (. .)))