( ( ( ( ( ( These elements ) ( have ( five ( valence electrons ) ) ) ) , ) but ) ( ( only four ) ( are ( necessary ( to ( ( form ( a ( covalent bond ) ) ) ( with ( the ( host semiconductor ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT These) (NNS elements)) (VP (VBP have) (NP (CD five) (JJ valence) (NNS electrons)))) (, ,) (CC but) (S (NP (RB only) (CD four)) (VP (VBP are) (ADJP (JJ necessary) (S (VP (TO to) (VP (VB form) (NP (DT a) (JJ covalent) (NN bond)) (PP (IN with) (NP (DT the) (NN host) (NN semiconductor))))))))) (. .)))
These elements have five valence electrons, but only four are necessary to form a covalent bond with the host semiconductor.
(ROOT (S (NP (NNP Carbon)) (VP (VBZ has) (NP (CD four) (JJ valence) (NNS electrons))) (. .)))