( ( A ( covalent bond ) ) ( ( is ( when ( atoms ( lose ( valence ( shell electrons ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN covalent) (NN bond)) (VP (VBZ is) (SBAR (WHADVP (WRB when)) (S (NP (NNS atoms)) (VP (VBP lose) (NP (JJ valence) (NN shell) (NNS electrons)))))) (. .)))
A covalent bond is when atoms lose valence shell electrons.
(ROOT (S (NP (NNP Valence)) (VP (VBZ is) (NP (NP (DT the) (NN term)) (PP (IN for) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (JJ covalent) (NNS bonds))))) (SBAR (S (NP (DT an) (NN atom)) (VP (MD can) (VP (VB form))))))) (. .)))
Valence is the term for the number of covalent bonds an atom can form.