( They ( then ( ( ( compare ( these groups ) ) ( with ( ( their positions ) ( on ( the ( periodic table ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (PRP They)) (ADVP (RB then)) (VP (VBP compare) (NP (DT these) (NNS groups)) (PP (IN with) (NP (NP (PRP$ their) (NNS positions)) (PP (IN on) (NP (DT the) (JJ periodic) (NN table)))))) (. .)))
They then compare these groups with their positions on the periodic table.
(ROOT (S (NP (NP (NNP Group) (CD 13)) (PP (IN of) (NP (DT the) (JJ periodic) (NN table)))) (VP (VBZ is) (ADVP (RB also)) (VP (VBN called) (NP (DT the) (DT the) (NN boron) (NN group)))) (. .)))