Question: why is the total mass of a helium nucleus not equal to the mass of its individual parts
[{'answer': 'mass defect', 'provenance': [{'wikipedia_id': '3043836', 'title': 'Nuclear binding energy', 'start_paragraph_id': 1, 'start_character': 583, 'end_paragraph_id': 1, 'end_character': 594, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '3043836', 'title': 'Nuclear binding energy', 'start_paragraph_id': 20, 'start_character': 0, 'end_paragraph_id': 20, 'end_character': 595, 'bleu_score': 0.9385289250847371, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 16252711990852111428}, 'section': 'Section::::Introduction.:Physics of nuclei.\n'}]}]