sentences: elements in the same group on the periodic table of elements have similar properties
docs: molecule is group_of_similar_atoms. hydrogen is element. one_of_chemical_elements is radon. one_of_chemical_elements is hydrogen. one_of_chemical_elements is oxygen. one_of_chemical_elements is chromium. one_of_chemical_elements is gallium. one_of_chemical_elements is nitrogen. each_element_in_periodic_table has number_associated_with different_elements has different_mechanical_properties