when do we considered a group to be a set
Group (mathematics) Many groups are simultaneously groups and examples of other mathematical structures. In the language of category theory, they are group objects in a category, meaning that they are objects (that is, examples of another mathematical structure) which come with transformations (called morphisms) that mimic the group axioms. For example, every group (as defined above) is also a set, so a group is a group object in the category of sets.
History of the periodic table In 1817, Johann Wolfgang Döbereiner, a chemist, began to formulate one of the earliest attempts to classify the elements.[8] In 1829, he found that he could form some of the elements into groups of three, with the members of each group having related properties. He termed these groups triads.[9]