The periodic ... table represent groups, or families, of elements. The elements in a group often look and behave similarly, because they have the same number of electrons in their outermost shell ...

In Bohr’s model of the atom, developed in 1913, electrons inhabit a series of concentric shells that encircle the nucleus. Bohr reasoned that elements in the same group of the periodic table ...

But to make the heaviest elements, researchers adopted a new brute force approach ... That pattern of shell filling is what gives the periodic table its shape, and the unusual filling may mean ...

filling the next shell up. Although he didn’t know about protons, Mendelev did notice gaps in his periodic table. He correctly interpreted these as four elements that had yet to be discovered.