25 ] with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium , respectively) shown in a single cell of the main table, between barium and hafnium , and radium and rutherfordium , respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods). Some periodic tables include a dividing line , or equivalent, between metals and nonmetals. [ 26 ] Periodic trends Electron configuration Approximate order in which shells and subshells are arranged by increasing energy according to the Madelung rule . The electron configuration or organisation of electrons orbiting neutral atoms shows a recurring pattern or periodicity. The electrons occupy a series of electron shells (numbered shell 1, shell 2, and so on). Each shell consists of one or more subshells (named s, p, d, f and g). As atomic number increases, electrons progressively fill these shells and subshells more or less according to the Madelung rule or energy ordering rule, as shown in the diagram. The electron configuration for neon , for example, is 1s 2 2s 2 2p 6 . With an atomic number of ten, neon has two electrons in the first shell, and eight electrons in the second shell—two in the s subshell and six in the p subshell. In periodic table terms, the first time an