2 Description of the table

• The table consists of 18 vertical groups and 7 horizontal periods.
• Elements are arranged in ascending atomic number; each element has one electron more than its predecessor in the period.
• Each period starts by filling a new energy level with one electron and then fills the sublevels in turn, ending with a noble gas.
• In a vertical group, the elements have similar electron configuration of the outer energy level, but different principal quantum number n.
• The period number equals the number of the outermost main level (n) of the element.

Electronic distribution of the first 30 elements (number of electrons in each sublevel):

(Chromium and copper are exceptions because half-filled (d⁵) and completely filled (d¹⁰) d sublevels are extra stable.)