A
period 5 element is one of the
chemical elements in the fifth row (or
period) of the
periodic table of the elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The fifth period contains 18 elements, beginning with
rubidium and ending with
xenon. As a rule, period 5 elements fill their 5s
shells first, then their 4d, and 5p shells, in that order, however there are exceptions, such as
rhodium.