In the
periodic table of the
elements, elements are arranged in a series of rows (or
periods) so that those with similar properties appear in a column. Elements of the same period have the same number of electron shells; with each
group across a period, the elements have one more proton and electron and become less
metallic. This arrangement reflects the
periodic recurrence of similar properties as the
atomic number increases. For example, the alkaline metals lie in one group (
group 1) and share similar properties, such as high reactivity and the tendency to lose one electron to arrive at a noble-gas electronic configuration. The periodic table of elements has a total of 118 elements.