A period in the periodic table is one of the horizontal rows , all of those elements have the same number of electron shells . Going across a period , each element has one more proton and is less metallic than its predecessor . Arranged this way , groups of elements in the same column have similar chemical and physical properties , reflecting the periodic law . For example , the alkali metals lie in the first column ( 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 . As of 2016 a total of 118 elements have been discovered and confirmed .

Question: the elements in each period have the same number of
Answer: electron shells