<P> A period in the periodic table is one of the horizontal rows, all of whose 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 . </P> <P> Modern quantum mechanics explains these periodic trends in properties in terms of electron shells . As atomic number increases, shells fill with electrons in approximately the order shown at right . The filling of each shell corresponds to a row in the table . </P>

What do the periods mean on the periodic table
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