About this periodic table
Elements are ordered by atomic number, the count of protons. Each row (period) adds an electron shell and each column (group) shares the same outer electrons, which is why elements in a column behave alike: lithium, sodium and potassium all react with water. Click any element for its atomic mass, group, period, block and electron configuration, or tap a family such as the halogens or noble gases to light it up. The lanthanides and actinides sit in the two rows below the main table.