Periodic table, computer artwork. This table shows the chemical elements arranged in order of atomic number (the number of protons in the nucleus). Elements with similar chemistry occur in the same group (vertical column). Some groups are colourcoded: alkali metals (yellow, left, except H); alkalineearth metals (purple with white text); transition metals (blue, centre); other metals (purple with yellow text, centre right); halogens (dark orange); inert gases (yellow, far right). The lanthanide and actinide series of elements (dark blue with white text) are shown in the lower two rows.