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 colour-coded: alkali metals (yellow, left, except H); alkaline-earth 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 are not shown.