The periodic table. The periodic table shows the known elements arranged in order of their atomic numbers (the number of protons in the atomic nu- cleus). Elements with similar chemistry occur in the same vertical column. Different groups of ele- ments are colour-coded: alkaline metals (orange, left); transition metals (magenta-blue, centre); semi-metals & non-metals (green); noble gases (dark blue, right); lanthanide series (magenta- green); actinide series (magenta-orange, bottom). Hydrogen stands alone. Periodic tables help scientists to predict how an element will behave chemically, and how the atoms of different elements combine together in molecules.