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A biological periodic system of the elements (PSE) indicating the essential elements. The essential elements for most forms of life are shown in black with the exception of chromium (Cr), which is shown with an upward diagonal pattern (see text), and the essential elements that are more restricted for some forms of life are shown in grey. Not shown are the f-group elements: lanthanides and actinides (asterisk after lanthanum (La) and actinium (Ac). The groups are numbered 1–18.
