The radical 钅 (金) indicates a metal element, and 不 gives the approximate pronunciation. Most Chinese element names follow this pattern: metal radical + phonetic component.
Left-side metal radical marking 钚 as a chemical element. Most modern element names in Chinese were coined by adding 钅 to a phonetic; this puts 钚 in the same coined-element series as 钠 sodium, 钾 potassium, 镁 magnesium, 铀 uranium.
Right 不 supplies the sound - bù matches the modern Chinese transliteration of 'plutonium' (Pu, /pu/), with 不 chosen for its bù reading rather than any 'not' meaning. A textbook example of the 20th-century elemental naming convention.
