碳

Left stone radical - pictograph of a rock under a cliff overhang. The indexing radical, marking 碳 as a mineral element. This is a relatively recent compound: 19th-century chemists needed a word for the element carbon and built it on the rock radical alongside 硫 (sulphur) and 硅 (silicon).

Right 炭 supplies the sound tàn exactly, and carries the perfect semantic anchor: 炭 means 'charcoal,' the everyday form of carbon people knew long before the element was isolated. The combination 'stone + charcoal' for the chemical element is one of the cleanest semantic-plus-phonetic coinings in modern scientific Chinese.