锶

Left metal radical - the side form of 金 simplified to five strokes. The indexing radical that marks 锶 as a metallic element. Modern scientific Chinese coined names for the periodic table by pairing this radical with a phonetic matching the international name: same pattern as 钙 (calcium), 钠 (sodium), 钾 (potassium).

Right 思 supplies the sound sī exactly - chosen as the closest single-syllable Mandarin match to the first syllable of 'strontium.' Pure phonetic borrowing; the 'thinking' meaning is irrelevant. This is a textbook example of how modern element names were coined: pick a phonetic that fits, slap on the radical for the element class.