分子结构

分子 (fēnzǐ, 'molecule,' literally 'divided particle') + 结构 (jiégòu, 'structure'). Note that 分子 has a homograph 分子 (fènzǐ) meaning 'member / element of a group' (积极分子 'activist,' 知识分子 'intellectual') - same characters, different tone on 分, completely different meaning. In a chemistry context it's always fēn.

Pronounce 分 as fēn (first tone) here, not fèn. Tone 1 = the chemistry/math meaning ('divide, fraction'); tone 4 = 'share, role.'