Samples of a particular compound all have the same elemental proportions by mass. Thomson’s cathode ray tube showed that atoms contain small, negatively charged particles called electrons.

Millikan discovered that there is a fundamental electric charge—the charge of an electron. The ancient Greeks proposed that matter consists of extremely small particles called atoms.

ion — electrically charged atom or molecule (contains unequal numbers of protons and electrons). electron — negatively charged, subatomic particle of relatively low mass located outside the nucleus. structural isomer — one of two substances that have the same molecular formula but different physical and chemical properties because their atoms are bonded differently. structural formula — shows the atoms in a molecule and how they are connected.