Charge and particle identity
An electron carries one unit of negative charge and has much less mass than a proton or neutron. In the Standard Model it is an elementary lepton, unlike composite nucleons. Electrons may be bound in atoms or appear in beams and ionized gases.
Bound states and ionization
Neutral atoms contain as many electrons as protons. Losing or gaining electrons makes ions; absorbing energy may instead excite an electron without removing it. Quantum states describe spatial probabilities, so textbook shells should not be interpreted as solid tracks along which tiny planets travel.
Electrons and current
Electron motion contributes to current in metals and semiconductors, but other charge carriers exist, including ions in liquids. In semiconductors, unoccupied electronic states behave as positively charged holes. A hole describes the collective electronic system; it is not an ordinary free positively charged electron.
Transitions and identification
Electronic transitions connect atomic energy differences with absorbed and emitted light. Their spectral signatures help identify substances. The same quantum framework also underlies energy bands and controlled carrier populations in semiconductors, connecting atomic physics with the operation of electronic devices.