JJ Thomson menemukan elektron pada tahun 1897 dengan mempelajari sinar katoda. Ia menunjukkan bahwa sinar tersebut merupakan aliran partikel bermuatan negatif yang jauh lebih kecil daripada atom. Elektron adalah partikel subatomik pertama yang ditemukan. Elektron membawa muatan negatif sebesar 1,602 x 10⁻¹⁹ coulomb dan memiliki massa sekitar 9,11 x 10⁻³¹ kilogram, sekitar 1/1836 massa proton.

Electrons occupy orbitals around the nucleus. In the quantum mechanical model, an orbital is a region of space where the electron is likely to be found. The shapes of orbitals are described by s, p, d, and f. Each orbital holds a maximum of two electrons with opposite spins. The arrangement of electrons in an atom is called its electron configuration. The outermost electrons, called valence electrons, determine chemical behavior.

Electrons are involved in chemical bonding, electricity, and magnetism. They flow through wires as electric current. They absorb and emit light when they jump between energy levels, which is the basis of spectroscopy. They are fundamental to the periodic table, which organizes elements by their electron configurations.

Electrons are not just particles. They exhibit wave-particle duality. They can diffract like waves and collide like particles. The double-slit experiment demonstrates this. The electron is a quantum object, and its behavior defies classical intuition.