Electrons are the tiny, restless particles that make chemistry happen. While protons and neutrons sit locked in the nucleus, electrons swarm around it in a cloud, and it is their arrangement that determines virtually everything about how an atom behaves — what it bonds with, how it conducts electricity, what color compounds it forms, and whether a substance is a gas, liquid, or solid. When you flip a light switch, it is electrons flowing through the wire. When iron rusts, it is electrons being transferred from iron to oxygen. Chemistry is, at its heart, the story of electrons.
Discovered by J.J. Thomson in 1897 using cathode ray tubes, the electron was the first subatomic particle ever identified. Thomson found that these rays consisted of particles about 1,836 times lighter than a proton, each carrying a negative charge. Despite being incredibly tiny (about 9.1 x 10⁻³¹ kg), electrons define the size of atoms. The electron cloud extends far beyond the nucleus, determining the atom's effective radius. If the nucleus were a basketball at center court, the nearest electrons would be orbiting somewhere in the parking lot.
Electrons do not orbit the nucleus like planets around the sun, despite what simple diagrams might suggest. Instead, they exist in quantum mechanical orbitals — three-dimensional probability clouds that describe where an electron is likely to be found. These orbitals come in different shapes labeled s, p, d, and f, and they fill up in a specific order as atoms get larger. The outermost electrons, called valence electrons, are the ones that participate in chemical bonding and reactions. This is why elements in the same column of the periodic table behave similarly — they have the same number of valence electrons.