Electron
An electron is a very light, negatively charged particle that moves around the nucleus of an atom and controls how atoms bond.
Definition
An electron is a tiny, negatively charged particle that moves around outside the nucleus, much lighter than a proton or neutron. It carries a charge of $-1$ ($-1.602 \times 10^{-19}\text{ C}$) and a rest mass of $9.109 \times 10^{-31}\text{ kg}$, about $1/1836$ of a proton. Electrons occupy energy levels, and the ones in the outermost level, the valence electrons, decide how an atom bonds; gaining or losing electrons turns an atom into an ion. In quantum mechanics electrons fill orbitals described by the quantum numbers $n$, $l$, $m_l$, and $m_s$, with the Pauli exclusion principle limiting each orbital to two electrons of opposite spin, and these configurations explain the periodic trends in ionization energy, electronegativity, and atomic size.
Example
A hydrogen atom has $1$ proton and $1$ electron, and a static shock is electrons jumping from one object to another. A neutral chlorine atom has $17$ electrons; gain one more and it becomes the chloride ion $Cl^-$ with $18$. Oxygen's configuration $1s^2 2s^2 2p^4$ leaves two unpaired $2p$ electrons, so it forms two covalent bonds, which is why water is $H_2O$ rather than $HO$ or $H_3O$.
Key Insight
Electricity in a wire is electrons on the move, the same particles that orbit atoms. Electrons, not protons, do all the work in chemistry: every reaction is a rearrangement of electrons. They also behave as both particles and waves, and their wavelength is what quantizes energy levels and lets electron microscopes resolve individual atoms.