While electrons have the same magnitude of charge as protons, they are far less massive. A proton weighs 1 atomic mass unit (amu), while an electron weighs around 1/1840 amu. In fact, electrons are small enough to exhibit properties of both particles and waves.
In 1897, the English scientist J. J. Thomson discovered the electron and proved it is a particle; in 1937 his son, G. P. Thomson won the Nobel Prize for Physics for proving the electron also behaves like a wave.
Many theories have evolved about atomic structure since the discovery of subatomic particles. It was once believed that the particles in atoms were arranged haphazardly like fruit in a Christmas pudding. A more elegant model was proposed by Danish physicist Niels Bohr, who suggested that electrons orbit the nucleus in the same way that planets orbit a star. In the Bohr model, the electrons fit into specific energy levels. The lowest energy level can contain up to two electrons, the next highest eight electrons and the one after that 18 electrons. In a stable, unexcited atom, a lower energy level must be filled up before an electron can populate the next level up. This model has long been taught to students and although it is very simplified it does accurately convey the idea that electrons can only exist at discrete levels or quanta. This is why the analysis of subatomic particles bears the name quantum mechanics.
For very small particles it is impossible to know both exactly where they are and exactly how fast they are moving. Precisely knowing one means it is impossible to know anything about the other. This is called Heisenberg's uncertainty principle.
Heisenberg's uncertainty principle means that electrons exist in a cloud or orbital around the nucleus. You can know that an electron is somewhere inside this orbital, but you can not know exactly where.
Each orbital can contain up to two electrons. The lowest energy orbital is called an s-orbital and the lowest energy level, called the K shell, comprises just one of these s-orbitals.
The second lowest level, called the L shell, is comprised of one s-orbital and three p-orbitals, meaning this shell can contain up to eight electrons. The third lowest shell, the M shell, can contain up to 18 electrons: one s-orbital, three p-orbitals and five d-orbitals.