Reality → Matter → Atoms → Electrons
Unlike protons and neutrons, electrons are elementary. They are negatively charged and their number in an atom equals the number of protons. Quantum mechanics use highly abstract and complex mathematical concepts to describe the diffused, yet highly organized, arrangement of electrons in the huge quasi-empty space around an atom's tiny nucleus [1] . While the contribution of the electron cloud to mass and matter normally is negligible [2] , the quantum mechanical model of the electron cloud explains largely the properties of elements [3] and the formation of molecules [4] .
The popular but now obsolete Bohr model incorporated for the first time quantization, but could not account for various spectral lines of atoms and was in conflict with the Maxwell equations, according to which the orbiting electrons would cause electromagnetic radiation, thus loose energy, and fall into the nucleus. De Broglie stipulated a standing wave for the electron, which solved the stability problem but failed to explain some other experimental observations. Heisenberg was first to give up the notion of trajectories for atom-bound electrons and used abstract mathematics to describe electron phenomena observed in experiments. Schrödinger managed to formulate an equation that uses quantum numbers as parameters for mathematical solutions describing the probability of finding the electron in a space called orbital. The awkward quantum mechanical atomic model is now generally accepted in physics and chemistry, fits well with experimental observations, and has been instrumental in advancing material sciences, micro-electronics, nanotechnologies, and life sciences.
Related:
The mass of an electron is only about 1/2000 of a proton's mass. This ratio refers to the mass of an electron at rest. Fast moving electrons can have considerable additional energy which, according to relativity, is equivalent to mass.
The configuration of electrons is used to explain the periodic table and the inferred key chemical and some physical properties (see Sheet for the quantum mechanical construction of the periodic table).
Valence electrons play a key role in chemical bonding. Based on the outdated but still popular electron shell model, the valence electrons are normally pictured as the 'outermost' electrons (for more modern concepts, see Energy level and band theory).