What is an electron?
An electron is a negatively charged subatomic particle that can be either bound to an atom or free (not bound). An electron that is bound to an atom is one of the three primary types of particles within the atom -- the other two are protons and neutrons.
Together, electrons, protons and neutrons form an atom's nucleus. A proton has a positive charge that counters the electron's negative charge. When an atom has the same number of protons and electrons, it is in a neutral state.
Electrons are unique from the other particles in multiple ways. They exist outside of the nucleus, are significantly smaller in mass and exhibit both wave-like and particle-like characteristics. An electron is also an elementary particle, which means that it is not made up of smaller components. Protons and neutrons are thought to be made up of quarks, so they are not elementary particles.
Shells, subshells and orbitals
In the early days of atomic study, scientists believed that an atom's electrons circled the nucleus in spherical orbits at specific distances, much like planets circle a sun. In this model -- referred to as the Bohr model -- the orbits furthest from the nucleus contain the greatest amount of energy. When an electron jumps from a higher energy orbit to a lower energy orbit, the atom releases electromagnetic radiation.
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