Electrons are negatively charged subatomic particles that are attracted to an orbit around the positively-charged nucleus of an atom. They reside in locations that are associated with energy levels called shells and are further organized into sub-shells and orbitals within each shell.
Electrons are found in specific locations outside of the nucleus. The shell in which an electron resides indicates the general energy level of the electron: those closer to the nucleus have less energy, while those that are further out, have more energy. The sub-shell describes the location and energy level of the electron more precisely, and the orbital describes the shape of an area of probability in which an electron orbits the nucleus. The electrons that are closest to the nucleus have the lowest amount of energy, and as the distance between electron and nucleus increases, so does the amount of energy that the electron carries. Farther from the nucleus, there is more space for electrons to orbit so that the outer shells can hold more electrons than the inner shells. The outermost electrons of an atom reside in the valence shell and are referred to as valence electrons. These electrons form ionic and covalent bonds with other atoms.
The electron was the first subatomic particle to be discovered. In the late 1890s, J. J. Thomson performed a series of experiments using cathode ray tubes that would lead to the discovery of the electron.
A cathode ray tube is a glass tube with two electrodes that are connected to a power source supplying electricity. A vacuum removes most of the air from the interior of the tube, and when the voltage is applied across the electrodes, a beam of particles travels from the negatively charged electrode (cathode) to the positively charged electrode (anode). The anode has a small hole so that the rays can pass through. A phosphor coating on the opposite end of the tube glows when the cathode rays strike it.
Thomson directed the cathode ray between two metal plates, one with a positive charge and one with a negative charge, and measured the position of the ray at the far end of the tube. When the ray passed between the two plates, it was deflected away from the negatively charged plate, bending in the direction of the positively-charged plate. Since like charges repel and opposite charges attract, this indicated that the particles making up the cathode ray possessed a negative charge. Further experiments to calculate the mass-to-charge ratio of the cathode particles revealed that the mass of each individual negatively-charged particle was tiny, about 1/2000 of the mass of any known atom. Thomson, therefore, concluded that there must be many electrons present in any given atom. Later, the discovery of protons and neutrons would explain the distribution of mass and overall neutral charge present in an atom.
