electrone

 Electron 

An electron is therefore considered nearly massless in comparison with a proton or a neutron, and the electron mass is not included in calculating the mass number of an atom. Any atom, however, may have more or fewer electrons than positive charges and thus be negatively or positively charged as a whole; these charged atoms are known as ions. Under ordinary conditions electrons are bound to the positively charged nuclei of atoms by the attraction between opposite electric charges. The rest mass of the electron is 9.1093837015 × 10−31 kg, which is only 1/1,836the mass of a proton. In a neutral atom the number of electrons is identical to the number of positive charges on the nucleus. Not all electrons are associated with atoms; some occur in a free state with ions in the form of matter known as plasma.

    Within any given atom, electrons move about the nucleus in an orderly arrangement of orbitals, the attraction between electrons and nucleus overcoming repulsion among the electrons that would otherwise cause them to fly apart. The electrons in orbitals closest to the nucleus are held most tightly; those in the outermost orbitals are shielded by intervening electrons and are the most loosely held by the nucleus. The detailed structural arrangement of electrons within an atom is referred to as the electronic configuration of the atom. As the electrons move about within this structure, they form a diffuse cloud of negative charge that occupies nearly the entire volume of the atom.