The electron configuration of an atom refers to the manner in which its electrons are arranged in the atom's orbital shells and sublevels, or subshells. The orbital configuration of an atom's electrons is a primary factor in determining its properties. The most significant electron orbit in relation to how the atom will react with other atoms is the outermost shell, or valence shell.
Electrons will fill up the orbitals of an atom in a specific order. This is because each electron has a negative electrostatic charge that repels the other electrons. The atom is striving to fit the electrons into an arrangement that minimizes the effect of the repulsive charges. No more than two electrons can occupy the same orbital, and when two electrons share an orbital, they must have opposing spins.
Electron configurations can be described by specifying the number of electrons that will be found in each of the atom's electron shells and within the orbitals in that shell's sublevels. The sublevels in an electron shell are noted as s, p, d, and f. These orbital sublevels and their respective electron configurations are reflected in the periodic table of the elements, which contains an s block, p block, d block and f block. An atom's electron configuration can be noted in three ways: in an orbital diagram, spdf notation or noble gas notation.