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Definition – the arrangement of electrons in an atom Each element has a unique electron configuration because each element has a different number of electrons Lowest energy arrangement of electrons for each element is known as the Ground State configuration To build up the electron configuration for any particular atom, first the energy levels of the orbitals are determined. Then the electrons are added to the orbitals one by one, according to three basic rules Remember each orbital can have at most 2 electrons An electron occupies the lowest-energy orbital that can receive it. • The energy level of an orbital is affected by two things • Distance from nucleus • Complexity of the orbital No two electrons in the same atom can have the same set of four quantum numbers. This means that the electrons must have opposite spins to occupy the same orbital Orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron All electrons in singly occupied orbitals must have parallel spins An unoccupied orbital is represented by a line With the orbital’s name written underneath the line Arrows are used to represent the electrons Up arrows are written first, followed by down arrows Eliminates the lines and arrows Coefficient gives the energy level Letter gives the sublevel Superscripts give the number of electrons at that sublevel 1s2 Helium has 2 electrons at the 1 s sublevel s sublevel has 1 orbitals › total of 2 electrons p sublevel has 3 orbitals › total of 6 electrons d sublevel has 5 orbitals › total of 10 electrons f sublevel has 7 orbitals › total of 14 electrons