Electron Configurations The location of electrons in orbit around the nucleus is what determines how an element will behave in chemical reactions. Therefore; electron configuration is everything to chemistry. There are three ways to show electron configuration: Orbital Notation Electron Configuration Notation Lewis Dot Notation Quantum Mechanical Model of the Atom (today’s model) Electrons DO occupy energy levels but their path is not definite Energy Increases You cannot say with certainty away from the that an electron will be at any nucleus point at any given time, although you do know that there is a HIGH PROBABILITY that the electrons are around the nucleus Energy Levels – regions around the nucleus where electrons are likely to be found Remember the ladder…each step is like an energy level Quantum Numbers Quantum Numbers – numbers that describe atomic orbitals and their electrons Orbitals – the place within an energy level where an electron will probably be found There are 4 quantum numbers! (1) PRINCIPAL Quantum Number – n describes the principal (main) energy levels of electrons in an atom the main energy levels are known as SHELLS increase in n = increase in energy maximum number of electrons (e-) in an energy level = 2n (2) ORBITAL Quantum Number – l determines the energy sublevel AND the shape of the orbital the number of sublevels is equal to the principal quantum number four shapes of the energy sub levels are: s = spherical p = dumbbell d = double dumbbell f = too complex to describe P Orbital S Orbital D Orbital (both) Each of the sublevels (s, p, d, f) contain orbitals Each orbital can hold 2 electrons s = 1 orbital = 2 e- p = 3 orbitals = 6 e- d = 5 orbitals = 10 e- f = 7 orbitals = 14 e- (3) MAGNETIC Quantum Number-ml describes the orientation (direction) of the orbital (electron cloud) in space (x, y or z axis) the number of orbitals = the number of different orientations (4) SPIN Quantum Number - ms describes the direction of the electron spin spins clockwise (+ ½) or counterclockwise (– ½) two electrons in the same orbital MUST HAVE OPPOSITE SPINS!!! Three Rules of Electron Configuration Aufbau Principle Pauli Exclusion Principle Hund’s Rule Aufbau Principle Electrons enter orbitals of the lowest energy first The main energy levels can have overlapping sublevels Example: n=1 and n=2 overlap s and p orbitals Pauli Exclusion Principle No two electrons can have the same four quantum numbers This limits the number of electrons in each orbital to 2. One spins clockwise and the other spins counterclockwise (+1/2, -1/2) Hund’s Rule When electrons occupy orbitals of equal energy, ONE electron enters each orbital until all are ½ filled with electrons spinning in the same direction (all up or all down), then the orbitals are filled with the opposite spinning electrons. .