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.