Electron Configurations

Objectives

 List the total number of electrons needed to fully occupy each main energy level  State the Aufbau principle, the Pauli exclusion principle, and Hund’s rule  Describe the electron configurations for the atoms of any element using orbital notation, electron-configuration notation, and when appropriate, noble-gas notation

Electron Configuration

 Arrangement of electrons in an atom  Atoms occupy the lowest energy arrangement — ground-state electron configuration

Rules

 1.

Aufbau principle : electrons enter orbitals of lowest energy levels first  Orbitals in same sublevel (s, p, d, f) are of equal energy  s sublevel has lowest energy  Sublevels in different main levels can overlap  1s is the lowest energy orbital

Notice the 4s sublevel is lower than 3d It has less energy than the 3d (this means that the 4s fills before 3d) http://www.chemguide.co.uk/atoms/properties/atomorbs.html

Text p. 105 Figure 4-16

“The order of the atomic orbitals can be somewhat difficult to remember. Fortunately, there is a mnemonic device that can help in this regard. The slide sequence below details this device.” http://www.iun.edu/%7Ecpanhd/C101webnotes/modern-atomic-theory/mnemonicdev.html

Rules

 2.

Pauli exclusion principle : quantum numbers of atomic orbitals may describe only one electron  Two electrons in the same orbital must have opposite spins  Spins are clockwise & counterclockwise 1s orbital

a

Rules

 3.

Hund’s Rule : when electrons occupy orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron with parallel spins  Sublevels add one electron to each orbital first   Second electrons added to the orbitals are then paired with an opposite spin p orbitals filling below b c

Orbital Notation

 Empty orbitals: ____  Name of orbital goes below line  One electron: ____  Two electrons: ____ Oxygen 8 electrons    1

s

2

s

2

p

 2

p

 2

p

Electron-Configuration Notation

 Eliminate lines and arrows  Number of electrons in a sublevel is the superscript Oxygen 8 electrons

1s 2 2s 2 2p 4

http://glencoe.mcgraw hill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/007874637x /514701/chem_ch05_t05_4.swf::Electron%20Configurations%20and%20Orbit al%20Diagrams%20for%20Elements%201–10

Practice

 Write the orbital notation for Al.

 Write the electron-configuration notation for Mn.

Practice

 The electron configuration of nitrogen is

1s 2 2s 2 2p 3

. How many electrons are present in nitrogen? What is the atomic number of nitrogen? Write the orbital notation for nitrogen.

Practice

 The electron configuration of fluorine is

1s 2 2s 2 2p 5

. What is the atomic number of fluorine? How many of its

p

orbitals are filled? How many unpaired electrons does a fluorine atom contain?