Download Chapter 4 Section 3

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
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


a
p orbitals filling below
b
c
Orbital Notation
Empty orbitals: ____
 Name of orbital goes below line
 One electron: ____
 Two electrons: ____

Oxygen
8 electrons
    
1s 2s 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
1s2 2s2 2p4
http://glencoe.mcgrawhill.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
1s2 2s2 2p3. 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
1s2 2s2 2p5. What is the atomic number
of fluorine? How many of its p orbitals
are filled? How many unpaired
electrons does a fluorine atom contain?