This document provides information about electron configuration and the principles that define how electrons are arranged in an atom's orbitals. It discusses the ground-state electron configuration as the most stable, lowest-energy arrangement of electrons in an atom. Three main rules that define electron configuration are described as the Aufbau principle, Pauli exclusion principle, and Hund's rule. The document also explains how electron configuration can be written using orbital diagrams or notation, and how the noble gas notation is used.

1. Name:……………………………………………
Grade: ……………………………………..
Subject: Chemistry
Date: ………………………………
CHAPTER 5 : Electrons in Atoms
Section 3 : Electron Configuration
 Ground-StateElectron Configuration
 The arrangement of electrons in an atom is called the atom’s electron
configuration. Because low-energy systems are more stable than
high-energy systems, electrons in an atom tend to assume the
arrangement that gives the atom the lowest energy possible.
 The most stable, lowest-energy arrangement of the electrons is called
the element’s
ground-state electron configuration.
 Three rules, or principlesdefinehow electrons can be arranged in
an atom’s orbitals.
1. Aufbau principle,
2. Pauli exclusion principle,
3. Hund’s rule
 The aufbau principle

2.  The aufbau principle states that each electron occupies the lowest
energy orbital available.
 The sequence of atomicorbitalsfrom lowestenergy to highest
energy.
 This sequence, known as an aufbaudiagram
 In the diagram, each box represents an atomic orbital.
 The Pauliexclusion principle:

3.  States that a maximum of two electronscanoccupya single
atomic orbital, but only if the electrons haveopposite spins
 An arrow pointing up represents the electron spinning in
one direction, and an arrow pointingdown represents the
electron spinning in the opposite direction.
 A box containing both up and down arrows represents a
filled orbital
 The maximum numberof electrons related to each principal
energy level equals 2 n 2
.
 Hund’srule
 Hund’s rule states that single electrons with thesame spin
must occupy eachequal-energyorbital before
additionalelectrons with opposite spins can occupy the same
orbitals.
 Example:
The sequence in which six electrons occupy three p orbitals is
shown below.
 Electron Arrangement

4.  Electron configuration can be presented by one of twomethods:
1. Orbital diagrams or
2. Electronconfiguration notation.
 Example:
 The orbital diagram for a ground-state carbon atom ( atomic
number =6),
 Electron configuration notation fora ground-state carbon
atom

5.  Example:Electron configuration notation and orbital diagram of
sodium atom
 Noble-gasnotation:
 The noble-gas notation uses bracketedsymbols.
 For example,
 [He]( atomic number =2) represents the electron configuration
for helium, 1 s 2
,
 [Ne]( atomic number =10) represents the electron configuration
for neon,
1 s 2
2 s 2
2 p 6
.

6.  Noble gas notation of sodium :
 ElectronConfiguration notation of sodium (atomic number
= 11) is
 Using noble-gas notation, sodium’s electron configuration can
be shortened to the form [Ne] 3 s 1 .
 Exceptionsto predicted configurations
 You can use theaufbau diagram to write correct ground-state electron
configurations

9.  Valenceelectrons are defined as electrons inthe atom’s outermost
orbitals—generally those orbitals associated with theatom’s
highest principal energy level.
 Example:

SulfuratomS contains16 electrons,

Noble-gas notationof S is [Ne] 3S 2
3 p 4
 Sulfur has six valence electrons.
 Electron-dot structures(Lewisstructure) :
Consists of the element’ssymbol,surrounded bydots representing
all of the atom’s valenceelectrons

12. Section 3 Review Questions :
.