Unit 3, Lesson 3 of 3
Name:
Date:
Atoms vs. Ions
OBJECTIVE: After this lesson, I will understand the difference between atoms and ions, and I will be able to compare and
contrast their relative sizes and properties.
Lewis Structures or Electron dot structures are a useful way of representing electrons. Rather than configuring all of the
electrons or showing the nucleus with the electrons arranged into various energy levels, Lewis Structures only show the
valence electrons.
Group
Valence
Electrons
Lewis Structure
of
Representative
Element
1
2
13
14
15
16
17
18
Na
Be
B
C
N
O
F
Ne
When elements form bonds, electrons are lost, gained, or shared. Atoms that gain or lose electrons form charged particles
called ions. Two different types of ions can form: Cations and Anions. The formation of ions is the result of each atom’s
desire for stability. We will define stability as having a complete outer shell, meaning a total of 8 valence electrons for most
elements. Atoms lose or gain electrons based upon the ease of their pathway to 8 valence electrons.
Cation:
Remember, ionization energy is the energy required to remove an electron from an atom—this process produces cations.
Elements in groups 1, 2, and 13 have the lowest ionization energy on the table, making it easiest for them to lose electrons.
Na

Mg

Na+
+
e-
K

Al

Anion:
Remember, electron affinity is the energy an atom releases when it gains an electron. The more energy released, the easier an
atom can gain an electron Elements in groups 15-17 have the greatest electron affinities on the table, making it easiest for
them to gain electrons.
Cl
+
O
+
e-


Cl
F
+

N
+

Unit 3, Lesson 3 of 3
Atoms vs. Ions
Group
Representative
Atoms
Ions
1
2
13
14
15
16
17
18
Na
Mg
Al
Si
P
S
Cl
Ar
[Na]+
[Mg]2+
[Al]3+
[P]3-
[S]2-
[Cl]-
Size
Comparison:
Atom
Vs.
Ion
The formation of ions, results in atoms taking on the electron configuration of a noble gas. When cations are formed, the
electron configuration is the same as the noble gas that precedes them on the table. For example, when sodium becomes the
sodium ion, its electron configuration changes from 2-8-1 to 2-8, the electron configuration of neon. When anions are
formed, their electron configurations are the same as the noble gas the follows them on the table. For example, when
chlorine becomes the chlorine ion, its electron configuration changes from 2-8-7 to 2-8-8, the configuration of argon. When
two atoms or ions have the same electron configuration, they are called isoelectronic.
Periodic Trend in Ionic Radii
Down a Group:
Across an Isoelectronic Series:
In addition to monatomic ions, polyatomic ions exist. A polyatomic ion is a group of atoms that collectively carries a charge.
Below is a list of some common polyatomic ions.