Ions
Section 5.5
IONS
An ion is simply a charged atom.
 Ions are formed as atoms lose or gain electrons to
achieve stability.
 To figure out how ions form, we must first start with its

Bohr – Rutherford diagram:
Protons = Atomic #
= 9p
Neutrons = Atomic mass – Atomic #
= 19 – 9
= 10n
Electrons = # Protons
= 9e
Fluorine - atomic number 9
9p
10n
+9p
-9e
0 = Total charge of zero
Fluorine - atomic number 9
9p
10n
+9p
-9e
0 = Total charge of zero
This structure is not stable as it does not
have a full valence shell.
Valence shell – the outermost electron shell
of an atom.
 To achieve a full valence shell, Fluorine must
gain an electron.

Fluorine atom (unstable)
9p
10n
Fluoride ion (stable)
-
Gain of an electron
+9p
-9e
0 = total charge of zero
9p
10n
+9p
-10e
-1 = total charge of negative 1

The ion formed is now stable because its
valence shell is full.

MORE EXAMPLES...
Oxygen – atomic number 8
28p
8n
Gains two electrons
+8p
-8e
0 = total charge of zero

8p
8n
+8p
-10e
-2 = total charge of neg 2
Is it easier to gain two electrons or lose
6?
Gain two
Magnesium – atomic number 12
+2
12p
12n
+12p
-12e
0 = total charge of zero

Loses two electrons
12p
12n
+12p
-10e
+2 = total charge of plus 2
Is it easier to gain six electrons or lose 2?
Lose two
Metals always form positive ions (they
give away their electrons to become
stable).
Non-metals always form negative ions
(they accept electrons to become stable).
Exception is noble gases because they do
not form ions (they’re already stable!)
Remember, being “stable” means having a
full valence shell.