Formation of Ions
Review: An ion is an atom that has lost or gained
electrons.
The atoms want to have a full outer shell (be like the
noble gases)
Therefore, most atoms will only form 1 type of ion,
for example
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Na only forms Na+
O only forms O2-
But how do we know what ion they will form?
Formation of Ions
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For electrons to have a full shell they can lose or
gain electrons – whichever is easier.
Group 1 atoms could either lose 1 electron or gain 7
electrons to have a full outer shell…which do you
think it will be?
Of course, its easier to lose one electron – so group
1 atoms form 1+ ions.
The other groups are similar….
So the periodic table can help us!
Valence Electrons
Valence
1
2
3
4
5
6
7
8
Formation of Ions
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Group 1 – form 1+ ions (they lose one electron)
Group 2 – form 2+ ions (they lose two electrons)
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Group 3 – 12 (Transition metals) – behave a little
strange, so the periodic table can’t help
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Group 13 – form 3+ ions (they lose 3 electrons)
Group 15 – form 3- ions (they gain 3 electrons)
Group 16 – form 2- ions (they gain 2 electrons)
Group 17 – from 1- ions (they gain 1 electron)
Group 18 – They don’t form ions!
What about group 14?
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Formation of Ions
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METALS form POSITIVE ions
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NON-METALS form NEGATIVE ions
Naming of Ions
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Naming Ions
 Positive
ions keep the element name and
just add the word ion.
• For example Na+ is the sodium ion
 Negative
ions drop the ending to the
element name and add “ide” (plus the word
ion)
• For example Br- is the bromide ion
A chemical formula tells us the type of
atoms and the number of atoms in a
molecule or a formula unit.
Chemical Formulas
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ionic compound
Formula represents one unit – (Formula
Unit)
 simplest ratio of the compound’s anions and
cations
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Binary Ionic Compounds
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Compounds are formed when one type of cation and
one type of anion bond together.
When writing formula units, the cation is written first
and the anion is written second.
The total charge must be ZERO. (Electrically neutral)
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The number protons must equal the number of
electrons
Luckily, we don’t have to count all the p+ and e-, the
ion charges give us the information we need.
Binary Ionic Compounds
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Compounds made of two different ions
Crossing Over Method
1.
2.
3.
4.
write two ion types with correct charges
make the anion’s charge, the cation’s
subscript
make the cation’s charge, the anion’s
subscript
simplify the ratio if possible
Practice Writing Formulas
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iron (III) sulfide
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Fe3+
S2Fe2S3
cadmium oxide
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Cd2+
O2Cd2O2
CdO
potassium nitride
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K1+
K3N
N3-
tin (IV) sulfide
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Sn4+
S2Sn2S4
SnS2
Compounds with Polyatomic
Ions
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A polyatomic ion is a group of covalently
bonded atoms that have a charge
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Ex. Sulfate - SO42-
the cation or anion could be a polyatomic ion
 The polyatomic ion still acts like one unit.
The formula of the ion never changes
 Example
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Na2SO4
MgSO4
Al2(SO4)3