Ionic Compounds

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Ionic Compounds

• Unit 4
• PreAP Chemistry
• McKinney HS

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What is an ionic compound?

• Composed of a metal and a nonmetal
• Solid formed from a lattice-style repetition of
  anions(-) and cations()
• Formula represents the ratio of cations to anions
  in the crystal lattice, and is called a formula
  unit
• Usually have high melting points
• Electrically non-conductive as a solid
• Conductive as molten liquids or in solution

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Formation of an ionic compound

• Combines a metal and a nonmetal through the
  transfer of electrons
• Example A compound made from potassium and
  chlorine

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Potassium has one valence electron and tends to
lose it to become a cation with a charge of 1
K
Cl
Chlorine has seven valence electrons and tends to
gain one to become an anion with a charge of -1
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K
Cl
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Potassium, being an alkali metal, has an
oxidation number of 1
-

K
Cl
Chlorine, being a halogen, has an oxidation
number of -1
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When these two ions combine to form an ionic
compound, they will combine in such a way that
the overall charge of the compound is zero. In
this case, the smallest ratio that adds up to
zero is one potassium ion and one chloride ion.
-

K
Cl
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When these two ions combine to form an ionic
compound, they will combine in such a way that
the overall charge of the compound is zero. In
this case, the smallest ratio that adds up to
zero is one potassium ion and one chloride ion.
KCl
Note that the metal ion is ALWAYS written first,
and that when there is only one ion of an
element, it is not necessary to place a subscript
1 next to that element.
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Calcium has two valence electron and tends to
lose both of them to become a cation with a
charge of 2
Ca
Cl
Chlorine has seven valence electrons and tends to
gain one to become an anion with a charge of -1
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Since calcium has two electrons to give, but
chlorine can only accept one, those electrons
must go to two separate chlorine atoms.
Cl
Ca
Cl
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-
Calcium, being an alkaline earth metal, has an
oxidation number of 2
Cl
2
Ca
-
Cl
Chlorine, being a halogen, has an oxidation
number of -1
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When these two ions combine to form an ionic
compound, they will combine in such a way that
the overall charge of the compound is zero. In
this case, the smallest ratio that adds up to
zero is one calcium ion and two chloride ions.
-
2
Ca
Cl
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When these two ions combine to form an ionic
compound, they will combine in such a way that
the overall charge of the compound is zero. In
this case, the smallest ratio that adds up to
zero is one calcium ion and two chloride ions.
CaCl2
Note that the metal ion is ALWAYS written first,
and that when there is only one ion of an
element, it is not necessary to place a subscript
1 next to that element.
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Hopefully by this point you can see that it is
not necessary to draw out the dot structures
before writing the formula for an ionic compound.
The dot structures provide us with the oxidation
number, and the oxidation numbers determine the
ratio of ions in the compound. But since we can
find the oxidation numbers of the representative
elements from the periodic table, we can skip the
step of drawing out the dot structures.
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Beryllium and oxygen

• Beryllium is an alkaline earth metal and has an
  oxidation number of 2 (it loses two electrons)
• Oxygen has an oxidation number of -2 (it gains
  two electrons)

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Be2
O2-
The smallest ratio that adds up to zero is one
beryllium ion to one oxide ion
BeO
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Lithium and nitrogen

• Lithium is an alkali metal and has an oxidation
  number of 1 (it loses one electron)
• Nitrogen has an oxidation number of -3 (it gains
  three electrons)

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Li
N3-
The smallest ratio that adds up to zero is three
lithium ions to one nitride ion
Li3O
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Aluminum and sulfur

• Aluminum has an oxidation number of 3 (it loses
  three electrons)
• Sulfur has an oxidation number of -2 (it gains
  two electrons)

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Al3
S2-
The smallest ratio that adds up to zero is two
aluminum ions to two sulfide ions
Al3S2