Ionic Bonding (Part II)

Ionic Compounds: The Compounds that Result

from Ionic Bonding

Compounds in which two or more ions are held together by electrical attraction.

The positive ions are called cations.The negative ions are called anions. Ionic compounds usually form large

crystals that can be seen with the naked eye.

Ionic compounds with smaller crystals may appear as powders.

What are Ionic Compounds?

All ionic compounds form crystals—the attraction between cations and anions is conducive to the ordered stacking that makes up crystals.

Ionic compounds tend to have high melting and boiling points—to break the positive and negative charges apart for either melting or boiling, it takes a huge amount of energy.

Properties of Ionic Compounds

All ionic compounds are very hard and brittle. It takes a lot of energy to pull ionic charges apart. If one gives a large crystal a strong enough impact, such as that from a hammer, one typically uses so much energy that the crystal doesn't break in only one place. Instead, it shatters.

When ionic compounds dissolve in water, they yield solutions that conduct electricity. The positive and negative ions that become separated from each other in a water solution allow electrons to flow much better than in plain water. Ionic compound are often called salts and salt-water conducts electricity. 

Properties of Ionic Compounds

Remember its based on the Octet Rule

Na Cl+ -

NaCl

Formation of an Ionic BondUsing Lewis Dot Diagrams

METAL + NONMETAL Important metal ions

Group 1A loses 1 electron (+1) Group 2A loses 2 electrons (+2) Group 3A loses 3 electrons (+3)

Important non-metal ions Group 7A gains 1 electron (-1) group 6A gains 2 electrons (-2) group 5A gains 3 electrons (-3) Group 4A gains 4 electrons (-4)

Ionic Compound

For a cation with a +1 charge and an anion with a -1 charge, it’s easy:Example: NaCl

For a cation with a +2 charge and an anion with a -2 charge, it’s also easy:Example: MgO.

Similarly, for a cation with a +3 charge and an anion with a -3 charge, it’s easy.

For Ionic Compounds, the Total Charge of the Cations Must

Balance the Total Charge of the Anions

One approach is basically mathematical:

For Ca (Group 2A) with a +2 charge and P (Group 5A) with a -3 charge, one needs (3 times +2) for the cations and (2 times -3) for the anions to balance. This is shown here with interlocking blocks: Ca3P2.

For Cations and Anions with Different Values of Charge, Balancing is More

Difficult

Ca2+

Ca2+

Ca2+

P3-

P3-

Ca 2+ and P 3-

3 2

Gives Ca3P2

But, Mg2+ and O 2- (Mg + O2 Reaction) 2 2

Gives Mg2O2 which must be changed to the simpler MgO!

Be careful when you use Criss Cross!

The Reaction of Ca and P (Another Method: Criss Cross)

Mg 2+ and C 4-

4 2

Gives Mg4C2

By convention, Mg4C2 must be changed to the simpler Mg2C, when both ion subscripts are divisible by 2.

Be careful when you use Criss Cross!

The Reaction of Mg and C (A Criss Cross Example with an Adjustment)

The reaction of aluminum and sulfur yields:

Aluminum is in Group 3A. Charge of 3+

Al3+

Phosphorus is in Group 6A. Charge of 2-

S2-

A 3+ and a 2- cannot balance simply:AlS would have a net +1 charge.

Another Example Problem For Cations and Anions with

Different Values

X

Al 3+ and S 2-

2 3

Gives Al2S3

But, Al 3+ and N 3- (Al + N2 Reaction) 3 3

Gives Al3N3 which must be changed to the simpler AlN!

The Reaction of Al and S (Method 1: Criss Cross)

For Al3+ and S2-, the lowest common denominator is 6 (plus or minus).

Therefore, one should try for a +6 balancing a -6.

2 times +3 is +6. So one needs 2 Al3+.3 times -2 is -6. So, one needs 3 S2-.Putting them together gives Al2S3.This method always works.

The Reaction of Al and S (Method 2: Mathematical

Balance)

For one Al3+ and one S2-, the combination would be AlS+ with a net charge of +1.

WRONG; it has to be neutral. For one Al3+ and two S2-, the combination would

be AlS2- with a net charge of -1.

WRONG; it has to be neutral. For two Al3+ and two S2-, the combination would

be Al2S22+ with a net charge of +2.

WRONG; it has to be neutral. For two Al3+ and three S2-, the combination

would be Al2S3 with a net charge of 0. CORRECT!

The Reaction of Al and S (Method 3: Trial and Error)

Use the correctly charged blocks for both the cation and anion.

Fit them together to form a rectangle.

The number of cation blocks is the subscript for the cation: Al2

The number of anion blocks is the subscript for the anion: S3

Put the components together into a proper chemical formula: Al2S3.

The Reaction of Al and S (Method 4: Charge-Balance Interlocking

Blocks)

S2-

S2-

S2-

Al3+

Al3+

Find the correct charges for the cation and anion from their Periodic Table Group.

Use the method that works best for you.Criss CrossMathematical BalanceTrial and ErrorCharge-Balance Interlocking Blocks

Check the result to see that it is balanced and in the simplest possible form.

Charge Balanced Ionic Compounds

The reaction of lithium with carbon.

Li4C

Practice Example

The reaction of aluminum with fluorine.

AlF3

Practice Example

Simple ionic compounds have two word names.

The first word is the name of the cation, which is just the name of the element.

The second word is the name of the anion, which is not simply the name of the element. This anion name is derived from the element name. Specifically, the last part of the element name is replaced with -ide.

Rules for Naming SimpleIonic Compounds

Oxygen becomes oxide.Hydrogen becomes hydride.Carbon becomes carbide.(The suffix part of the element name beginning with a vowel—a, e, i, o, u, or y—is replaced with the suffix -ide).

Examples of Anion Names

NaBrMg3P2

AlNNa2O

Naming Examples

• Sodium bromide

• Magnesium phosphide

• Aluminum nitride

• Sodium oxide