IONIC BONDS AND IONIC COMPOUNDS

Section 7.2

Forming Ionic Compounds

• Since ions have charges, ions with opposite charges will be attracted to each other. The force that pulls them together (and holds them together) is an electrostatic force known as an ionic bond.

Ionic Bonding

• In an ionic bond, one element has a much stronger attraction for electrons than the other atom, so an electron is essentially transferred to the more electronegative atom.

• The result is 2 or more anions and cations bonded together – an ionic compound.

Realplayer\7.2 Formation of ions and ionic bonding.flv

Comparing Electronegativities• Whether or not a metal and a nonmetal

will form an ionic bond can be determined by comparing their electronegativities.

• If the difference between the two electronegativity values is 2.0 or more, then they will ionize and form an ionic bond.

• Consider calcium (1.0) and fluorine (4.0): END = 4.0-1.0 = 3.0 ionic bond

Forming Ionic Compounds

• When ionic bonding occurs and a new ionic compound is formed, the new substance can be represented with a chemical formula.

• A chemical formula shows the types of atoms in the substance, as well as how many of each type are in one unit of the substance.

• One unit of an ionic compound is referred to as a formula unit (NOT a molecule).

Why a “Formula Unit?”

• An ion is actually attracted to any oppositely charged ion nearby; this creates strong attractive forces (bonds) in multiple directions and is why ionic compounds have specific crystalline structures (more on this later).

• A formula unit is the lowest whole-number ratio of ions in an ionic compound.

Considering Table Salt

• As can be seen in the diagram to the right, there is one chloride ion for each sodium ion (a 1:1 ratio), which is why table salt has the formula NaCl.

Using Electron Dot Structures• It is also easily seen from the

electron dot structures of sodium and chlorine that they would combine in a one-to-one ratio.

Balancing Charges

• Another possible way to figure out the chemical formula of one formula unit of an ionic compound is to consider the charges on the ions. Figure out how many of each are needed for the charges to be balanced (make sure when you are finished that you have the simplest whole-number ratio).

• Since sodium ion and chloride ion have+1 and -1 charges (respectively), they balance with one of each.

Now you try it.

Use electron dot structures to figure out the chemical formulas of the ionic compounds formed when the following elements combine:Magnesium and chlorine

Confirm with their charges:

Now you try it.

Aluminum and chlorine

Confirm with their charges:

Now you try it.

Aluminum and sulfur

Confirm with their charges:

Properties of Ionic Compounds• Most ionic compounds are

crystalline solids at room temperature. This results from the strength of the attraction between the ionic particles.

• The crystalline structure is a direct result of the repeating 3-dimensional patterns that come from the charges as well as the sizes of the ions involved.

Coordination Numbers

• When ions form a crystal, a cation is attracted to all nearby anions.

• The coordination number tells the number of ions of opposite charge that surround an ion in a crystal.

• What is the coordination number of sodium ions in sodium chloride?

Coordination Numbers

• What is the coordination number of cesium ions in cesium chloride?

Coordination Numbers

• What is the coordination number of titanium ions in titanium dioxide?

• What is the coordination number of oxide ions?

HINT: Look here!

Properties of Ionic Compounds• Solid ionic compounds do not conduct

electricity, even though they are composed of charged particles, because the ions are locked rigidly into their crystal lattice – they are not free to move, and neither are their electrons.

• Melting an ionic solid allows the ions to flow, so charged particles can move and liquid ionic compounds are excellent conductors of electric current. (However, this would take extreme temperatures!)

Properties of Ionic Compounds• Ionic compounds will also dissolve in

water. When they dissolve, they become dissociated into their ions in the solution.

Properties of Ionic Compounds• Since the ions are

free to move around in the water, salt-water solutions are excellent conductors of electric current as well, and they do not require extreme temperatures (like melting would).

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