Chapter 7

7.2: Ionic Bonds & Ionic Compounds

The Formation of Ionic Bonds

Objectives Describe the formation of ionic bonds and

the structure of ionic compounds. Generalize about the strength of ionic

bonds based on the physical properties of ionic compounds.

Categorize the formation of an ionic bond as exothermic or endothermic.

The Formation of Ionic Bonds

A positively charged cation is attracted to a negatively charged anion.

The electrostatic force of attraction holding oppositely charged particles together is called an IONIC BOND.

Ionic Compound Defined as a compound having an ionic

bond It is a compound that is composed

entirely of ions yet has a “0” overall charge.

Ionic Compounds A binary ionic compound contains only 2

different elements – a metal cation and a nonmetal anion (ex. NaCl)

Forming Ionic bonds Even though they contain charged particles,

ionic compounds are electrically neutral. This means, the number of electrons

lost by a cation must equal the number of electrons gained by the anion.

The electrical charges of the cations and anions must balance (the total of all the charges has to add up to zero).

Example: NaClSee Table 4, p. 211

Na Cl

[Na]+ [ Cl ]-

NaClShowing ionic compound formation

using electron dot structures

The Formation of Ionic Bonds Consider the formation of the ionic compound,

calcium fluoride, using electron configurations. Calcium is a group 2A metal: [Ar] 4s2

How many electrons will calcium lose to gain a noble gas configuration?

What is its charge?

Fluorine is a group 7A nonmetal: [He] 2s22p5

How many electrons will fluorine gain to attain a noble gas configuration?

What is its charge?

The Formation of Ionic Bonds The number of electrons lost must equal the

number of electrons gained; the total of the charges must add to zero. The # of electrons lost does not equal the # gained. The total of the charges does not equal 0 (+2 + -1 = +1).

Therefore, another F ion is needed – each F will gain 1 electron, for a total of two. That means, the # of electrons lost will equal the # gained; the total of the charges will be 0.

The Formation of Ionic Bonds One Ca+2 will form an ionic bond with

two F-.

The formula for the compound will be CaF2 – the subscript of 2 below F means there are 2 F- in the compound.

The name of the compound will be a combination of the names of the 2 ions that make it up - calcium fluoride.

Practice worksheet

Properties of Ionic Compounds The cation & anion have a regular,

repeating pattern; forming an ionic crystal

There is no such thing as 1 unit of NaCl – Large numbers of ions are found together in a crystal, in a ratio that balances the charges.

For example, in NaCl, cations and anions are in a 1:1 ratio

Ionic Compounds

Ionic Compounds

The formula, NaCl, is an empirical formula – through the subscripts, it gives the smallest possible ratio of ions – 1 Na+ to 1 Cl-

This ratio is determined by the number of electrons transferred in ionic bonding

Ionic Crystals

Each type of crystal has a characteristic shape.

NaCl Calcium carbonate

Ionic crystal

The 3-d geometric arrangement of particles is called a crystal lattice.

Ionic compounds have different crystal lattice structures because they have differently charged ions making up that structure.

Properties of Ionic Compounds Melting points, boiling points, and hardness

depend on the strength of the attraction between the ions in the compound. They are generally high numbers.

Properties of Ionic Compounds Dissolve readily in water because they dissociate:

NaCl --> Na+(aq)+ Cl-(aq) In a solid, ions are locked into position and

electrons cannot flow freely—solid ions are poor conductors of electricity.

Ionic compounds, in a liquid state or in aqueous solution, are good conductors of electricity since electrons can move freely.

Ions in aqueous solution that conduct electricity are called electrolytes.

Properties of Ionic Compounds Ionic crystals are hard, yet they are

brittle. This figure demonstrates how and why

crystals break when an external force is applied.

The Formation of Ionic Bonds During any chemical reaction, energy is

either absorbed or released. If energy is released, the reaction is

exothermic.

The Formation of Ionic Bonds

The formation of ionic compounds from positive and negative ions is almost always exothermic.

The attraction of the positive ion for the negative ion forms a more stable system that is lower in energy than the individual ions.

The Formation of Ionic Bonds

The energy required to separate one mole* of ions in an ionic compound is referred to as the lattice energy.

The more negative the lattice energy, the stronger the force of attraction, the harder to separate the ions.

* I mole = 6.02 x 1023 ions

TrendsTrends in Lattice Energy In smaller ions, the ions are closer in

space and require a greater lattice energy to separate. See NaF vs. NaI in Table 6, p. 217

Compounds of ions with larger ionic charges will have greater lattice energy. Compare NaCl and SrCl2