1.11 The Formation of Ionic Compounds

pp. 29 - 35

Types of Bonds

• The force of attraction holding two atoms or ions together in a compound is known as a chemical bond.

• There are two types of bonds: ionic and covalent.

• Ionic bonding occurs between metals and non-metals.

• Covalent bonding occurs between two non-metals.

Properties of Ionic Compounds

• They are solids at ambient (normal) temperatures and they all have definite geometries.

• They are hard and brittle.

• They have relatively high melting and boiling points. For example, sodium chloride melts at 801 °C.

• They conduct electricity as molten liquids but not as solids.

• They conduct electricity when dissolved in water.

(a) sodium chloride

calcium carbonate mercury(II) sulfide

Formation of Ions

Ion charge - the electric charge that an atom takes on when it loses or gains electrons.

An atom or group of atoms that has lost or gained electrons is called an ion.

Electrons have a negative charge, and so an atom that loses electrons becomes a positive ion. An atom that gains electrons becomes a negative ion.

Octet Rule

• When elements form ionic compounds, changes occur in the arrangement of electrons in the outer orbit

• Electrons are gained or lost so that element can have a stable electron arrangement of the closest noble gas.

• In order for a compound to be stable, it must have a completely filled outer electron shell – aka stable octet

Metals• Tend to have 1, 2, or 3 valence electrons (electrons in the

outer orbit)• They lose electrons when they combine with other elements

to form positive ions (cations): note the t in the word think +• They lose electrons, thus they have the same electron

arrangement as the Noble gas a row above them• E.g. Sodium:

N 12

P 11

N 12

P 11

Non-Metals

• Non-metals – Tend to have 4, 5, 6, or 7 valence electrons in. • They gain electrons to form negative ions (anions) • They gain electrons, thus they have the same electron

arrangement as the Noble gas in the same row. • E.g. Fluorine

N 10

P 9N 10

P 9

Lewis Symbols• Since it is only the valence shell electrons that take part in

chemical reactions, a Lewis symbol drawing is used to depict the electrons in the atoms valence shell.– Lewis Symbol – A diagram composed of a chemical

symbol and dots, depicting the valence electrons of an atom or ion.

• Electrons are drawn on the four sides of the symbol. • Each side can hold two electrons. • The electrons are filled in one at a time until all the spaces

are filled with one electron and then electrons are paired up. • It’s like seats on the train. You don’t sit right beside someone

if there are double empty seats all around you.

Lewis Symbols for Ions

• Draw the Lewis symbol for the ion showing all of the electrons in the outer shell. If there are none, just draw the symbol.

• Then draw square brackets around the symbol and in the upper right corner write the charge on the ion.

• E.g. Potassium, Calcium, Nitrogen, Oxygen, Chlorine (neutral and ion)

Polyatomic Ions

• Ions that are composed of one atom are called monatomic ions

• When an ion is composed of more than one atom, it is called a polyatomic ion

• Table 1 on p. 33 lists some common polyatomic ions and the back of your periodic table lists more.

The Formation of Ionic Compounds

• Using Lewis symbols as follows:

• The bond between a positive and negative ion is an ionic bond.

The Structure of Ionic Compounds

• Large numbers of cations and anions arrange themselves into crystals.

• The rigid arrangement of ions is called a crystal lattice structure.

• The smallest repeating unit in an ionic crystal structure is called the formula unit

Explaining the Properties of Ionic Compounds

• They have high melting points because their ions are held together by strong electrostatic forces (ionic bonds).

• They are hard because their bonds resist being “stretched.”

• A piece of sodium chloride is easily cracked or fractured because, when an outside force strikes the crystal, the crystal lattice structure is offset as like charges are brought together.

• They are electrolytes because, when dissolved in water, ions separate (dissociate) from the crystal by water molecules releasing free-floating ions. Ions are able to move, and carry electric charges, through the water

Homework

• Read section 1.11 (pp. 29 - 35)

• Answer the following questions:– p. 35 # 1, 3, 4abc, 5abc, 7– worksheet