Chemistry Matters –

Chapter 6 :Ionic Bonding

But …Atoms of Noble Gases are different …

Atoms of noble gases are unreactive or stable.

They exist as individual atoms.

This means that they do not combine with other atoms to form compounds.

Are all atoms reactive?

Most of them are!

What are Noble Gases?

Noble gases are elements that are found in Group 8 (also called Group 0) of the Periodic Table.

helium, neon, argon, krypton, xenon, radon.

Examples:

Why are Noble Gases unreactive?

What do they have in common?

Each noble gas has a full valence shell.

The outer shells of noble gases are shown here.

What are the electrons in the outer shells also called?

Valence electrons

What are valence electrons?

The shell that is furthest from

the nucleus which contains

electrons is called the outer

shell or the valence shell.

The electrons in this shell are called valence electrons (or valency electrons).

Helium atoms are unreactive because …

Helium has two valence electrons; this is the maximum number that can occupy the 1st (K-) shell.

We say that it as a duplet configuration. This is a stable configuration (structure).

Atoms of the other noble gases have eight valence electrons.

This is called an octet configuration; this structure is stable.

An atom is stable if it has a duplet or octet configuration.

A duplet or octet configuration is also known as a noble gas structure or a noble gas configuration.

How do we know if an atom is stable or unstable?

So why are other atoms reactive?

Atoms react in order to have the noble gas structure (duplet or octet) (in order to become stable!!)

How do atoms achieve the noble gas structure?

Atoms of elements (besides the noble gases) react to achieve the noble gas structures.

When atoms lose or gain electrons, they form ions.

They do so by losing, gaining or sharingvalence electrons.

What type of ions do metals form?

Metals form positively charged ions (cations).

How do atoms of metals become positive ions (cations)?

Atoms of metals lose valence electrons to form positive ions (cations).

It takes less energy to lose fewer electrons.

Why don’t they gain electrons instead?

Hint: Do metal atoms have few or many valence electrons?

Few; between 1-3 valence electrons.

Example 1: Formation of a Sodium Ion

A sodium atom has 11 protons and 11 electrons. It is neutral (has a net charge of zero).Its electronic configuration is (2, 8, 1).

The sodium ion that is formed hasone positive charge because it has11 protons but 10 electrons.

11+

11+

11-

0

Formation of a Sodium Ion

To attain an octet configuration, a sodium atom (Na) loses 1 valence electron.

+

It forms a sodium ion (Na+).

Formation of a Calcium Ion

A calcium atom (Ca) has an electronic configurationof (2, 8, 8, 2)

It forms a calcium ion (Ca2+).

To attain an octet configuration, the atom loses 2 valence electrons.

2+

Example 2:

(2, 8, 8)

What type of ions do non-metals form?

Non-metals form negatively charged ions (anions).

-

How do atoms of non-metals become negative ions (anions)?

A chlorine atom (Cl) gains an electron

to form a chloride ion (Cl- ). The chloride ion has an octet configuration.

Example 1:

2-Formation of an Oxide Ion

The oxygen atom has an electronic configuration of (2, 6). It gains twoelectrons to form an oxide ion (O2–).

Example 2:

Forming Ionic Bonds

When metals react with non-metals, an ionic compound is formed.

Example:Sodium reacts with chlorine to form an ionic compound called sodium chloride.

When SODIUM meets CHLORINE…

Na (2,8,1)

Sodium atom

Cl (2,8,7)

Chlorine atom

Na+ (2, 8)Sodium ion

Chloride ion

Cl- (2,8, 8)

How do we show ionic bonding?The diagram here shows the formation of an ionic bond in sodium chloride. It is called a ‘dot-and-cross’ diagram.

Na (2,8,1)

Sodium atom

A valence electron is transferred to the outermost shell of a chlorine atom

Cl (2,8,7)

Chlorine atom

Na+ (2, 8)

Sodium ion Chloride ion

Cl- (2,8, 8)

Positive ions and negative ions are attracted to each other and move closer – an ionic bond is formed. Electrostatic

attraction

Steps involved in the formation of an ionic compound

1. Formation of positive ions

Each sodium atom (Na) loses its single valence electron to form a positively charged sodium ion (Na+).

Na Na+ + e

Example:

2. Formation of negative ions

Each chlorine atom gains an electron from a sodium atom to form a negatively charged chloride ion.

Cl -Cl + e

Steps involved in the formation of an ionic compound

Example:

3. Formation of ionic bonds

Positive sodium ions and negative chloride ions are attracted to one another by electrostatic attraction to form sodium chloride.

NaClNa+ + Cl -

Steps involved in the formation of an ionic compound

Dot-and-Cross diagram of an ionic compound, e.g. sodium chloride

The dots represent the electrons of one atom, while crosses represent the electrons of another atom.

This is the electron transferred from sodium to chlorine.

Formation of Ionic Compounds:Example 2: Magnesium Chloride.

Each magnesium atom losestwo electrons to form a magnesium ion. Each chlorine atom gains an

electron to become a chloride ion.

Electrostatic attraction arises between magnesium and chloride ions. Magnesium chloride is formed.

How to write Chemical Formulae of Ionic Compounds

The formula of an ionic compound is constructed by balancing the charges on the positive ions with those on the negative ions.

All the positive charges must equal all the negative charges in an ionic

compound.

Example 1: Formula of Magnesium Oxide

How to write Chemical Formulae of Ionic Compounds (Method 1)

Mg2+

Magnesium ion

Since the charges balance, the formula is MgO.

O2–

Oxide ion

Example 2: Formula of Aluminium Oxide

How to write Chemical Formulae of Ionic Compounds: Method 1

Al3+

Aluminium ion

To balance the charges, there must be 2 Al3+

and 3 O2– .So, the formula is Al2O3

O2–

Oxide ion

1. write down the ions with the charges:

2. Move the values m and n diagonally (but without the charges).

The formula is XnYm.

How to write Chemical Formulae of Ionic Compounds: Method 2

X m+ Yn–

X m+ Yn–

Example 1: Formula of Magnesium Oxide

How to write Chemical Formulae of Ionic Compounds (Method 2)

Mg2+

Magnesium ion

Write the formula as MgO.

O2–

Oxide ion

Mg2 O2

Reduce to the simplest ratio Mg1 O1

Example 2: Formula of Aluminium Oxide

How to write Chemical Formulae of Ionic Compounds: Method 2

Al3+Aluminium ion

So, the formula is Al2O3

O2– Oxide ion

Al2 O3

Sodium ions (Na+) and Chloride ions (Cl-) arrange into a 3-D lattice structure (crystal).

The formula NaCl for sodium chloride shows that for every sodium ion present in a piece of salt, there is one chloride atom present (ratio of Na+ : Cl- = 1: 1)

sodium ion

chloride ion

Structure of Ionic Compounds Ionic compounds form giant ionic structures /

giant lattice structures/ crystal lattice structures

In a lattice, millions of sodium and chloride ions are arranged in an orderly manner.

These ions are held in place by strong forces of electrostatic attraction (ionic bonds) throughout the entire lattice.

Physical Properties of Ionic Compounds

1. Volatility (non-volatile) - high melting and boiling points

2. Solubility - most are soluble in water but not in organic solvents (e.g. ethanol)

3. Electrical conductivity - conducts electricity in molten and aqueous states, but not in the solid state.

Why do ionic compounds have high melting and boiling points (i.e. low volatility)?

Millions of ions in the lattice are held together by strong forces of electrostatic attraction between the ions. A large amount of energy is needed to overcome these many strong bonds.

As a result, ionic compounds:- are solids at room temperature and pressure. - Have very high melting points and boiling points.

Solubility

Ionic compounds are usually soluble in water.

(Ionic compounds are insoluble in organic solvents. Examples of organic solvents : ethanol, petrol and turpentine.)

Electrical Conductivity of ionic compounds

Ionic compounds do not conduct electricity in the solid state because the ions are not free to move about.

Bulb does not light up.

SOLID ionic compound, e.g. sodium chloride

When an ionic compound is melted or dissolved in water, it can conduct electricity.

This is because the ions are FREE to move in the molten state or in aqueous solution.

Electrical Conductivity of ionic compounds

Bulb lights up.

Molten or aqueous ionic compound