chemical bonding chapter 6. contents you will learn about: ionic bonds – electron transfer...

Chemical BondingChemical Bonding

Chapter 6

Chapter 6

contentscontents

You will learn about:

• Ionic bonds – electron transfer

• Covalent bonds – electron sharing

• Properties of ionic and covalent compounds

You will learn about:

• Ionic bonds – electron transfer

• Covalent bonds – electron sharing

• Properties of ionic and covalent compounds

The noble gases

(Group 0 elements) are also known as inert gases. They are gases.

The noble gases

(Group 0 elements) are also known as inert gases. They are gases.

Noble Gas Electronic Configuration

1. When the outer shell of an element is completely filled with electrons, the element is very stable.

2. Atoms of noble gases in Group 0 are very stable. Hence, they do not react with other elements.

3. Noble gas configurations are very stable because they have fully filled outer/valence shells.

Noble gases (except for helium) have 8 electrons in their outer shells.

Noble gases (except for helium) have 8 electrons in their outer shells.

helium

neon

argon

He

Ne

Ar

Arrangement of electrons in the outer shell

Examples

2

2.8

2.8.8

Duplet structure

Octet structure

Octet structure

• The tendency for atoms to surround themselves with a stable ‘octet’ of electrons is called the octet rule.

• Helium has only 2 electrons in its outer shell , but is stable as it too has a fully filled outer shell of electrons.

The maximum number of electrons that can be accommodated in the 1st shell is 2. This is called the duplet rule.

Other elements do not have the stable electronic configuration where the outer shell is fully filled with electrons forming either the duplet or octet structure.

Thus, they react with one another by losing, gaining or sharing their valency electrons to achieve the stable noble gas electronic configuration during a chemical reaction.

( valency electrons are electrons in the outer shell used in chemical bonding)

Chemical bonding

Ionic bonding

Covalent bonding

By losing or gaining of electrons (i.e transferring of electrons)

Takes place between atoms of metals and non-metals

By sharing of electrons between atoms of non-metals

Covalent substances

Ionic compounds

An ion is a charged particle formed from an atom or a group of atoms by the loss or gain of electrons .It can be either positively or negatively charged.

atoms form ions by gaining or losing electrons to obtain the stable electronic structure of a noble gas

An ion is a charged particle formed from an atom or a group of atoms by the loss or gain of electrons .It can be either positively or negatively charged.

atoms form ions by gaining or losing electrons to obtain the stable electronic structure of a noble gas

Formation of Cations (positive ions) and Anions (negative ions)Formation of Cations (positive

ions) and Anions (negative ions)

negative ionadd

electrons

positive ion

neutral atom

remove electrons

electrons = protons

more electrons

than protons

more protons than

electrons

-

+

Electronic configuration of Metals & Non-metals

Electronic configuration of Metals & Non-metals• MetalsMetals tend to loselose their valence electrons to achieve

the stable noble gas electronic configuration as they have few valence electrons.

• Non-metalsNon-metals tend to gaingain electrons to achieve the stable noble gas electronic configuration as they have a lot of electrons in their valence shell

Note : only the valence (outer) electrons are involved in bonding; those in the complete inner shells do not take part.

Formation of Cations

Li atom loses an electron readily and become a lithium ion, Li+

Examples

Na atom 2,8,1 Na+

ion2,8

Na atom loses an electron and becomes a positive ion, sodium ion, Na+ which have the stable noble gas electronic structure as Neon (2,8).

11 protons11 electrons12 neutrons


loses 1 electron

+

Na Na

[all +ve charges (protons) balanced by -ve charges (electrons)]

11 protons (+11) and

10 electrons (-10)

overall charge = (+11 -10) = +1

NeutralCharged (+1)

Al atom 2,8,3 Al3+ ion 2,8

Al atom loses 3 electrons and becomes a positive ion, aluminium ion, Al3+ which have the stable noble gas electronic structure as Neon (2,8).



loses 3 electrons

3+

Al Al

[all +ve charges (protons) balanced by -ve charges (electrons)]


10 electrons (-10)

overall charge = (+13 -10) = +3

NeutralCharged (+3)

Now You Try!

• Draw diagrams to show the formation of (a) A potassium ion

(b) A calcium ion

Formation of AnionsCl atom gains 1 electron from a metal atom and becomes a negative ion,

named as chloride ion, Cl- which has the same noble gas electronic structure as Argon (2,8,8).



chlorine atom2,8,7

Cl

chloride ion2,8,8

gains 1 electronx

¯

Cl

Charged (-1)


18 electrons (-18)

overall charge = (+17 -18) = -1

Neutral

Now You Try ………

Draw diagrams to show the formation of (a)a fluoride ion (b) an oxide ion (c) a nitride ion

Cations- more examples

12 10 19 18Number of electrons

20191212No of protons

Mg Mg2+ K K+Symbol

Magnesium Magnesium atomatom

MagnesiumMagnesiumionion

PotassiumPotassiumatomatom

PotassiumPotassiumatomatom

Number of neutrons

12 12 20 20

2412 12

24 39

193919

Note: A positive ion still has the same number of protons

and neutrons as its atom, but, it will have less

electrons than protons.

16 16 8 8Number of neutrons

Anions – more examples

SulphurSulphuratomatom

Sulphide Sulphide ionion

Oxygen Oxygen atomatom

Oxide Oxide ionion

881616No of protons

S S2- O O2-Symbol

16 18 8 10Number of electrons

32 3216 16

16 168 8

Note: A negative ion still has the same number of

protons and neutrons as its atom, but, it will

have more electrons than protons.

Ionic bonding – involves the transfer of electrons from one atom to another so that each can achieve the noble gas electronic configuration.

Ionic bonding – involves the transfer of electrons from one atom to another so that each can achieve the noble gas electronic configuration.

usually formed between atoms of metals and non-metals

positive and negative ions are formed after the transfer of electrons

The oppositely charged ions are attracted to each other by strong electrostatic force of attraction.

This strong electrostatic force of attraction between the oppositely charged ions is called ionic bond.

Example

ionic bondsionic bonds

ionic bonds – electron transferionic bonds – electron transfer

Example 1 Formation of sodium chloride through ionic bonding

sodium atom, Na2.8.1

chlorine atom, Cl2.8.7

electron transfer

Diagrammatic Representation of Ionic BondingDiagrammatic Representation of Ionic Bonding

G:\pure chem\chemical bonding\A04-2-19Animation ionic and Covalent Bonding.MOV

An ionic compound, sodium chloride ( NaCl ) is formed. An ionic compound, sodium chloride ( NaCl ) is formed.

has the same electronic structure as

the noble gas, neon

has the same electronic structure as the noble gas, argon

sodium ion, Na+

2.8

++

chlorine ion, Cl-

2.8.8

--

Na Cl

‘DOT and CROSS’ Diagram

- electron

of Na

- electron

of Cl

Example 2 Formation of magnesium chloride

through ionic bonding

Example 2 Formation of magnesium chloride

through ionic bonding

chlorine atom, Cl

2.8.7

electron transfer


chlorine atom, Cl

2.8.7

electron transfer

magnesium atom, Mg

2.8.2

Mg

ClCl


has the same electronic structure as

the noble gas, neon


magnesium ion, Mg2+

2.8

2+2+

chloride ion, Cl-

2.8.8

--


chloride ion, Cl-

2.8.8

--

Cl Mg Cl

Ionic bonds are formed between atoms of metals and non-metals in compounds.

Examples include:

Ionic bonds are formed between atoms of metals and non-metals in compounds.

Examples include:

ionic bondsionic bonds

Compounds with ionic Compounds with ionic bondsbonds ElementsElements

NameName FormulaFormula MetalsMetals Non-Non-metalsmetals

sodium chloride

NaCl sodium chlorine

magnesium oxide

MgO magnesium oxygen

potassium iodide

KI potassium iodine

calcium bromide

CaBr2 calcium bromine

Structures of Ionic CompoundsStructures of Ionic Compounds

Ionic compounds have giant ionic lattice structure.

Properties of ionic compoundsProperties of ionic compounds

Have high melting points (above 250oC) and high boiling points (above 500oC)

Reason: The ionic bonds (electrostatic force of attraction) between the ions are very strong . A very large amount of heat energy is needed to overcome these strong bonds.

This also explains why all ionic

compounds are solids at

room temperature.

Have high melting points (above 250oC) and high boiling points (above 500oC)

Reason: The ionic bonds (electrostatic force of attraction) between the ions are very strong . A very large amount of heat energy is needed to overcome these strong bonds.

This also explains why all ionic

compounds are solids at

room temperature.

I. Boiling points and Melting PointsI. Boiling points and Melting Points

++++

++++

++++

++++

++++

++

++

-- -- --

-- -- --

-- -- --

-- --

-- --

II. Solubility II. Solubility

Ionic compounds are usually soluble in water but insoluble in organic solvents.

Reason: water molecules can separate the positive ions from the negative ions, causing them to dissolve.

Exceptions: silver chloride, barium sulphate are ionic compounds which are insoluble in water.

Organic solvents eg petrol, alcohol and turpentine

Ionic compounds are usually soluble in water but insoluble in organic solvents.

Reason: water molecules can separate the positive ions from the negative ions, causing them to dissolve.

Exceptions: silver chloride, barium sulphate are ionic compounds which are insoluble in water.

Organic solvents eg petrol, alcohol and turpentine

properties of ionic compoundsproperties of ionic compounds

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

When the substance is in molten state (melted in liquid form)) or aqueous state ( when dissolved in water), it can conduct electricity.

Reason : In these states, the ions are free to move. The moving ions conduct electricity.

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

When the substance is in molten state (melted in liquid form)) or aqueous state ( when dissolved in water), it can conduct electricity.

Reason : In these states, the ions are free to move. The moving ions conduct electricity.

III Electrical conductivityIII Electrical conductivity

Covalent bond is a bond formed by the sharing of electrons between atoms of non-metals.

After bonding, each atom attains the stable noble gas electronic configuration.

Covalent bond is a bond formed by the sharing of electrons between atoms of non-metals.

After bonding, each atom attains the stable noble gas electronic configuration.

What is covalent bond?What is covalent bond?

Why must atoms of non-metals share electrons while atoms of metals and non-metals form ions?

Why covalent bonds are formed between non-metal atoms?

• For elements with 4 valency electrons, gaining or losing 4 electrons to achieve a noble gas electronic configuration requires a large amount of energy. Thus, the non-metallic elements combined by sharing of electrons to form molecules.

Valency electrons - are electrons in the outermost

shell used to form bonds.

Valency - is the number of electrons an atom uses to

form bonds

Valence electrons - are electrons in the outermost shell

The molecules formed can be

(i) Simple covalent molecules like H2, O2, H2O, CO2, NH3, CH4, HCl, N2, Cl2, etc or

(ii)Giant covalent molecules (or macromolecules)( which is a three dimensional network of atoms bonded together by covalent bonds to form a giant molecule ) like diamond, graphite, silicon dioxide, etc

Covalent Covalent MoleculeMolecule

Chemical Chemical formulaformula

Electronic Structure of Molecule Electronic Structure of Molecule (‘Dot and Cross’ Diagram)(‘Dot and Cross’ Diagram)

Structural Structural diagramdiagram

hydrogen gas H2

Other ways to represent :

H H

H H+

H atom H atom

H H

H2 molecule

H H

Formation of Covalent Bonds Formation of Covalent Bonds

x

o

Single covalent bond

Covalent Covalent MoleculeMolecule FormulaFormula Electronic Structure of Molecule Electronic Structure of Molecule

(‘Dot and Cross’ Diagram)(‘Dot and Cross’ Diagram)

Simple Way of Simple Way of Showing the Showing the

BondsBonds

water H2OH

H

OH

O

H

Other ways to rep :

or H O

xx

xx xx H




BondsBonds

methane CH4H

H

C H

H

H C

H

H

H

Other ways to rep :

orx

x

xx HH C

H

H




BondsBonds

carbon dioxide

CO2

O C OOCO

Other ways to rep :

x xo ox x or x O xoC ox O x xx xx


(‘Dot and Cross’ Diagram)(‘Dot and Cross’ Diagram)Structural Structural diagramdiagram

oxygen O2

O OOO

Other ways to rep : x x o o

or xo O xo O x x o o

Double bond

(2 pairs of electrons)

Covalent Covalent MoleculeMolecule

FormulaFormula Electronic Structure of Molecule Electronic Structure of Molecule (‘Dot and Cross’ Diagram)(‘Dot and Cross’ Diagram)

Structural Structural diagramdiagram

nitrogen N2

N NNN

Other ways to rep : x x o o

or xo N xo N xo

Triplebond

(3 pairs of electrons)

Example 1 :Iodine is a simple diatomic covalent molecule.

It has a simple molecular structure.

Reason : There exists weak intermolecular forces of attraction, between the iodine molecules, also known as weak Van der Waals’ forces. These weak forces of attraction requires only a small amount of heat enerrgy to overcome.

Example 1 :Iodine is a simple diatomic covalent molecule.

It has a simple molecular structure.

Reason : There exists weak intermolecular forces of attraction, between the iodine molecules, also known as weak Van der Waals’ forces. These weak forces of attraction requires only a small amount of heat enerrgy to overcome.

(a) Simple Molecular Structure(a) Simple Molecular Structure

Structure of Covalent SubstancesStructure of Covalent Substances

Example 2 Methane is also a simple covalent molecule.

Four covalent bonds (C-H) are held together by strong forces of attraction. However, weak Van der Waals’ forces between methane molecules hold them together loosely. Therefore, methane exists as a gas at room temperature and pressure.

Example 2 Methane is also a simple covalent molecule.

Four covalent bonds (C-H) are held together by strong forces of attraction. However, weak Van der Waals’ forces between methane molecules hold them together loosely. Therefore, methane exists as a gas at room temperature and pressure.

Covalent molecules have strong covalent bonds between atoms, but the Van der Waals’ forces which exist between separate molecules are weak.

During melting or boiling, the molecules do not break up into atoms, but merely move further apart.

Thus, simple covalent molecules are volatile, i.e., they have low melting points and boiling points, as not much heat energy is needed to overcome the Van der Waals’ forces.

Covalent molecules have strong covalent bonds between atoms, but the Van der Waals’ forces which exist between separate molecules are weak.

During melting or boiling, the molecules do not break up into atoms, but merely move further apart.

Thus, simple covalent molecules are volatile, i.e., they have low melting points and boiling points, as not much heat energy is needed to overcome the Van der Waals’ forces.

volatilityvolatility

Properties of simple covalent compounds


Molecules do not break up into atoms but merely move further apart during melting

and boiling

G:\pure chem\chemical bonding\A09-0-19Ionic MgO and Covalent bond SO2.MOV

Most covalent molecules are insoluble in water but are usually soluble in organic solvents.

Exceptions: Alcohol and sugar are covalent compounds which are soluble in water.

Some covalent molecules dissolve in water because of chemical reactions. E.g., chlorine.

Most covalent molecules are insoluble in water but are usually soluble in organic solvents.

Exceptions: Alcohol and sugar are covalent compounds which are soluble in water.

Some covalent molecules dissolve in water because of chemical reactions. E.g., chlorine.

solubilitysolubility



Covalent substances (elements or compounds) do not conduct electricity whether in the solid or molten state. This is because they do not contain ions or free electrons.

Exceptions: carbon, in the form of graphite, conducts electricity. Covalent compounds such as hydrogen chloride and hydrogen sulphide also conduct electricity when dissolved in water.

Covalent substances (elements or compounds) do not conduct electricity whether in the solid or molten state. This is because they do not contain ions or free electrons.

Exceptions: carbon, in the form of graphite, conducts electricity. Covalent compounds such as hydrogen chloride and hydrogen sulphide also conduct electricity when dissolved in water.

electrical conductivityelectrical conductivity

Differences between Ionic and Covalent Bonding

Differences between Ionic and Covalent Bonding

Ionic Bond Covalent Bond

•Formed between metals and non-metals.

•Formed by transferring of electrons.

•Formed between non-metals and non-metals.

•Formed by sharing of electrons.

properties of ionic and covalent compounds


PropertyProperty Ionic CompoundsIonic CompoundsCovalent Compounds Covalent Compounds

(with simple (with simple Molecular Structure)Molecular Structure)

Boiling point high low

Electrical conductivity

do not conduct electricity when solid; conduct when

molten or dissolved in water

do not conduct electricity in any form

Solubility in Water

usually soluble usually insoluble

Solubility in organic solvents

insoluble soluble

summary of main differences in properties between ionic compounds and covalent compounds

summary of main differences in properties between ionic compounds and covalent compounds

chemical bonding chapter 6. contents you will learn about: ionic bonds – electron transfer...

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