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Chapter 6. Chemical Bonding. contents. 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.

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Chemical Bonding

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Chemical bonding

Chapter 6

Chemical Bonding


Chemical bonding

contents

  • You will learn about:

  • Ionic bonds – electron transfer

  • Covalent bonds – electron sharing

  • Properties of ionic and covalent compounds


Chemical bonding

The noble gases

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


Noble gas electronic configuration

When the outer shell of an element is completely filledwith electrons, the element is very stable.

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

Noble gas configurations are very stable because they have fully filledouter/valence shells.

Noble Gas Electronic Configuration


Chemical bonding

He

Ne

Ar

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

Examples

Arrangement of electrons in the outer shell

2

Duplet structure

helium

neon

Octet structure

2.8

argon

Octet structure

2.8.8


Chemical bonding

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.


Chemical bonding

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)

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


Chemical bonding

Chemical bonding

Covalent bonding

Ionic bonding

By sharing of electrons between atoms of non-metals

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

Takes place between atoms of metals and non-metals

Covalent substances

Ionic compounds


Chemical bonding

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

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

-

negative ion

add electrons

more electrons than protons

neutral atom

electrons = protons

+

positive ion

remove electrons

more protons than electrons


Electronic configuration of metals non metals

Electronic configuration of Metals & Non-metals


Electronic configuration of metals non metals1

Metalstend to lose their valence electrons to achieve the stable noble gas electronic configuration as they have fewvalence electrons.

Non-metalstend to gain 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.

Electronic configuration of Metals & Non-metals


Formation of cations

Formation of Cations

Examples

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


Chemical bonding

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).

+

loses 1 electron

Na

Na

2,8

Na+ ion

Na atom

2,8,1

11 protons

10 electrons

12 neutrons

11 protons

11 electrons

12 neutrons

Charged(+1)

Neutral

11 protons (+11) and

10 electrons (-10)

 overall charge = (+11 -10) = +1

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


Chemical bonding

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).

3+

loses 3 electrons

Al

Al

2,8

Al3+ ion

Al atom

2,8,3

13 protons

10 electrons

14 neutrons

13 protons

13 electrons

14 neutrons

Charged(+3)

Neutral

13 protons (+13) and

10 electrons (-10)

 overall charge = (+13 -10) = +3

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


Now you try

Draw diagrams to show the formation of

(a) A potassium ion

(b) A calcium ion

 Now You Try!


Formation of anions

Cl 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).

¯

x

Cl

Formation of Anions

gains 1 electron

Cl

chlorideion

chlorine atom

2,8,7

2,8,8

17 protons

17 electrons

18 neutrons

Charged (-1)

17 protons (+17) and

18 electrons (-18)

 overall charge = (+17-18) = -1

17 protons

18 electrons

18 neutrons

Neutral


Now you try1

Draw diagrams to show the formation of

a fluoride ion (b) an oxide ion (c) a nitride ion

Now You Try ………


Cations more examples

Potassium

atom

Magnesium atom

Magnesiumion

Potassium

atom

39

24

24

39

Symbol

Mg2+

K

K+

Mg

12

19

19

12

No of protons

12

12

19

20

Number of electrons

10

19

18

12

Number of neutrons

12

20

20

12

Cations- more examples

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

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

electrons than protons.


Anions more examples

Sulphur

atom

Sulphide ion

Oxygen atom

Oxide ion

No of protons

16

16

8

8

Number of neutrons

16

16

8

8

Anions – more examples

Symbol

S2-

O

O2-

32

16

S

32

16

16

8

16

8

Number of electrons

16

18

8

10

Note: A negative ion still has the same number of

protons and neutrons as its atom, but, it will

have more electrons than protons.


Chemical bonding

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.


Chemical bonding

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

Example


Chemical bonding

ionic bonds – electron transfer

ionic bonds


Chemical bonding

chlorine atom, Cl

2.8.7

Diagrammatic Representation of Ionic Bonding

Example 1 Formation of sodium chloride through ionic bonding

electron transfer

sodium atom, Na

2.8.1


Chemical bonding

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

-

+

-electron

of Na

- electron

of Cl

Na

Cl

sodium ion, Na+

2.8

chlorine ion, Cl-

2.8.8

has the same electronic structure as the noble gas, argon

has the same electronic structure as the noble gas, neon

‘DOT and CROSS’ Diagram


Chemical bonding

electron transfer

electron transfer

ionic bonds – electron transfer

Example 2 Formation of magnesium chloride

through ionic bonding

Mg

Cl

Cl

magnesium atom, Mg

2.8.2

chlorine atom, Cl

2.8.7

chlorine atom, Cl

2.8.7


Chemical bonding

-

-

2+

Cl

Mg

Cl

chloride ion, Cl-

2.8.8

magnesium ion, Mg2+

2.8

chloride ion, Cl-

2.8.8

has the same electronic structure as the noble gas, argon

has the same electronic structure as the noble gas, neon

has the same electronic structure as the noble gas, argon

ionic bonds – electron transfer


Chemical bonding

ionic bonds

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

Examples include:


Chemical bonding

Structures of Ionic Compounds

Ionic compounds have giant ionic lattice structure.


Chemical bonding

-

-

-

+

+

-

-

+

+

+

-

-

+

-

+

-

-

+

+

+

-

-

+

-

+

Properties of ionic compounds

I.Boiling points and Melting Points

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

Reason: The ionic bonds (electrostatic force of attraction) between the ionsare 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.


Chemical bonding

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


Chemical bonding

properties of ionic compounds

III Electrical conductivity

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

  • When the substance isin 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.


Chemical bonding

What is covalent bond?

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

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

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.

Why covalent bonds are formed between non-metal atoms?

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

Simple covalent moleculeslike 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

The molecules formed can be


Chemical bonding

H

H

Formation of Covalent Bonds

Single covalent bond

+

H

H

H atom

H atom

H

H

x

o

H2 molecule


Chemical bonding

Other ways to rep :

or

HO

xx

H

x

x

xx

O

H

H

H

O

H


Chemical bonding

H

Other ways torep :

or

H

H

C

H

H

H

C

H

H

H

x

x

x

C

H

H

x

H


Chemical bonding

C

O

O

O

O

C

Other ways to rep :

x xoox x

orx O xoC ox O x

xx xx


Chemical bonding

O

O

O

O

Double bond

(2 pairs of electrons)

Other ways to rep :

x x o o

or xo

O xoO

x x o o


Chemical bonding

N

N

N

N

Triplebond

(3 pairs of electrons)

Other ways to rep :

x x oo

or xo

N xoN

xo


Chemical bonding

Structure of Covalent Substances

(a) Simple Molecular Structure

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.


Chemical bonding

  • 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.


Chemical bonding

Properties of simple covalent compounds

volatility

  • 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.

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


Chemical bonding

Properties of simple covalent compounds

solubility

  • 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.

electrical conductivity

  • 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.


Chemical 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.


Chemical bonding

properties of ionic and covalent compounds

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


Chemical bonding

properties of ionic and covalent compounds


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