In this section you will… Describe Ionic and Covalent Bonds Explain Electronegativity Determine the Type of Bond.

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Technical Science

Introduction to Chemistry

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Chemical Bonds

● In this section you will…

–

Describe Ionic and Covalent Bonds

–

Explain Electronegativity

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Describe Ionic

and Covalent Bonds

● Atoms want fill the S & P orbitals of their outer

energy levels to become more stable.

● A chemical reaction is a change in the electron

configuration of 2 or more atoms as a result of the

atoms filling their outer energy levels.

● The attraction between 2 or more atoms as the

result of chemical reaction is a chemical bond.

● Two or more atoms bonded together is a molecule.

● A compound is a pure substance composed of the

same type of molecules.

Review

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Describe Ionic

and Covalent Bonds

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● Chemical bonding is important because virtually all

elements are found in nature as molecules rather

than in elemental form.

● In other words, there are sodium atoms in many

substances in your body right now.

● However, there is no sodium metal in elemental or

unreacted form.

● The sodium atoms in your body have reacted and

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Describe Ionic

and Covalent Bonds

3 of 12

● There are two different ways that atoms can fill

their outer energy levels. Therefore, there are two

different types of chemical bonds.

● Chemical bonds are classified as either Ionic or

Covalent.

● Let’s take a look at an example of each type of

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Describe Ionic

and Covalent Bonds

Ionic Bonds

4 of 12

Sodium Chloride is an Ionically bonded substance. It is formed from a Sodium atom

and a Chlorine atom.

Sodium

Na

Cl

Chlorine Sodium has 1 electron in its

outer energy level.

Chlorine has 7 electrons in its outer energy level.

Sodium wants to give up its outer electron.

Chlorine wants to get another electron.

Notice:

The electron was completely Exchanged from the sodium to the chlorine.

Sodium is now a

positively charged ion: Na+1 Chlorine is now a

negatively charged ion: Cl -1

+1 -1

Opposite charges attract.

The positive Sodium Ion is pulled toward the negative Chlorine Ion.

Na

+1

Cl

-1

The attraction that holds the positive ion to the negative ion

is called an Ionic Bond. We have created a

Molecule of a new compound: Sodium Chloride.

Sodium Chloride NaCl

The molecule was the result of a chemical reaction and it is held together with an Ionic Bond. Sodium is a highly reactive metal.

If you were to put it in water it would explode.

Chlorine is a highly reactive nonmetal.

It is extremely poisonous, even deadly, if it is inhaled.

But with the exchange of a single electron, we have created a molecule of ordinary table salt.

This shows the importance of chemical reactions

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Describe Ionic

and Covalent Bonds

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Ionic Bonds

● Explain how the ionic compound Beryllium Oxide,

BeO, is formed from Beryllium and Oxygen.

Α

Beryllium from Group 2 wants to give up 2

electrons. Oxygen from Group 6 wants to get 2

electrons.

When Be gives up 2 e

-

, it becomes an ion: Be

+2

.

When O gets 2 e

-

, it becomes an ion: O

-2

.

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Describe Ionic

and Covalent Bonds

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Ionic Bonds

● How is the ionic compound Magnesium Fluoride,

MgF

₂

, formed from Magnesium and Fluorine.

Α

Mg (Group 2) wants to give 2 e

-

but F (Group 7)

wants to get only 1 e

-

.

So, we need 2 fluorine atoms

to accept both of magnesium’s electrons.

When Mg gives up 2 e

-

, it becomes an ion: Mg

+2

.

When each F gets 1 e

-

, it becomes an ion: F

-1

.

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Describe Ionic

and Covalent Bonds

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Ionic Bonds

● An ionic bond is formed when the electrons are

exchanged from one atom to another.

● In the process, positive and negative ions are

formed.

● The attraction between the oppositely charged ions

is an ionic bond.

● This chemical reaction results in a molecule of a

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Describe Ionic

and Covalent Bonds

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Ionic Bonds

● Ionic compounds have other important properties.

What happens when an ionic compound like Sodium Chloride, NaCl,

is dissolved in water?

Na+ Cl

-When an ionic compound dissolves in water the ions separate from each other.

Na+

Cl

Na+ Cl -Na+

Cl

Na+ Cl -Na+

The ions are able to separate from each other because the electrons were

exchanged.

Water molecules surround the ions and keep the ionic

bonds from reforming. The tendency of ionic compounds to break up into ions when dissolved in

water is important. The hardness of water is determined by the amount

of dissolved Calcium and Magnesium ions. Many substances are transported in your body

as dissolved ions. These ions are called electrolytes. It is the ions in a solution that

allow water to conduct electricity. The ions carry

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Describe Ionic

and Covalent Bonds

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Covalent Bonds

Water is a Covalently bonded substance.

It is formed from 2 Hydrogen atoms and 1 Oxygen atom. Oxygen has 6 electrons in its

outer energy level.

Each Hydrogen has 1 electron in its outer energy level.

Oxygen wants to get 2 electrons. Each Hydrogen wants to

get 1 electron to fill its outer level. Notice:

All the atoms want to get electrons but no one

wants to give electrons. The atoms enter into a partnership where they

share electrons.

Oxygen will share one electron with each Hydrogen.

Each Hydrogen will share one electron with Oxygen.

The 2 shared electrons from the Hydrogen atoms spend part of

their time with Oxygen. This fills Oxygen’s outer level.

The 2 shared electrons from Oxygen spend part of their time

with the Hydrogen atoms. This fills Hydrogens’ outer level

Notice:

The electrons were not gained or lost.

The electrons were shared. The sharing of electrons between

atoms is called a Covalent Bond. This sharing holds the atoms together creating a molecule. The formation of this bond creates

a new compound with different properties from the original

substances.

Hydrogen and Oxygen are very reactive gases. But when they react, a new very stable substance,

water, is created.

This shows the importance of chemical reactions

and chemical bonds.

O

Oxygen

H

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Describe Ionic

and Covalent Bonds

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Covalent Bonds

● Explain how the covalent compound Methane, CH

₄

is formed from Carbon and Hydrogen.

Α

Carbon from Group 4 wants to get 4 e

-

but

Hydrogen from Group 1 wants to get only 1 e

-

.

So,

we need four Hydrogen atoms to share each of their

electrons with one carbon.

Carbon shares each of its 4 outer electrons with

one hydrogen and each Hydrogen shares its

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Describe Ionic

and Covalent Bonds

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Covalent Bonds

● A covalent bond is formed when the electrons are

shared between two atoms.

● This sharing of electrons is what holds the the

atoms together.

● Notice there are no ions formed because electrons

are shared not exchanged.

● This chemical reaction results in a molecule of a

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Describe Ionic

and Covalent Bonds

12 of 12

Covalent Bonds

● Covalent compounds have other important

properties.

What happens when an covalent compound like

Sugar, C₆H₁₂O₆, is dissolved in water?

When a covalent compound dissolves in

water the atoms stay together as a molecule. The carbon, hydrogen, &

oxygen atoms don’t separate because they are

sharing electrons. No ions are released

because covalent compounds aren’t composed of ions. The tendency of covalent

compounds to stay as molecules when dissolved

in water is important. Covalent compounds generally don’t dissolve in

water as well as ionic compounds.

The larger the covalent molecule the less likely it is

to dissolve.

Covalent molecules dissolved in water do not conduct electricity because there are

no ions to carry the charge.

C C C C C C O

O O

O O O

H H

H H H H

H

H H H

H H

C₆H₁₂O₆ C₆H₁₂O₆

C₆H₁₂O₆ Sugar Molecule

=

C₆H₁₂O₆

C

6H12O6

C₆H₁₂O₆

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1 of 7

Explain Electronegativity

● Now you know the difference between an

Ionic and a Covalent Bond. But…

● Why do some atoms react and form ionic bonds

while other atoms react and form covalent bonds?

● The answer is Electronegativity.

● Electronegativity is a measure of how hard an atom

pulls on the electrons in it’s outer energy level.

(The bigger the number the harder the pull.)

● Let’s look at the electronegativities of the atoms and

see if we can explain the differences.

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2 of 7

Explain Electronegativity

● The table below shows the electronegativities of the

elements.

● Remember, electronegativity is a measure of how

hard an atom pulls on the electrons in its outer level.

H 2.1 He … Li 1.0 Be 1.5 B 2.0 C 2.5 N 3.0 O 3.5 F 4.0 Ne … Na 1.0 Mg 1.2 Al 1.5 Si 1.7 P 2.1 S 2.4 Cl 2.8 Ar … K 0.9 Ca 1.0 Sc 1.2 Ti 1.3 V 1.4 Cr 1.6 Mn 1.6 Fe 1.6 Co 1.7 Ni 1.8 Cu 1.8 Zn 1.7 Ga 1.8 Ge 2.0 As 2.2 Se 2.5 Br 2.7 Kr … Rb 0.9 Sr 1.0 Y 1.1 Zr 1.2 Nb 1.2 Mo 1.3 Tc 1.4 Ru 1.4 Rh 1.4 Pd 1.4 Ag 1.4 Cd 1.5 In 1.5 Sn 1.7 Sb 1.8 Te 2.0 I 2.2 Xe … Cs 0.9 Ba 1.0 La 1.1 Hf 1.2 Ta 1.3 W 1.4 Re 1.5 Os 1.5 Ir 1.6 Pt 1.4 Au 1.4 Hg 1.4 Tl 1.4 Pb 1.6 Bi 1.7 Po 1.8 At 2.1 Rn … 1

2 3 4 5 6 7 8

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3 of 7

Explain Electronegativity

2 3 4 5 6 7 8

Electronegativities of the Elements

● What happens to the electronegativity when…

-

You move down through a group ?

-

You move across a period to the right?

A

Electronegativity decreases.

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4 of 7

Explain Electronegativity

● Why does electronegativity decrease as you move

down a group on the Periodic Table?

Fr

0.9

Cs

0.9

Rb

0.9

K

0.9

Na

1.0

Li

1.0

H

2.1

Group 1

Hydrogen

Lithium

Sodium

Electronegativity is a

meausure of how hard an

atom pulls on the electrons

in its outer energy level

This pull comes from the

positive charge on the

protons in the nucleus.

As we move down through

a Group the outer

electrons get further away

from the nucleus.

Hydrogen’s outer level = 1

st

Lithium’s outer level = 2

nd

Sodium’s outer level = 3

rd

etc.

The further the electrons are

from the nucleus the weaker

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5 of 7

Explain Electronegativity

● Why does electronegativity increase as you move

across a period to the right on the Periodic Table?

Li

1.0

Be

1.5

B

2.0

C

2.5

N

3.0

O

3.5

F

4.0

Ne

…

Period 2

Why does Electronegativity

increases as you move across a

period (to the right) on the

Periodic Table?

Lithium Fluorine

Before we answer that, we need

to answer another question.

What keeps electrons in

orbit around the atom?

The positively charged protons

in the nucleus pull the

negatively charged electrons.

Opposites Attract

How many positive charges

(protons) do Lithium and

Fluorine have in their nuclei?

3 Protons _{9 Protons}

In what Energy Level

are the outer electrons for

Lithium and Fluorine?

2nd Energy Level 2nd Energy Level

The protons for both

Lithium and Fluorine are

pulling on electrons that are in

the 2

nd

Energy Level.

Fluorine has 9 protons with

which to pull while

Lithium has only 3 protons.

So…

Fluorine can pull the electrons

harder and therefore it has a

larger electronegativity than

Lithium.

Electronegativity gets larger as

you move to the right on a

period because…

You are adding more protons to

the nucleus which gives a larger

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7 of 7

Explain Electronegativity

● What is electronegativity?

● Why does electronegativity decrease as you go

down a Group?

● Why does electronegativity increase as you move to

the right across a period?

Α

Its a measure of an atom’s pull on its electrons

Α

Electrons are further from the nucleus so the pull is

weaker

Α

There are more protons with which to pull and the

electrons are at the same distance.

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1 of 12

Determine the

Type of Chemical Bond

Home

● You know the difference between an Ionic and a

Covalent Bond

● You also know what electronegativity is and why

different atoms have different values.

● But we still haven’t answered our question about

why some atoms react to form ionic bonds and

some form covalent bonds.

● All we know is that it has something to do with

the electronegativity of the atoms involved.

● In the next slides we will see what happens when

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2 of 12

Determine the

Type of Chemical Bond

● What type of bonds will form when an atom with a

large electronegativity reacts with an atom with a

small electronegativity?

Na

Cl

Chlorine

Large Electronegativity 2.8

Sodium

Small Electronegativity 1.0

The atom with the large electronegativity (strong pull) will take the outer electron

from the atom with the small electronegativity (weak pull). When electrons are exchanged, as they were between sodium and chlorine,

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Determine the

Type of Chemical Bond

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● What type of bonds will form when an atoms with

similar electronegativities react?

O

Oxygen

Electronegativity = 3.5

H

Hydrogen

Electronegativity = 2.1 each

Both atoms have a similar electronegativity which means

that their pull on electrons is also fairly equal.

Since neither atom is strong enough to pull electrons away from the other, the atoms must

share the electrons. When electrons are shared,

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Determine the

Type of Chemical Bond

4 of 12

● Ionic Bonds form when the electronegativities of

the atoms are very different.

● Covalent Bonds form when the electronegativities

of the atoms are similar.

● But, how do we decide how similar or how

different the electronegativities are?

● There are some “Rules of Thumb” that will help

you make those decisions.

● Keep in mind these rules work most of the time

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Determine the

Type of Chemical Bond

5 of 12

● One method is to look at the type of element that is

reacting.

2 3 4 5 6 7 8

When Metals react with Nonmetals, Ionic Bonds tend to form

Metals

Nonmetals

Metals and Nonmetals tend to have different electronegativities. Stronger nonmetals will take electrons from the

weaker metals forming Ionic Bonds When nonmetals bond with each other,

Covalent Bonds tend to form. Nonmetals tend to have similar electronegativities. Since their pull on

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Determine the

Type of Chemical Bond

6 of 12

● A second method is to look at the positions on the

Periodic Table of elements that are reacting.

2 3 4 5 6 7 8

Atoms that are far apart on the Periodic Table have a bigger difference in electronegativity and

tend to form Ionic Bonds

Atoms that are close together on the Periodic Table have a smaller difference in electronegativity and

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Determine the

Type of Chemical Bond

7 of 12

● The most reliable method is to actually calculate the

difference between the electronegativities.

2 3 4 5 6 7 8

● If the difference is 1.7 or more, it is Ionic.

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Determine the

Type of Chemical Bond

8 of 12

● Is Sodium Chloride (NaCl) Ionic or Covalent?

Na

+1

Cl

-1

Electronegativity

Chlorine = 2.8 Sodium = 1.0 Difference = 1.8

2 3 4 5 6 7 8

If the difference is 1.7 or more, it is an Ionic Bond.

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Determine the

Type of Chemical Bond

9 of 12

● Is Water (H

₂

O) Ionic or Covalent?

Electronegativity

Oxygen = 3.5 Hydrogen = 2.1 Difference = 1.4

2 3 4 5 6 7 8

If the difference is less than 1.7, it is a Covalent Bond.

Water (at 1.4) is Covalent.

O

H

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Determine the

Type of Chemical Bond

10 of 12

● Use the 1.7 Rule to determine if the molecules are

Ionic or Covalent.

2 3 4 5 6 7 8

NaF

SO

2

Fluorine = 4.0 Sodium = 1.0 Difference = 3.0

Ionic

Oxygen = 3.5 Sulfur = 2.4 Difference = 1.1

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Determine the

Type of Chemical Bond

11 of 12

● Use the 1.7 Rule to determine if the molecules are

Ionic or Covalent.

2 3 4 5 6 7 8

CaBr

2 _{Calcium = 1.0}Bromine = 2.7

H

2

Difference = 1.7

Hydrogen = 2.1 Hydrogen = 2.1 Difference = 0.0

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Determine the

Type of Chemical Bond

12 of 12

● List the differences between Ionic and Covalent

Bonds on the table below.

Ionic

Covalent

Exchange Electrons Share Electrons

Different Electronegativities Similar Electronegativities

Formed from Ions No Ions Involved

Electrolyte Non-Electrolyte

Conducts Electricity Does Not Conduct Electricity

Forms Ions in Water Stays as Molecules in Water

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