1 of 43 © Boardworks Ltd 2009€¦ · 12 of 43 © Boardworks Ltd 2009 Strength of metallic bonding: ion size Element Ionic radius (nm) Melting point (K) Li Na K Rb Cs 0.076 0.102

1 of 43 © Boardworks Ltd 2009


Types of bonding: OVERVIEW

an ionic bond

occurs between

a metal and

non-metal atom

(e.g. NaCl)

a covalent

bond occurs

between two

non-metal atoms

(e.g. I2, CH4)

a metallic bond

occurs between

atoms in a metal

(e.g. Cu)

There are three types of bond that can occur between atoms:


Ionic bonding


Charge on the ions

Metals lose electrons to form positive ions (cations) while

non-metals gain electrons to form negative ions (anions).

The elements in groups 4 and 8 (also called group 0) do

not gain or lose electrons to form ionic compounds.

1 2 3 4 5 6 7 8/0

1+ 2+ 3+ N/A 3- 2- 1- N/A

Na+ Al3+ N/AN/A N3- O2- F-Mg2+

Group

Charge

Example

The number of electrons gained or lost by an atom is

related to the group in which the element is found.


Representing ionic bonding


Covalent bonding


Metallic bonding


Strength of metallic bonding: ion charge

The strength of metallic bonding depends on two factors:

1. the charge on the metal ions

1. The charge on the metal ions

The greater the charge on the metal ions, the greater the

attraction between the ions and the delocalized electrons,

and the stronger the metallic bonds. A higher melting

point is evidence of stronger bonds in the substance.

2. the size of the metal ions.

Na Mg Al

1+ 2+ 3+

371 923 933

Element

Charge on ion

Melting point (K)


Strength of metallic bonding: ion size

Element

Ionic radius

(nm)

Melting

point (K)

Li Na K Rb Cs

0.076 0.102 0.138 0.152 0.167

454 371 337 312 302

2. The size of the metal ions

The smaller the metal ion, the closer the positive

nucleus is to the delocalized electrons. This means

there is a greater attraction between the two, which

creates a stronger metallic bond.


Types of bonding

© Boardworks Ltd 200714 of 50



What are bonds?

Compounds are formed when different elements chemically

react and form bonds with each other.

Some compounds, like water (H20),

have small, simple structures with

just a few atoms bonded together.

Others compounds, like DNA, have large, complex structures

containing thousands or even millions of bonded atoms.

Elements are the simplest substances that exist on Earth.

Each element is made up of just one type of atom, usually

joined to other atoms of the same element by bonds.

This forms molecules such as chlorine (Cl2).


Why do atoms form bonds?

Bonds involve the electrons in the outer shells of atoms.

Filled electron shells are very stable, and unreactive.

(do not form bonds)

1st shell holds a maximum of 2 electrons

2nd shell holds a maximum

of 8 electrons

3rd shell holds a

maximum of 8 electrons

Each shell has a maximum number of electrons that it can

hold. Electrons fill the shells nearest the nucleus first.


Why do atoms form bonds?

The atoms of noble gases have completely

full outer shells and so are stable.

The atoms of other elements have

incomplete outer electron shells

and so are unstable.

This makes the noble gases very unreactive

and so they do not usually form bonds.

By forming bonds, the atoms of these

elements are able to have filled outer

shells and become stable.


What are the types of bonding?

Different types of bonds are formed depending on the types

of atoms involved:

All bonds involve electrons and all bonding involve changes

to the number of electrons in the outer shells of atoms.

ionic bonding

– occurs between metal and non-metal atoms.

covalent bonding

– occurs between non-metals atoms only.

metallic bonding

– occurs between metal atoms only.

How do you think electrons are involved in ionic bonding?



From atoms to ions

How can reactive metal atoms become stable positive ions?


How do atoms form ions?

An ion is an atom or group of atoms that has

an electrical charge, either positive or negative.

Atoms have an equal number of protons and electrons and

so do not have an overall charge.

Atoms with incomplete outer electron

shells are unstable. By either gaining or

losing electrons, atoms can obtain full

outer electron shells and become stable.

When this happens, atoms have an unequal number of

protons and electrons and so have an overall charge.

This is how atoms become ions.

How does an atom become a positive or negative ion?


Positive and negative ions?

The electron configuration of an atom shows how many

electrons it must lose or gain to have a filled outer shell.

An atom that loses electrons has more protons

than electrons and so has a positive overall charge.

This is called a positive ion.

An atom that gains electrons has more electrons

than protons and so has a negative overall charge.

This is called a negative ion.

Atoms with a nearly empty outer shell, will lose electrons

to obtain a full outer shell.

Atoms with a nearly full outer shell, will gain electrons

to obtain a full outer shell.


How do atoms form positive ions?

An atom that loses one or more electrons

forms a positive ion.

Positive ions have a small ‘+’ symbol and a number by this

to indicate how many electrons have been lost.

This number is usually the same as the number of electrons

in the atom’s outer shell. For example:

Metal atoms, such as sodium, magnesium

and iron, form positive ions.

lithium ion [ 2 ]lithium atom 2.1

aluminium atom 2.8.3 aluminium ion [ 2.8 ]

magnesium atom 2.8.2 magnesium ion [ 2.8 ] = Mg2+

= Li+

= Al3+


How is a sodium ion formed?

2.8.1

(partially full outer shell)

11 protons = +11

11 electrons = -11

Total charge = 0

Sodium atom:

11 protons = +11

10 electrons = -10

Total charge = +1

Sodium ion:

loses

1 electron

+

[2.8]

(full outer shell)

Na Na


How is a magnesium ion formed?

2.8.2


12 protons = +12

12 electrons = -12

Total charge = 0

Magnesium atom:

12 protons = +12

10 electrons = -10

Total charge = +2

Magnesium ion:

[2.8]2+

(full outer shell)

2+

Mg Mg

loses

2 electrons


How do atoms form negative ions?

Negative ions have a small ‘-’ symbol and a number by this

to indicate how many electrons have been gained to fill their

outer shell. For example:

An atom that gains one or more electrons

forms a negative ion.

The name of the ion is slightly different to the atom’s name.

Non-metal atoms, such as chlorine,

oxygen and nitrogen, form negative ions.

chlorine atom chloride ion [ 2.8.8 ] = Cl-

oxygen atom oxide ion [ 2 ]

nitrogen atom nitride ion [ 2 ]

2.8.7

2.6

2.5 = N3-

= O2-


How is a fluoride ion formed?

2.7


9 protons = +9

9 electrons = -9

Total charge = 0

Fluorine atom:

9 protons = +9

10 electrons = -10

Total charge = -1

Fluoride ion:

[2.8]-

(full outer shell)

-

F F

gains 1

electron


[2.8.8]2-

(full outer shell)

2.8.6


How is a sulfide ion formed?

16 protons = +16

16 electrons = -16

Total charge = 0

Sulfur atom:

16 protons = +16

18 electrons = -18

Total charge = -2

Sulfide ion:

2-

S S

gains 2

electrons


Building an ion


Calculating ion charges

What is the charge on the ion of each element?

+2 +2+1 -3 -1

2.8.8.2 1 2.8.5 2.7 2.2

calcium hydrogen phosphorus fluorine berylliumElement

Electron shells

Charge on ion


Electron configuration of ions

The number of protons and neutrons however, remain

different for each element.

This means that each ion has different properties.

For example, oxygen and fluorine both gain electrons to

become negative ions. The resulting ions have the same

electron configuration:

F

-2-

O

When different elements gain or lose electrons to complete

their outer electron shell, they can create ions that have the

same electron configuration.


Comparing electron configurations


What is a Polyatomic ion?

Ions can be made up of a single atom or a group of atoms. An

ion made up of a group of atoms is called a Polyatomic ion.

hydroxide

sulfate

nitrate

carbonate

ammonium

-1

-2

-1

-2

+1

Ion Formula Charge Atoms present

NH4+

CO32-

NO3-

SO42-

OH- O

O OO

O

O O O

OOO

H

H H HH

C

N

N

S

What atoms are present in the following compound ions?


Comparing positive and negative ions



Ions and attraction

Why do some ions attract while others repel?


What is ionic bonding?

Compounds that contain ions are called ionic

compounds. These compounds are usually

formed by a reaction between a metal and a

non-metal.

Why do these substances react together and form bonds?

The metal and non-metal atoms have incomplete outer

electron shells and so are unstable.

Electrons are transferred from each metal atom to each

non-metal atom. The metal and the non-metal atoms form

ions with completely full outer shells and become stable.

The positive and negative ions are strongly attracted to each

other. This electrostatic attraction is called ionic bonding.


How are ionic bonds formed?

Sodium chloride is an ionic compound formed by the reaction

between the metal sodium and the non-metal chlorine.

Sodium has 1 electron

in its outer shell.

Chlorine has 7 electrons

in its outer shell.

2.8.7 [2.8.8]-

+

Cl Cl

-

2.8.1 [2.8]+

Na Na

By losing this electron,

it has a filled outer shell

and forms a positive ion.

By gaining an electron

from sodium, it has a

filled outer shell and

forms a negative ion.


How are ionic bonds formed?

The positive sodium ions and the negative chloride ions

are strongly attracted to each other.

-

Cl

It is this electrostatic attraction that forms ionic bonds in

sodium chloride and other ionic compounds.

Na

+


Formation of an ionic bond


Ions and ionic bonding – summary



What is the ratio of ions?

Sodium chloride (NaCl) and magnesium oxide (MgO) are

simple ionic compounds.

Both compounds have a 1:1 ratio of metal ions to non-metal

ions, which is shown by the formula of each compound.

Na Cl Na+ Cl-1 electron

Mg Mg2+O O2-2 electrons

In each compound, the metal needs to lose the same

number of electrons that the non-metal needs to gain.


What is the ratio of ions in sodium oxide?

What is the ratio of ions needed to make sodium oxide?

Sodium (2.8.1) needs to lose 1 electron to form a sodium ion

but oxygen (2.6) must gain 2 electrons to form an oxide ion .

Na

Na

O O2-

Na+

Na+

1 electron from each Na atom

Two sodium atoms are required for each oxygen atom

and so the ratio of sodium ions to oxide ions is 2:1.

From this ratio, the formula of sodium oxide is Na2O.


What is the ratio of ions in magnesium chloride?

What is the ratio of ions needed for magnesium chloride?

Magnesium (2.8.2) needs to lose 2 electrons to form a

magnesium ion but chlorine (2.8.7) needs to gain 1 electron

to form an chloride ion.

Mg

Cl

Mg2+

Cl

Cl-

Cl-

1 electron for each Cl atom

Two chlorine atoms are required for each magnesium atom

and so the ratio of magnesium ions to chloride ions is 1:2.

From this ratio, the formula of magnesium chloride is MgCl2.


How to write the formula of an ionic compound

To work out the formula of an ionic compound:

1. Write down the symbol for each element

– the metal is always written first.

2. Calculate the charge for each type of ion.

3. Balance the number of ions so that the positive

and negative charges are balanced and equal zero.

This gives the ratio of ions.

4. Use the ratio to write down the formula of the ionic

compound.

The formula of a compound uses chemical symbols and

numbers to show the ratio of atoms of each element present.


Formula of aluminium bromide

What is the formula of aluminium bromide?

Al Br

+3 -1

AlBr3

3 bromide ions are needed

for each aluminium ion

1 : 3

Symbol

Balance the

number of ions

Ion charge

Ratio of ions

Formula

Al

Br

Al3+

Br

Br

Br-

Br-

Br-

1 electron for each bromine

atom


Formula of aluminium oxide

What is the formula of aluminium oxide?

Al O

+3 -2

Al2O3

2 aluminium ions are

needed for 3 oxide ions

2 : 3

Symbol

Balance the

number of ions

Ion charge

Ratio of ions

Formula

Al Al3+

AlAl3+

O O2-

O O2-

O O2-

2 electrons for each oxygen atom


More ionic formulae

What are the formulae of all the possible ionic compounds

from combinations of these metals and non-metals.

metals

non-

metalsLi Ca Na Mg Al K

F

O

N

Br

S

Cl

LiF CaF2 NaF MgF2 AlF3 KF

Li2O CaO Na2O MgO Al2O3 K2O

Li3N Ca3N2 Na3N Mg3N2 AlN K3N

LiBr CaBr2 NaBr MgBr2 AlBr3 KBr

Li2S CaS Na2S MgS Al2S3 K2S

LiCl CaCl2 NaCl MgCl2 AlCl3 KCl


What is the ionic formula?



What is an ionic lattice?

In an ionic compound, when any macroscopic sample ions

react together, countless atoms will transfer electrons to form

countless oppositely charged ions.

These oppositely charged species being produced in close

proximity are drawn together into an giant, 3D, ordered, solid,

three-dimensional array of cations and anions called a

crystal lattice

The structure of the ionic lattice

affects the properties of the

ionic compound.

The smallest whole number

cation-to-anion ratio in this

structure represents the

chemical formula for the

ionic compound


Why do ionic compounds form crystals?

Ionic compounds such as sodium chloride, form crystals, with

a cubic shape. This is due to the structure of the ionic lattice.

All ionic compounds form lattices and crystals when solid.


Heating ionic compounds

Why are ionic compounds solid at room temperature and

have high melting points and boiling points?

Larger ionic charges produce stronger ionic bonds and so

much more heat is required to break the ionic bonds in

magnesium oxide than in sodium chloride.

sodium chloride

magnesium oxide

CompoundIon

charges

Melting

point (oC)

Boiling

point (oC)

1+ and 1-

2+ and 2-

801 1,413

2,852 3,600

strong

ionic bonds

hold ions

together

Ionic bonds are strong and a

lot of heat is needed to break

them.


Do ionic compounds conduct electricity?


How can ionic compounds conduct electricity?

When liquid (molten), the ions

can break free of the lattice

and are able to move. The ions

are charged particles and so

can carry an electric current.

As solids, ionic compounds

cannot conduct electricity

because their ions are

bonded together in the lattice.

ions in solid state

cannot move

ions in molten state

can move and conduct

electricity

Ionic compounds are usually soluble in water because water

molecules have a slight electrical charge and so can attract

the ions away from the lattice. When dissolved, the ions are

free to move and can carry an electric current.


Why are ionic compounds brittle?

Ionic compounds are brittle – they shatter when they are hit.

When the lattice is hit, a layer

of ions is shifted so that ions

with the same charges are

lined up together.

These like charges repel

each other and so split

the ionic lattice causing

it to shatter.

repulsion

force

Ionic bonds are strong, so why does this happen?


Electronegativity is defined as the ability of an atom in a

molecule to attract electrons to itself.

Electronegativity is a function of two

properties of isolated atoms:

• The atom’s ionization energy (how strongly

an atom holds onto its own electrons)

• The atom’s electron affinity (how strongly

the atom attracts other electrons)

For example, an element which has a large (negative)

electron affinity and a high ionization (always endothermic, or

positive for neutral atoms)…

…Will attract electrons from other atoms and resist having

electrons attracted away

We say these atoms are “highly electronegative”

Electronegativity


Electronegativity trends: across a period

Electronegativity increases across a period because:

1. The atomic radius decreases.

2. The charge on the nucleus increases without

significant extra shielding. New electrons do not

contribute much to shielding because they are added

to the same principal energy level across the period.


Electronegativity trends: down a group

2. Although the charge on the nucleus increases,

shielding also increases significantly. This is

because electrons added down the group fill new

principal energy levels.

Electronegativity decreases down a group because:

1. The atomic radius increases.


Electronegativity is dictated by:

The number of protons in the nucleus

• across a period you are increasing the number of protons, but filling electrons in the same Bohr quantized energy level. You are only filling sub-shells, so electronegativity increases from left to right

The distance from the nucleus

• down groups, you are placing electrons into new quantizedenergy levels, so moving further away from the attractivepower of the nucleus. Outer shell becomes further awayfrom the nucleus.

The amount of shielding by the inner electrons

• level of shielding upon bonding electrons increases down groups, and adds to the reduction in electronegativity. Shielding is caused by repulsion of electrons for each other.


Ions & Atomic Radius

Negative Ions

As extra electrons are added to a neutral atom

(eg O to make O-2) the ion has the same positive

nuclear charge (due to protons), and an

increased number of electrons surrounding the

nucleus.

• The electrostatic repulsion increases

• The volume occupied by the electrons

increases

NEGATIVE IONS are LARGER

than the corresponding neutral atom


Ions & Atomic Radius

Positive Ions

As electrons are removed from a neutral atom

(eg Mg to make Mg+2) the ion has the same

positive nuclear charge (due to protons), and a

decreased number of negative electrons

surrounding the nucleus.

• The electrostatic repulsion decreases

• The volume occupied by the electrons

decreases

POSITIVE IONS are SMALLER

than the corresponding neutral atom


Electronegativity & Ionic Bonds

• Formed between two atoms with large

differences in their ionization energies

and electronegativity's

• An electronegativity difference of

greater than 1.6 can be classified as an

ionic bond

• In this case we can essentially say

electrons are transferred from one

atom to another


Summary of Ionic Bonding




Worksheet KEY


Worksheet Key


True or false?



Glossary

bond – A strong force that joins atoms or ions together in molecules and giant lattices.

compound ion – An ion made up of a group of atoms, rather than one single atom.

ionic bond – The electrostatic force of attraction between oppositely charged ions.

ionic compound – A compound made up of ions.

ionic lattice – A giant 3D structure of closely packed, oppositely-charged ions.

negative ion – An atom or group of atoms that has gained electrons and so has a negative charge.

noble gas – An element that has a full outer electron shell and so is very stable and unreactive.

positive ion – An atom or group of atoms that has lost electrons and so has a positive charge.


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1 of 43 © Boardworks Ltd 2009€¦ · 12 of 43 © Boardworks Ltd 2009 Strength of metallic bonding: ion size Element Ionic radius (nm) Melting point (K) Li Na K Rb Cs 0.076 0.102