1
Topic 3 – National Chemistry Summary Notes
Bonding, Structure and Properties of Substances
Covalent Bonds
Most atoms do not exist as single atoms. They are mainly found combined with
other atoms in compounds.
They are held together in these compounds by chemical bonds. Atoms prefer to
be bonded to each other because this gives them a stable electron
arrangement.
A stable electron arrangement is achieved when the outer energy level (electron
level or electron shell) is full. The noble gases all have full outer energy levels
and are very stable. Remember you were told in the last topic that chemical
reactions are all to do with outer electrons? If the outer energy level is full
then the atom is un-reactive.
Most atoms need 8 electrons in the outer energy level to be full (the exceptions
are Hydrogen and Helium who only need 2). This is sometimes described as a
stable octet.
You can use target diagrams, like the one shown below for sodium, to show the
arrangement of electrons in an atom:
The electron arrangement for sodium is : 2,8,1
You will need to be able to draw similar diagrams for the first 20 elements.
LI 1
2
In this first part of the topic we are going to look at how two non-metal atoms
form bonds called covalent bonds. When two or more non-metal atoms are
joined by covalent bonds we call them molecules.To help us we are going to use
simple diagrams showing just the outer electrons of atoms rather than target
diagrams.
When non-metal atoms (like the example for hydrogen shown below) form
covalent bonds they do this by overlapping their outer energy levels in order to
share one pair of electrons.
This gives each atom a full outer energy level (electron level).
Note: In some textbooks energy levels are also called electron shells.
Covalent Bonds in Elements
There are a group of 7 elements in the Periodic Table that exist as two atoms
joined together by covalent bonds in a Diatomic Molecule. They are called The
Diatomic Elements and are shown below:
Iodine - In
Oxygen - October
Chlorine - Children
Bromine - Buy
Nitrogen - Nasty
Hydrogen - Halloween
Fluorine - Food
LI 2 E
A diatomic molecule is a
molecule containing two
non-metal atoms joined
together by covalent bonds.
A diatomic element is a
when a diatomic molecule is
formed from two identical
atoms
3
When you look at the outer energy levels of each element you can see how many
electrons their atoms would have to share in order to fill up their outer energy
level.
Hydrogen has room for one electron to fill up its’ outer energy
level, so it will share one electron pair.
Chlorine, Bromine, Iodine and Fluorine all have 7 electrons in the
outer energy level and therefore have room for one more, so they will also share
one electron pair.
Every time a pair of electrons is shared a covalent bond is made. The bonds
between these atoms are described as single bonds because only one pair of
electrons are shared:
The single covalent bond can also be represented using element symbols and
lines:
Cl-Cl, H-H, Br-Br, I-I, F-F one dash represents a single bond.
4
Double and Triple Covalent Bonds
Oxygen atoms have room to share two electron pairs. This means that a double
covalent bond is formed between two oxygen atoms. You can draw an overlapping
orbital diagram to show this sharing:
O=O
Nitrogen atoms have 5 outer energy level electrons so they need to share 3
pairs of electrons in order to fill the level. This requires a triple bond:
How would you show two nitrogen atoms joining up to form a diatomic molecule
using symbols and dashes?
Answer:
5
Covalent Bonds in Compounds
Note: You should remember from earlier work that a compound is a substance
made up of two or more different elements joined together.
As a general rule if a compound is made up of non-metal elements only then it
will have covalent bonding. Examples:
Methane- CH4
or
Ammonia – NH3
or
Water – H2O
or
Note: Covalent bonds are very strong. A covalent bond is when the two
positive nuclei involved in the bond are held tightly together by their
common attraction for the shared pair of electrons.
LI 3
E
6
Shapes of Covalent Molecules
Molecules are three dimensional structures with specific shapes. We need to
know the shapes of some of the simple two element molecules:
Molecule Arrangement of Atoms Shape
Hydrogen oxide
(water)
Bent (and flat or planar)
Hydrogen fluoride
Linear
Nitrogen hydride
(ammonia)
Pyramidal
Carbon hydride
(methane)
Tetrahedral
The arrangement of atoms in methane and ammonia are trying to represent the
real shape using a perspective diagram.
LI 4 E
2
7
Discrete Molecular Covalent versus Giant Network Structures
Most covalent substances exist as molecules with a definite number of atoms
present eg methane CH4. These are called discrete covalent molecular
structures. Some covalent substances, however, take the form of huge
repeating structures, called giant covalent networks.
Example 1 : Diamond – a covalent network element
Diamond contains only carbon atoms joined together by covalent bonds.
The carbon atoms are held very tightly giving Diamond great hardness and a
very high melting point.
Example 2 : Quartz – a covalent network compound
Quartz contains atoms of silicon and oxygen in the ratio of one silicon to two
oxygen. Its’ formula is SiO2 and it’s proper name is silicon dioxide.
The arrangement of atoms in quartz makes the structure very rigid and gives it
a high melting and boiling point. To melt a covalent network you have to break
that network of covalent bonds which are very strong!!!
LI 5 E
8
Ionic Bonds
Ionic bonds are a special bond that forms between a metal atom and a non-metal
atom.
The bonds form when a metal atom transfers electrons(s) to a non-metal atom.
This means the atoms are able to achieve the same stable, full outer energy
level arrangement as the nearest noble gas.
Ions
Atoms which have lost or gained electrons become charged particles called
ions.
How do you know what kind of ion an atom will become?
Atoms in groups 1,2 and 3 all LOSE electrons to become positively
charged.
Atoms in groups 5,6 and 7 all GAIN electrons to become negatively
charged.
Atoms in group 4 do not usually form ions!!
In General
Metals lose electrons forming positive ions.
Non-metals gain electrons forming negative ions.
Activity!! Complete the following:
Group
Number
1 2 3 4 5 6 7 0
Charge
on ion
+
example
Na+
LI 6
9
How Does An Ionic Bond Form?
Your teacher will show you how to use target diagrams to illustrate how metal
atoms react with non-metal atoms to form ionic bonds.
Atoms are electrically neutral (no charge) but electrons are negatively charged.
When metal atoms lose electrons becoming positively charged and non-metal
atoms gain electrons to become negatively charged these oppositely charged
ions attract each other forming a strong bond.
This is often described as an electrostatic force of attraction between
positive and negative ions.
LI 7
E
The sodium atom
gives away its’ outer
electron to the
chlorine atom.
The sodium atom is
now a positive ion
with a full outer
energy level.
The chlorine atom is
now a negative ion
with a full outer
energy level
10
Ionic Lattice Structures
A tiny amount of an ionic solid compound contains millions of ions. These millions
of ions are held together by ionic bonds, that is :
“The electrostatic force of attraction between positive and negative ions”
These forces are so great that the ions group together with positive ions
surrounding negative ions and negative ions surrounding positive ions.
This produces a regular, geometric structure called an Ionic Lattice.
The lattice structure of a sodium chloride crystal.
In an ionic lattice the ions are held tight and cannot move.
LI 8 E
Ionic compounds form lattice structures
of oppositely charged particles.
11
Melting Points of Substances
The bonding present in a substance has a big effect on a number of different
properties. Melting points (and boiling points) are an important property. How
easy it is to melt or boil a substance determines what the physical state will be
at room temperature. ( room temperature is aprox 25oC )
Physical States of Ionic Compounds
Have a look at page 6 of the Data Booklet. Find the melting points and boiling
points of the following substances and then decide what state they will be at
room temperature:
As you can see all these ionic compounds are solids at room temperature. In fact
all ionic compounds are solids at room temperature.
The high melting and boiling points are due to the strong ionic bonds
which need to be broken. A lot of heat energy has to be put in to allow
the ions to break free from each other and move around. Therefore all
ionic substances are solids at room temperature.
Name of Compound Melting Point oC Boiling Point oC Physical State at
Room Temperature
Barium chloride
Calcium oxide
Magnesium chloride
Potassium iodide
LI 9
E
12
Physical States of Covalent Compounds
Most covalent compounds have low melting points and can be found as gases,
liquids and soft solids at room temperature.
Name of Compound Melting Point oC Boiling Point oC Physical State at
Room Temperature
Water (hydrogen
oxide)
Methane (carbon
hydride)
Carbon dioxide
Silicon dioxide **
** Discrete Covalent Molecular compounds all have low melting and
boiling points but The Giant Covalent Network compounds like
silicon dioxide have very high melting points because very strong
covalent bonds have to be broken in order to melt the compound.
To melt a Discrete Molecular Covalent compound there are only
weak forces of attraction between molecules that are easy to
break.
Comparing Electrical Conductivity of Substances
A substance can conduct electricity if it can allow some kind of charged
particles to flow through it.
An electric current (electricity) is a flow of charged particles.
From practical work in science you may remember that all metals conduct
electricity but what about non-metal elements, ionic compounds and covalent
compounds?
Using a simple electrical circuit with a light bulb to indicate whether the
substance conducts electricity you will test a range of solids first and fill in the
following table:
E
LI 10
13
Electrical Conductivity of Solid Substances:
Name of
Substance
Metallic
Bonding
(please tick)
Covalent
Bonding
(please tick)
Ionic
Bonding
(please tick)
Does it Conduct
Yes/No
Sulphur
Copper
Iron
Tin
Carbon
(graphite)
Wax (contains
carbon and
hydrogen)
Sodium
Chloride
Lead bromide
Sugar (carbon,
hydrogen and
oxygen)
The solid to be tested has
to be attached in series in
the circuit using wires and
crocodile clips.
14
Summary of Conductivity of Solids
1. All the metals do conduct electricity.
2. The only non-metal that can conduct is carbon in the form of graphite.
3. Ionic compound do not conduct as solids.
4. Covalent compounds do not conduct as solids.
Electrical Conductivity of Liquid Substances
A range of liquids were tested. These included solutions of both ionic and
covalent substances, some pure liquids and a teacher demonstration of two
“melts”. “Melts” simply means solids that have to be heated first to melt them
before they were tested using a simple circuit. They can also be described as
“molten”.
Substance Ionic Bonding
(please tick)
Covalent
Bonding
(please tick)
Does it
Conduct
Yes/No sugar solution
(carbohydrate)
sodium chloride
solution
copper chloride
solution
hexane
(carbon hydride)
nickel sulphate
solution
molten wax
molten lead
bromide
ethanol
LI11
15
Summary of Conductivity of Liquids
1. Ionic substances DO conduct when in solution.
2. Ionic substances DO conduct when molten.
3. Covalent solutions, pure liquids and melts do NOT conduct.
Problems With Testing Conductivity of a Melt!!
Testing conductivity of a melt can often give false positive results. After
watching the teacher demonstration, draw a labelled diagram of the apparatus
used and clearly indicate and explain the possible source of error.
Explanation:
Circuit to Test Conductivity of a Melt:
16
Explanation of Conductivity of Substances
Why do covalent substances not conduct in any state or in solution?
Covalent substances do not contain any charged particles. They are made up of
neutral atoms joined together. Electricity cannot flow unless there are charged
particles able to flow (move).
Why do ionic substances only conduct in solution or melt?
Ionic substances do contain charged particles but they are not able to move in a
solid, therefore ionic solids cannot conduct.
Ionic solutions do conduct because dissolving the substance has broken down
the ionic lattice. The ions are now free to move.
Ionic melts conduct because the addition of heat energy has allowed the ions to
vibrate more, overcoming the electrostatic forces holding them together and
shake free of each other. The ions are free to move.
Why do metals conduct in any state?
We will be looking at metals in more detail later in the course but they have a
special structure where their outer energy level electrons are loosely shared
between atoms in a metallic lattice. These electrons are easy to move and since
they are charged particles their movement allows electricity to flow through
easily. Some text books describe these electrons as a “sea of delocalised
electrons”.
Metallic lattice
LI12
E
17
Summary of Physical State and Conductivity of Structures
Type of Structure Physical
State at
Room
Temperature
Conductivity
Liquid Solid
Ionic
High melting
point solids
Discrete Covalent
Molecular
Low melting
point, gas,
liquids and soft
solids
Giant Covalent
Network
Very high
melting point
solids
Metallic
Wide range of
melting points
but all solids
except mercury
18
Topic 3 – Pupil Self-evaluation Note: if the success criteria is in italics then this is extension level work.
Learning Intention
Success Criteria
“ I can:” 1 I am going to find out how
atoms can join together in a
covalent bond.
draw target diagrams to show electron arrangements of atoms.
draw outer electron diagrams to show overlapping energy levels.
state that a covalent bond is a shared pair of electrons.
state that atoms share electrons to get a full, stable, outer energy
level.
state that non-metal atoms form covalent bonds.
state that when two or more non-metal atoms are joined together by
covalent bonds then a molecule if formed.
2 I am going to find out about
the diatomic molecules.
name the 7 diatomic elements describe what “diatomic element” and “diatomic molecule” means. draw diagrams to represent single, double and triple covalent bonds.
3 I am going to find out about
how compounds are held
together by covalent bonds.
draw simple diagrams of two element compounds.
state that covalent bonds are very strong.
describe what holds a covalent bond together. 4 I am going to find out about
the shapes of simple
covalent molecules.
describe the shapes of water, methane, ammonia and hydrogen fluoride molecules.
draw perspective diagrams of these molecules. 5 I am going to find out about
different covalent
structures.
describe what a Giant Network Covalent and a Discrete Molecular Covalent structure is like.
give an example of a compound that exists as Giant Network Covalent state that melting point and boiling point are related to structure.
6 I am going to find out how state that ionic bonds form between a non-metal atom and a metal
E
19
atoms can change in order
to join together in ionic
bonds.
atom.
state that atoms become ions by losing or gaining electrons.
state what charge an ion will have from groups 1 to 0.
7 I am going to find out how
an ionic bond forms.
draw diagrams to explain how an ionic bond forms.
explain that an electrostatic force of attraction holds the ionic bond together.
8 I am going to be able to
describe an ionic lattice.
explain that an ionic solid has a regular, geometric structure called an ionic lattice and describe it.
9 I am going to be able to use
the data booklet to find out
the melting and boiling
points of substances and
relate this to physical state
and type of bonding.
use the data booklet to find the melting and boiling points of selected
substances.
state that all ionic substances are solids at room temperature.
explain why ionic substances are all solids.
10 I am going to find out how
electrical conductivity of a
solid substance relates to
its’ bonding.
describe how I could test a solid substance for electrical conductivity.
state the conductivity of metal solids.
state the conductivity of covalent or ionic solids.
11 I am going to find out how
electrical conductivity of a
liquid substance is related
to the bonding.
state the conductivity of ionic substances when in solution.
state the conductivity of covalent substances as solution or as liquid.
state that ionic melts conduct but covalent melts do not conduct.
explain a problem that can occur when testing melts for conductivity.
12 I am going to be able to
describe why a substance
conducts.
explain that for an ionic substance to conduct then the ions need to be free to move.
describe conductivity in a metal as a flow of electrons.