Download Molecules and formulae - Deans Community High School

West Lothian Council
Lorna C. Webster
1
Atomic numbers
Read
You already know that elements are made up of tiny particles
called atoms. The atoms of different elements are not the
same. For example, iron atoms are a different size and weight
from gold atoms.
Every element has been given a special number called the
atomic number. The atomic number can be found on a periodic
table for example on pages 1 and 8 of the data book.
Look up a data book page 8. You can see from the key what
each piece of information means.
key
Atomic number
Name of element
symbol
4
Beryllium
Be
You can see from the key that the element called berylllium has
an atomic number of 4.
Look up the box for phosphorus and you will see that it has an
atomic number of 15.
key
Atomic number
Name of element
symbol
15
Phosphorus
P
Write a heading and try the work below.
1. What are the tiny particles called that elements are made
from?
Lorna C. Webster
2
2. Are iron atoms the same as gold atoms? Explain your answer.
3. What pages in the data book would you look at to find the
atomic number for an element?
4. Copy the boxes above to show how you can work out the
atomic number for beryllium.
5. Copy the tables below and fill them in with 20 elements of
your choice.
Element
name
atomic
number
Element
name
atomic
number
Ask your teacher if your should do the sheet called “data book
exercise”.
Lorna C. Webster
3
Molecules and bonds
Read
Some substances are made up of molecules. A molecule is a
group of atoms that are joined together. The join between the
atoms is a kind of chemical glue called a bond.
A molecule of hydrogen would look like:
H
atom
H
bond
atom
A molecule of water would look like:
O
H
H
The bond between the atoms are very strong.
Write a heading and try the work below.
1. Explain what a molecule is?
2. Explain what a bond is?
3. Draw a molecule of water and label the atoms and the bonds.
4. Are the bonds between the atoms weak or strong?
Lorna C. Webster
4
Read
A beaker of water does not only contain one molecule it contains
billions of molecules. There is chemical glue holding the
different molecules together. This glue is also called bonds
but the bonds between molecules are only weak.
weak bond
O
atom
H
H
O
H
atom
H
O
atom
H
H
Strong bond
Continue your work by answering the questions below.
5. A beaker of water contains many molecules of water. What
is the chemical glue called that holds them together.
6. Copy the diagram above to show the atoms, strong bonds and
weak bonds in a beaker of water.
7. Copy and complete the sentences below.
A molecule is a small group of a______ joined together
by b_______. The bonds between atoms are s_______
but the bonds between molecules are w______.
Lorna C. Webster
5
Substances and ions
Read
Not all substances are made from molecules. Some substances
are made from particles called ions. An ion is a charged
particle. An ion can be positively or negatively charged.
Substances that are made from ions are called ionic compounds
and contain both positive and negative ions joined together by
strong bonds.
Your teacher may show you a model of the ions in an ionic
compound.
ions
Strong bonds
Write a heading and try the work below.
1. What is an ion?
2. What type of charge can an ion have?
3. What type of compounds are made from ions?
4. Are the bonds between ions strong or weak?
5. Collect a copy of the ionic compound diagram and choose two
colours to colour in the positive and negative ions before you
stick it into your jotter.
Lorna C. Webster
6
Read
Substances can be solids, liquids or gases at room temperature.
In the next experiment you will find out a rule for the state of
ionic and molecular compounds.
6. Copy the table below and fill it in using display 4.
Substance
Molecular or
ionic
Solid, liquid
or gas
salt
water
oxygen
polystyrene
fertiliser
petrol
sulphur
baking soda
methane
epsom salts
7. Copy and complete the rule below.
Ionic substances are always s________ at room
temperature but molecular substances can be s______,
l________ or g_____ at room temperature.
Lorna C. Webster
7
What about conducting electricity?
Read
You should already know from S1 and S2 that metals can
conduct electricity but what about whether ionic substances or
molecular substances conduct electricity. The aim of the next
experiment (1.39) is to find out if ionic substances or
molecular substances can conduct electricity.
The experiment we will use will be set up as shown in the
diagram below.
power
pack
4V AC
leads
A
ammeter
carbon
electrodes
substance being
tested
Write a heading and try the work below.
1. What is the aim of the experiment (what are you trying to
find out?
2. What do you think will happen?
3. Draw a diagram of the experiment before you start.
4. Carry out the experiment by going to each station and
connecting up the circuit. Then design a table for your
results.
5. Now write a conclusion (what did you find out about the ionic
and molecular substances?).
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8
What about melting and boiling?
Read
Ionic and molecular substances are not only different in
conduction of electricity, they are also different in how they
melt or boil. The stronger the bonds in a substance, the
more heat is needed to make the substance melt.
Write a heading and try the work below.
1. John has two chemicals labelled A and B. He knows that the
bonds in substance A are stronger than the bonds in
substance B. Which one will he need to heat up more to make
it melt?
2. Copy the table below and complete it using the melting and
boiling point chart.
Name of
substance
Phenol
molecular
or ionic?
melting point boiling point
in C
in C
Calcium oxide
Naphthalene
Sodium bromide
3. Which type of substance always has high melting points and
boiling points?
4. Collect and complete the types of substances summary
sheet and stick it into your jotter.
Lorna C. Webster
9
Molecules and formulae
Read
You have now looked at molecular and ionic substances and the
last section of this booklet is about molecules and formula.
A formula is just a short way to show how many atoms are
present in a molecule. There are two rules that we need to
remember about writing formula:

Numbers always go after the symbols

We do not write the number one
Look at the molecule that is shown below.
H
H
H
C
C
H
H
O
H
This molecule has two carbon atoms, six hydrogen atoms and
one oxygen atom so it will have the formula:
C2H6O
All that we have done is counted the atoms and put in numbers
after the symbols.
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10
Write a heading and try the work below
1. What are the two rules for writing formula?
2. Copy the molecule that is show on page 10 and explain why it
has the formula C2H6O.
3. Use a data book to copy and complete the table below.
Colour of atom
Black
White
Red
Green
Yellow
Blue
element name
Carbon
Hydrogen
Oxygen
Chlorine
Sulphur
nitrogen
element symbol
C
4. Now look at the molecules that your teacher has put around
the classroom and for each one:
 Draw it
 Colour it in
 Work out its formula.
5. Which molecules show elements?
6. Explain your answer to question 5.
7. Which molecules have the same formula but a different
shape?
8. Collect a model kit and build five of your own molecules. The
rules are that you must use the grey bonds and make sure all of
the holes on the atoms are full. For each one draw it colour it
in and write it’s formula.
Your teacher may now ask you to do experiment 1.37. Follow
the instructions on the card and present your answers in a table
with two headings.
Lorna C. Webster
11
Compound names revisited
Read
You already know that the name of a chemical tells us about the
elements that are present. If the name ends in ide the
chemical contains two elements. The first part of the name
tells us one of the elements. The second part of the name gives
us a clue to the name of the second element.
eg sodium chloride contains the elements sodium and chlorine
(we need to check a data book for the full name of the second
element).
eg magnesium oxide contains the elements magnesium and
oxygen.
Write a heading then copy and complete the sentences below
to make sure you understand how to do this before you
start the next page.
Magnesium chloride contains the elements ______________
and __________.
Sodium oxide contains the elements ______________ and
______________.
Potassium chloride contains the elements ____________ and
_____________.
Barium oxide contains the elements ___________ and
____________.
Nitrogen hydride contains the elements ___________ and
_____________.
Carbon sulphide contains the elements ____________ and
_____________.
Lorna C. Webster
12
Names and prefixes
Read
The name of an object sometimes tells us about the object. For
example a tricycle has three wheels; an octopus has eight legs.
a triangle has
3 sides
a pentagon has
5 sides
The little bit that has gone at the beginning of the word is
called a prefix. Prefixes have different meanings.
prefix
meaning
mono
1
di
2
tri
3
tetra
4
penta
5
hex
6
hept
7
oct
8
The prefix tells us more about the chemical but does not
change the elements that are in it. For example carbon
tetrachloride is the same as carbon chloride so it will still
contain carbon and chlorine; carbon dioxide will still contain
carbon and oxygen.
Write a heading and try the work below.
1. How do you know that a tricycle has three wheels?
2. What does the word octopus tell you about the animal?
3. What are the little bits that go infront of words called?
4. Copy the table above that shows the meanings of prefixes 1
to 8.
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13
5. Copy and complete the sentences below
Sulphur dioxide contains ___________ and __________.
Sulphur trioxide contains _________ and ____________.
Carbon monoxide contains ____________ and ___________.
Nitrogen
dioxide
contains
andoften
____________.
If we know
the name
of a ___________
substance we can
work out its
Phosphorus
pentachloride
___________
and _______.
formula even
if we cannot contains
see the atoms
to count them.
Silicon tetraflouride contains ____________ and __________.
For example you might already have heard the name of a gas
Formula and prefixes
Read
Sometimes we can work out the formula of a molecule even if
we cannot see the molecule to count the atoms. We can do this
if the name has a prefix in it.
Carbon tetrachloride contains carbon and chlorine but tetra
means four so this tells us that there are four chlorine atoms.
There is not a prefix infront of the carbon part of the name so
this means that there is only one carbon atom.
The symbol for carbon is C
and the symbol for chlorine is
chlorine
Cl. The
carbon
The formula for this molecule
will be
Lorna C. Webster
14
CCl4
You might have heard of a poisonous gas called called carbon
monoxide. The diagram below shows you how you can use the
name to work out the formula of this gas.
Carbon monoxide
This tells us the
molecule has a
carbon atom in it.
This tells us
there is one
oxygen
This tells us the
molecule has
oxygen in it.
So the formula will be CO (remember we don’t write one’s in
chemistry).
Another example is shown below
Carbon dioxide
This tells us the
molecule has a
carbon atom in it.
So the formula will be
This tells us
there are
two oxygen
atoms.
This tells us the
molecule has
oxygen in it.
CO2
Write a heading and try the work below.
1. Copy the diagram showing how to work out the formula for
the posonous gas called carbon monoxide.
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15
2. Copy and complete the table below
name
carbon dioxide
elements
present
carbon and
oxygen
formula
carbon and
chlorine
nitrogen and
oxygen
CCl4
CO2
carbon monoxide
sulphur dioxide
sulphur monochloride
carbon tetrafluoride
Phosphorus trihydride
Uranium hexafluoride
diphosphorus trioxide
dinitrogen oxide
Diphosphorus pentoxide
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16
NO2