Year 10 Syllabus Revision– Suggested Answers 15-22
CHEMISTRY
15.
Describe an appropriate model that has been developed to describe atomic structure.
Give characteristics of an appropriate model that has been developed to describe
atomic structure.
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The modern day model of the atom is one that has been developed over about 200 years of
experimentation and discovery.
The widely accepted model of the atom has the following features:
(a) an extremely small and very dense, positively charged nucleus containing protons and
neutrons. The positive charge is attributed to the protons (neutrons are neutral in charge).
For interest:
atoms range in size from about 1 – 5 angstroms (Å) – 10-10m. On this scale the nucleus would
be 10-4 Å (10-14m).
since protons and neutrons are about 2000 times the mass of an electron over 99.9% of the
mass of the atom is found in the nucleus. In density terms this would equate to approximately 1013
– 1014g/cm3 (or 1 matchbox full of nuclear material weighing over 2½ billion tons).
The charge of a proton is +1.602 x 10-19C (C = coulombs – unit of charge)
(b) the rest of the atom is mostly empty space in which the electrons orbit the nucleus in electron
shells (energy levels). The electrons of an atom are arranged in their respective shells in a
particular order. This is known as electronic configuration. There is a basic mathematical
expression that can be used to determine the maximum number of electrons that each shell can
hold, ie.
Max.No. electrons = 2n2 (where n = shell number)
Shell 1 = 2 x 12 = 2
Shell 2 = 2 x 22 = 8
Shell 3 = 2 x 32 = 18
Shell 4 = 2 x 42 = 32
NOTE: for the first 20 elements the third shell will only hold
a maximum of 8 electrons. This is known as octet
stability.
For interest:
to get an idea of the vastness of this empty space in an atom picture the Sydney Cricket
Ground. A cricket ball in the centre would represent the size of the nucleus. The playing area
would represent the overall size of the atom and fruit flies would be electrons.
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Diagrammatically the atom can be represented as follows:
A sodium atom
Protons = 11 (P)
Neutrons = 12 (N)
Electrons = 11 – 2,8,1 (
nucleus
11P
12N
)
electron shells
16.
Describe features of and the location of protons, neutrons and electrons in the atom.
Give the characteristic features of and the location of protons, neutrons and electrons
in the atom.
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All atoms are made up of three subatomic particles known as protons, neutrons and electrons.
The main features of these subatomic particles and their location in the atom are:
Particle
Charge
Relative Mass
Proton
Positive (+ve)
1 A.M.U
Neutron
Electron
Neutral (no charge) 1 A.M.U
Negative (-ve)
1/2000 A.M.U
Location
Nucleus
Nucleus
Electron shells
NOTE: A.M.U = Atomic Mass Unit
17. Identify the atom as the smallest unit of an element and distinguish between atoms and
molecules.
Recognise the atom as the smallest unit of an element and use this characteristic to highlight the
differences between atoms and molecules.
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18.
By definition an element is a: “pure substance composed of only one type of atom”.
Elements are essentially indivisible in that they cannot be broken down into simpler or different
substances.
By this reasoning the element gold would only consist of gold atoms, the element oxygen would only
consist of oxygen atoms, the element sodium would only consist of sodium atoms and so on.
Some elements are capable of existing as single atoms (although impossible to see). This individual
atom would therefore show the characteristic properties of that element.
However, some elements cannot exist as a single atom. Instead they will combine with like atoms to
form molecules. A molecule is defined as: “the simplest part of a chemical substance that can exist by
itself and display the unique properties of that substance”.
When elements exist as molecules they are said to be diatomic – two similar atoms. Eg. Hydrogen
(H2), oxygen (O2), nitrogen (N2), fluorine (F2), chlorine (Cl2), bromine (Br2), iodine (I2).
Sometimes compounds also form molecules. These molecules differ from diatomic elements in that
they have combinations of different elements. Molecules of compounds are represented by molecular
formulas. Eg. Water (H2O), methane (CH4), carbon dioxide (CO2), hydrogen peroxide (H2O2), ethanol
(C2H6O).
Distinguish between elements, using information about the numbers of protons, neutrons
and electrons.
Highlight differences between elements, using information about the numbers of protons,
neutrons and electrons.
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There are around 100 different elements known to exist with about 92 of these occurring
naturally.
Every element differs from the others in terms of the numbers of subatomic particles that
make up each atom of that element.
The main particle that determines each different element is the proton. No two elements have
the same number of protons. The number of protons in the nucleus of an atom of any element
is known as its atomic number. The Periodic Table lists all the known elements in order of
increasing atomic number.
All atoms are neutral in charge. This is achieved by the fact that the number of electrons (-ve
charge) is equal to the number of protons (+ve charge).
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The number of neutrons in an atom of a particular element is not a fixed value. The sum of the
protons and neutrons in the nucleus of an atom of any element is known as its atomic mass or
mass number.
Some atoms of the same element may have different numbers of neutrons. Such atoms are
known as isotopes. Eg.
Carbon has three main isotopes –
12
6
C,
13
6
C,
14
6
C.
In the atomic symbol the top number is the atomic mass and the bottom number is the atomic number.
To determine the number of protons you simply subtract the atomic number form the atomic mass, ie.
12
6
C – number of neutrons = 12 – 6 = 6
13
6
C – number of neutrons = 13 – 6 = 7
14
6
C – number of neutrons = 14 – 6 = 8
For this reason the atomic mass for most elements is not a whole number. The value is calculated as an
average of the masses of the different isotopes for that element.
19. Identify that a new compound is formed by rearranging atoms rather than by creating matter.
Recognise and show that a new compound is formed by rearranging atoms rather than by creating
matter.
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In the 1700s a French scientist by the name of Antoine Lavoisier conduct a set of carefully controlled
experiments about masses in chemical reactions. The results of his experiments showed that the total
mass of all substances present after a chemical reaction (products) was equal to the total mass of the
substances before the reaction (reactants).
This important fact became known as the Law of Conservation of Mass (or Matter), which states
that: “matter (atoms) can neither be created nor destroyed only change during chemical processes”.
This means that during a chemical reaction the total number and type of any atom present before the
reaction must be the same as after the reaction.
As such new compounds formed during a chemical reaction are only rearrangements of atoms already
present in the reaction mixture.
For example, it is commonly known that two molecules of hydrogen will react with one molecule of
oxygen to produces two molecules of water as shown in the following chemical equation:
2H2 + O2  2H2O
By careful inspection it can be seen that both sides of this equation contain 4 atoms of hydrogen and 2
atoms of oxygen and that water is simply a rearrangement of these atoms from their initial gaseous
molecules. Other examples include:
Hydrochloric acid + sodium hydroxide  sodium chloride + water
HCl
+
NaOH
Atoms Before Reaction
H=2
Cl = 1
Na = 1
O=1

NaCl
+ H2 O
 Atoms after reaction
H=2
Cl = 1
Na = 1
O=1
Sulfuric acid + aluminium carbonate  aluminium sulfate + carbon dioxide + water
3H2SO4
+
Al2(CO3)3
Atoms Before Reaction

Al2(SO4)3
+
 Atoms after reaction
3CO2
+ 3H2O
H=6
S=3
O = 21
Al = 2
C=3
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H=6
S=3
O = 21
Al = 2
C=3
In all cases matter is conserved and products are rearrangements of the atoms from the reactants.
20. Classify compounds into groups based on common chemical characteristics.
Arrange compounds into appropriate groups based on common chemical characteristics.
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There are two main types of compounds. These are ionic and covalent (or molecular) compounds.
A comparison of the chemical properties of these two types of compounds includes:
Ionic Compounds
Form when metals and non-metals bond
together.
Involve the transfer of electron/s from one atom
to another.
Results in ions forming crystal structures of
arrays of oppositely charged ions.
Strong forces of electrostatic attraction between
ions makes them solid at room temperature and
have relatively high melting and boiling points.
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Covalent Compounds
Form when non-metals bond together.
Involve the sharing of pairs of electrons between
the two atoms.
Results in the formation of molecules that have
weak forces of attraction between them.
Weak forces of attraction between molecules
means that they can be gas, liquid or low
melting point solids at room temperature.
Common examples of these compounds include:
-
Ionic
sodium chloride (NaCl)
magnesium oxide (MgO)
copper sulfate (CuSO4)
aluminium sulfide (Al2S3)
calcium carbonate (CaCO3)
potassium iodide (KI)
-
Covalent
carbon dioxide (CO2)
water (H2O)
ammonia (NH3)
methane (CH4)
ethanol (C2H6O)
sucrose (sugar) (C12H22O11)
21. Identify a range of common compounds using their common names and chemical
formulae.
Recognise a range of common compounds using their common names and chemical formulae.
Common name
Salt
Sugar
Alcohol
Water
Vinegar
Bicarb soda
Carbon dioxide
Ammonia
Methane
Hydrochloric acid
Sulfuric acid
Nitric acid
Rotten egg gas
Copper sulfate
Sodium hydroxide
Formula
NaCl
C12H22O11
C2H6O
H2O
CH3COOH
NaHCO3
CO2
NH3
CH4
HCl
H2SO4
HNO3
H2S
CuSO4
NaOH
Chemical name
Sodium chloride
Sucrose
Ethanol
Dihydrogen oxide
Acetic (ethanoic) acid
Sodium bicarbonate
Carbon dioxide
Nitrogen trihydride
Carbon tetrahydride
Hydrogen chloride
Dihydrogen sulfate
Hydrogen nitrate
Dihydrogen sulfide
Copper sulfate
Sodium hydroxide
22.
Describe the role of indicators.
Provide an outline of the role of indicators.
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Chemical indicators are substances that change colour in different solutions to indicate whether
the solution is acidic or basic/alkaline (and sometimes neutral).
Most indicators will only change between two colours, ie. one colour in acids and a different
colour in bases (alkalis).
However, indicators like universal indicator may go through several colour changes as the
solution goes from strongly acidic through neutral to strongly basic.
Indicators like universal can also give the pH of a solution. The pH scale gives a numerical value
as to the strength of an acidic or alkaline solution. The pH scale is as shown below:
Strongly
Acidic
0
1
Weakly
Acidic
2
red
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3
4
5
orange
Neutral
6
7
Weakly
Basic
8
9
yellow green green/blue
(colour of universal indicator)
Strongly
Basic
10
11
blue
12
13
purple
Some common indicators and their colour changes include:
Indicator
Litmus
Phenolphthalein
Bromothymol blue
Methyl orange
Phenol red
Colour in Acid
Red
Colourless
Yellow
Red
Yellow
Colour in Base
Blue
Red
Blue
Yellow
Red
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