Download The structures of atoms

Double Certificate GCSE Science Pack 3
Unit 2
The structures of atoms
There are only about 109 different kinds of atoms which make up the entire universe (see Unit 3).
Every atom has the same basic structure and is in two parts:
Nucleus
Electron ‘cloud’
containing ‘orbiting’
electrons
Chemical reactions involve interactions between the electron clouds of different atoms. The cloud
is made up of shells – like the inside of an onion.
e-
The second shell can hold up to eight electrons
e-
e-
The first shell can hold up to two electrons
e-
e-
e-
e- e-
The third shell can hold up to eight electrons
e- ee-
e-
Nucleus
e-
e-
e-
e-
e-
e-
The three innermost electron shells – e- represents one electron
Since electrons have almost no mass, practically all the mass of an atom
is made up of the protons and neutrons in its nucleus. However, the
nucleus of an atom has no influence on chemical activity. The nucleus
is made up of a cluster of protons and neutrons. Each proton has one
positive (+) charge; each neutron has no charge.
Proton
Neutron
The units of mass for atoms are atomic mass units (amu).
• The mass number of an atom = the number of protons + neutrons in the nucleus.
• The atomic number of an atom = the number of protons in the nucleus.
• The mass number - the atomic number = the number of neutrons in the nucleus.
Particle
Mass (amu)
Charge
proton (p)
1
1
neutron (n)
1
0
electron (e)
almost nil
-1
The nucleus of an atom of sodium contains 11 protons and 12 neutrons. There are 11 electrons in its
shells – 2 in the first, 8 in the second and 1 in the outer. The atom of sodium can be represented by
the symbol 23
11Na where 11 is the atomic number and 23 is the atomic mass.
The total number of (-) electron charges in the cloud exactly equals the number of (+) proton
charges in the nucleus. So there is no overall charge on an atom – it is neutral. When different kinds
of atoms react with each other, they upset this neutral condition by exchanging or sharing electrons
from their clouds – see Units 12 and 13.
Pearson Publishing, Chesterton Mill, French’s Road, Cambridge CB4 3NP
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Double Certificate GCSE Science Pack 3
Unit 2
5 Atoms of different elements can join
together to form chemical compounds. To
do this the atoms:
Questions
Tick the box next to the correct answer.
1 Atoms of elements:
A
B
C
D
n
n
n
n
have an overall positive charge
are neutral overall
have an overall negative charge
are always charged either positive
or negative
A
B
C
D
join their nuclei together
share their outer electrons
exchange nuclei
bond protons with their
partner’s electrons
n
n
n
n
6 Atoms of different elements must have:
2 The atomic number of an element tells you:
A
B
C
D
the number of protons in its nucleus
the number of neutrons in its nucleus
the number of electrons in its nucleus
the number of electrons in its shells
n
n
n
n
3 An atom of aluminium can be represented
by the symbol 27
13Al. Which line in the table
gives the correct number of protons,
neutrons and electrons in an atom of
aluminium?
Protons
Neutrons
Electrons
A
13
14
13
B
14
13
27
C
13
13
14
D
14
27
13
A
n
B
n
C
n
D
n
4 The mass number of an atom is the number
of:
A
B
C
D
protons in the nucleus
neutrons in the nucleus
all the particles in the atom
the number of protons and
neutrons together
n
n
n
n
Pearson Publishing, Chesterton Mill, French’s Road, Cambridge CB4 3NP
A different mass numbers
B the same number of neutrons in
their nuclei
C different numbers of electrons in
their shells
D different numbers of protons in
their nuclei
n
n
n
n
7 How many neutrons are there in an atom of
fluorine 199 F?
A
B
C
D
9
19
10
28
n
n
n
n
8 If a neutral atom of an element has 14
electrons and 19 neutrons, its atomic
number is:
A
B
C
D
5
14
19
33
n
n
n
n
6
Double Certificate GCSE Science Pack 3
Unit 9
Water and solutions
Three-quarters of the Earth’s surface is covered by water. Water is sometimes called a ‘universal
solvent’ because so many substances can be dissolved in it. Almost all chemical reactions in biological
systems take place in solution in water (in aqueous solution (aq)). Water also has great importance as
a solvent and as a coolant in industry.
The water cycle
The ‘water cycle’ is what keeps an approximately constant amount of water available for us to use.
Used in factories
and homes
Rain, hail and snow
return water to the surface
Water table
Plants transpire
Some water enters
the ground, forming a
saturated layer of rock
Water evaporates
from sea and land
Where the water table intersects
the surface springs occur
Water treatment
plant
Hardness in water
As water ‘cycles’ through the soil, in particular, it dissolves minerals and salts and causes ‘hardness’;
this makes it difficult for soap to form a lather and also produces a scum.
SO42-
Cl-
Ca2+
HCO3-
Mg2+
Permanent hardness
Temporary hardness
Softened by:
Softened by boiling
A Adding ‘washing soda’ (Na2CO3)
(cheap and convenient)
B Using ion exchange resins (eg Permutit
and others) which remove Ca2+ and
Mg2+ (inexpensive but tedious)
Ca(HCO3)2 (aq)
CaCO3 (s) + H2O(l) + CO2 (g)
Scale
C Distillation (very expensive)
Salts and minerals in the soil
Advantages and disadvantages of hard water
Both types of hardness are found in most water supplies to varying degrees. ‘Hard’ water causes
problems with ‘scaling’, which narrows pipes and reduces the efficiency of immersion heaters, and
with poor lather formation from the use of soaps and detergents. ‘Hard’ water tastes nicer, however,
and is healthier in several ways since it contains minerals which are good for the body.
Pearson Publishing, Chesterton Mill, French’s Road, Cambridge CB4 3NP
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Double Certificate GCSE Science Pack 3
Unit 9
Questions
6 Permanently hard water:
Tick the box next to the correct answer.
1 The water cycle shows:
A the network of rivers and canals
in the UK
B how water is purified
C the steps in recycling water
over the Earth
D how climates change due to
sea breezes
n
n
n
25%
50%
75%
100%
n
n
n
n
A calcium and magnesium ions
dissolved in the water
B isotopes of oxygen in the water
molecules
C bacteria in the water
D the presence of any metal ions
in the water
n
n
n
n
8 Hard water has some advantages over soft
water. One of these is that hard water:
n
n
n
n
A dissolves substances more
quickly than soft water
B is more efficient in central
heating systems
C boils quicker than soft water
D contains minerals useful to the body
n
n
n
n
9 Which of the following statements is not
correct?
4 Water is said to be ‘hard’ when it:
A can be easily compressed
B is not easy to find in drought
conditions
C contains many impurities
D makes it difficult to get lather
with soap
n
n
n
7 Hardness in water is caused by:
3 Water is sometimes called a ‘universal
solvent’ because:
A the human body is over 90% water
B so many substances can be dissolved
in it
C so much of the Earth’s surface is
covered by it
D it is essential for all plant and
animal life
n
n
2 What percentage of the Earth’s surface is
covered by water?
A
B
C
D
A can be softened by adding
‘washing soda’
B can never be softened
C must be boiled to remove hardness
D means that there are impurities
in the water
n
n
n
n
A Using soft water is cheaper than
using hard water
B Temporarily hard water can be
softened by boiling
C Scum is the result of using soap
with hard water
D Soft water is better for plants than
hard water
n
n
n
n
5 Temporarily hard water is caused by calcium
and magnesium ions and also:
A
B
C
D
hydrogencarbonate ions
other metal ions
minerals which cause scale
sulphate and chloride ions
n
n
n
n
Pearson Publishing, Chesterton Mill, French’s Road, Cambridge CB4 3NP
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