Download Ex. 06 Answer

21st Century Combined Science (Chemistry Part)
Suggested answers to in-text activities and unit-end exercises
Topic 2 Unit 6
In-text activities
Discussion (page 32)
Atoms of elements in the same group have the same number of outermost shell electrons. The
number equals to the group number of the elements.
Discussion (page 33)
Atoms of elements in the same period have the same number of occupied electron shells. The
number equals to the period number of the elements.
Decision Making (page 35)
1 a) Order by the relative atomic mass alone may give some indication of the sequential order.
The order is: R A E Q M D G L X J B T Z.
b) Order given by state at room temperature: A B E G J T L Q D M R X Z.
Order given by electrical conductivity: E B A G J D L M Q R T X Z.
Order given by reactivity: D R A E G J L Q T Z B M X.
The suggested table could be:
R
A
G
J
E
*
B
Q
L
T
M
X
Z
D
#
2 Gaps appear as shown above. The first missing element * (between E and B) would be expected
to be a solid, golden in colour, with very high electrical conductivity and medium reactivity. It
would probably have a relative atomic mass between 16 and 20.
The second missing element # (after R and D) would be a colourless gas, with nil electrical
conductivity and high reactivity. It would probably have a relative atomic mass between 24 and
28.
3 a) The new element of atomic mass 25.8 is most probably the element that would fit into the
second gap # described above.
b) See answer in 2.
Checkpoint (page 37)
1 a) Period 4, Group VII
b) 35
c) Bromine
Suggested answers to in-text activities and unit-end exercises
Topic 2 Unit 6
1
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21st Century Combined Science (Chemistry Part)
d) Fluorine / chlorine
2 a) Across a period, the elements show a gradual change (increase) in non-metallic
character.
Across a period, the elements show a gradual change (decrease) in atomic size.
b) 5
c) 2
d) Metals — a, b, c, d
Metalloid — e
Non-metals — f, g, h, i, j
any one
Checkpoint (page 44)
a) The reactivity of Group 1 elements increases down the group.
b) To prevent it from reacting with the air.
c) i) Their atoms have the same number of outermost shell electrons.
ii) Their atoms have different number of occupied electron shells.
d) • Potassium floats on the surface of water while magnesium sinks.
• Potassium burns with a lilac flame while magnesium shows no reaction.
Checkpoint (page 50)
a) X
b) i) W and Y
ii) Their atoms have the same number of outermost shell electrons.
iii) Both react with hydrogen to give hydrogen halides.
Checkpoint (page 52)
1 The same as silicon tetrachloride.
2 a) Alkaline earth metals
b) p = 2
q=8
r=2
c) Solid
d) C.
The reactivity of alkaline earth metals increases down the group.
Checkpoint (page 57)
1 a)
Suggested answers to in-text activities and unit-end exercises
Topic 2 Unit 6
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© Jing Kung. All rights reserved.
21st Century Combined Science (Chemistry Part)
b) By losing two electrons.
c)
d) 2 positive charges.
2 At — Group VII
Ba — Group II
Cs — Group I
Ga — Group III
P — Group V
Se — Group VI
3 An atom of X has an electronic arrangement of 2,8,8,1. There are 4 occupied electron shells.
Hence X belongs to Period 4 of the periodic table.
Unit-end exercises (pages 61-67)
1
Suggested answers to in-text activities and unit-end exercises
Topic 2 Unit 6
3
© Jing Kung. All rights reserved.
21st Century Combined Science (Chemistry Part)
2 a) Carbon
b) Neon
c) Lithium
d) Fluorine
e) Oxygen
3 a) i) 2,2
ii) 2,8,2
b) Their atoms have the same number of outermost shell electrons.
4 Students may use pathways other than the one shown below.
5 a ) X — proton
Y — neutron
Z — electron
b) Group VII
c) Period 2
6D
Suggested answers to in-text activities and unit-end exercises
Topic 2 Unit 6
4
© Jing Kung. All rights reserved.
21st Century Combined Science (Chemistry Part)
7D
8B
9D
10 B
(2) The reactivity of halogens decreases down the group.
11 A
Element X is phosphorus.
12 B
(1) Barium belongs to Group II. Hence its atom has 2 outermost shell electrons.
(2) The reactivity of Group II elements increases down the group. Hence barium is more reactive
than calcium.
13 D
14 a) Across a period, the elements show a gradual change (increase) in non-metallic character.
Across a period, the elements show a gradual change (decrease) in atomic size.
any
one
b) Boron / silicon
c) i) Their atoms have different number of occupied electron shells.
ii) Their atoms have the same number of outermost shell electrons.
d) Both of them react with water vigorously.
Hydrogen and an alkaline solution are formed in both cases.
e) The balloon will fall because the density of krypton is higher than that of air.
f) The electronic arrangement of an atom of Y is 2,8,6. Hence Y belongs to Group VI of the
periodic table.
15 a)
Element
State at room temperature and pressure
Colour
Chlorine
gas
greenish yellow
Bromine
liquid
reddish brown
Iodine
solid
black
b) Halogens
c) Their atoms have the same number of outermost shell electrons.
d) Decreases
Suggested answers to in-text activities and unit-end exercises
Topic 2 Unit 6
5
© Jing Kung. All rights reserved.
21st Century Combined Science (Chemistry Part)
e)
16 a) Noble gases
b) Each noble gas atom has an octet structure (or a duplet structure) in its outermost shell.
c) Argon does not react with the metal filaments in light bulbs.
d) In advertising signs
17 a)
Metals
f, h
Metalloids
a, g
Non-metals
b, c, d, e
b) 2
c) f
d) c
e) e
f) Alkali metals
18 a) Alkali metals
b)
c) Its melting / boiling point is relatively low when compared with other metals.
d) i) Rubidium is more reactive than potassium.
ii) Store rubidium in paraffin oil.
iii) • Use a safety screen during experiment.
• Wear safety glasses.
• Wear protective gloves.
Suggested answers to in-text activities and unit-end exercises
Topic 2 Unit 6
any one
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