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New 21st Century Chemistry

Suggested answers to in-text activities and unit-end exercises

Topic 2 Unit 5

In-text activities

Checkpoint (page 4)

Element

Symbol

State at room temperature

 

 

and pressure

 

 

 

Aluminium

Al

solid

 

 

 

Calcium

Ca

solid

 

 

 

Hydrogen

H

gas

 

 

 

Sodium

Na

solid

 

 

 

Chlorine

Cl

gas

 

 

 

Potassium

K

solid

 

 

 

Oxygen

O

gas

 

 

 

Sulphur

S

solid

 

 

 

Discussion (page 5)

 

Physical property

Difference

 

 

 

 

 

Melting and boiling points

The melting and boiling points of metals are high while those of

 

 

 

non-metals are low.

 

 

 

 

 

Electrical conductivity

Metals are good conductors while non-metals are poor conductors.

 

 

 

 

 

Heat conductivity

Metals are good conductors while non-metals are poor conductors.

 

 

 

 

 

 

 

 

Checkpoint (page 7)

 

 

 

1 a) Mercury, sodium.

 

 

b) Silicon

 

 

c) Carbon, oxygen

 

 

d) Carbon

 

2 X is a non-metal.

It has a low melting point and does not conduct electricity. Y is a metal.

It is shiny and can be bent or hammered into shape. Z is a metalloid.

It has the property of a non-metal (a brittle solid) and the property of a metal (can conduct electricity).

Suggested answers to in-text activities and unit-end exercises

1

Topic 2 Unit 5

 

© Jing Kung. All rights reserved.

New 21st Century Chemistry

Checkpoint (page 12)

1

Atom

 

Number of protons

Number of neutrons

Number of electrons

 

 

 

 

 

 

 

 

 

 

 

37 Li

 

3

 

 

4

 

 

3

 

1327 Al

 

13

 

 

14

 

 

13

 

2040 Ca

 

20

 

 

20

 

 

20

 

2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Atom

 

Atomic

Mass

 

Number of

 

 

 

 

 

number

number

 

 

 

 

 

 

 

 

protons

neutrons

electrons

 

 

 

 

 

 

 

 

 

 

 

 

Fluorine

 

9

19

9

10

9

 

 

 

 

 

 

 

 

 

 

 

 

Boron

 

5

11

5

6

5

 

 

 

 

 

 

 

 

 

 

 

 

Phosphorus

 

15

31

15

16

15

 

 

 

 

 

 

 

 

 

 

 

 

Checkpoint (page 16)

 

 

 

1

Relative atomic mass of magnesium =

24 78.6 25 10.126 11.3

24.3

 

 

 

100

 

2

Let the relative abundance of 510 B and

511 B be y % and (100 – y )% respectively.

Relative atomic mass of boron = weighted average relative isotopic mass of the isotopes

10.8 10 y 11 (100  y) 100

1080 10y 1100 11y y =20

the relative abundance of 105 B is 20% and that of 115 B is 80%.

3 X has isotopes of different masses. Its relative atomic mass is the weighted average relative isotopic mass of all the isotopes on the 12C = 12.00 scale.

Checkpoint (page 20)

 

 

1 a)

b)

c)

Suggested answers to in-text activities and unit-end exercises

2

Topic 2 Unit 5

 

© Jing Kung. All rights reserved.

New 21st Century Chemistry

2

Element

Description

Electronic arrangement

 

 

 

A

Its atomic number is 8.

2,6

 

 

 

B

Its atom has 1 completely filled shell

2,7

 

and 7 electrons in the second shell.

 

 

 

 

C

Its atom has 20 electrons.

2,8,8,2

 

 

 

3 a) An atom of X has an electronic arrangement of 2,8,5. An atom of X has 15 electrons. Hence the atomic number of X is 15.

b) X is phosphorus.

STSE Connections (page 21)

1 a) Rutherford’s scattering experiment

b)When thin sheets of metal foil were bombarded with positively charged particles, most of the particles went through the metal foils.

2 Neutrons are neutral particles and thus more difficult to detect than charged particles.

Unit-end exercises (pages 26-30)

1

Suggested answers to in-text activities and unit-end exercises

3

Topic 2 Unit 5

 

© Jing Kung. All rights reserved.

New 21st Century Chemistry

2 a) False

The most common isotope of hydrogen (11 H) does not contain neutron. b) False

The isotope has 6 protons and 6 neutrons.

c)False

Electrons that occupy the shells are negatively charged.

d)True

The maximum number of electrons the second electron shell can hold is 2(2)2, i.e. 8.

3 a)

Atom

Atomic

Mass

 

Number of

 

Electronic

 

number

number

 

 

 

arrangement

 

protons

neutrons

electrons

 

 

 

 

 

 

 

A

3

6

3

3

3

2,1

 

 

 

 

 

 

 

B

8

18

8

10

8

2,6

 

 

 

 

 

 

 

C

11

23

11

12

11

2,8,1

 

 

 

 

 

 

 

D

16

32

16

16

16

2,8,6

 

 

 

 

 

 

 

E

8

16

8

8

8

2,6

 

 

 

 

 

 

 

F

13

27

13

14

13

2,8,3

 

 

 

 

 

 

 

b) A — lithium

B — oxygen

C — sodium

 

c) B and E

 

 

 

 

4

 

 

 

 

 

 

 

 

 

 

 

Element

 

Description

 

Name

Symbol

 

 

 

 

 

A

Its atomic number is 7.

 

nitrogen

N

 

 

 

 

 

B

Its atom has 12 protons.

 

magnesium

Mg

 

 

 

 

 

C

Its atom has 18 electrons.

 

argon

Ar

 

 

 

 

 

D

The electronic arrangement of

 

potassium

K

 

its atom is 2,8,8,1.

 

 

 

 

 

 

 

 

 

5 a)

 

b)

 

 

c)

Suggested answers to in-text activities and unit-end exercises

4

Topic 2 Unit 5

 

© Jing Kung. All rights reserved.

New 21st Century Chemistry

d)

e)

6D

7B

8B Number of neutrons = mass number ― atomic number

=267 ―107

=160

9C

10D Let the relative abundance of 6X and 7X be (100 – y )% and y % respectively.

Relative atomic mass of X = 6.93 = 6 (100  y) 7 y 100

693 600 6y 7y y = 93

11 a) A is a non-metal. It is a gas and does not conduct electricity.

B is a metal. It is a shiny solid and a good conductor of electricity.

b)C is mercury. C is a liquid at room temperature and pressure.

12• It is a good conductor of electricity.

• It is a good conductor of heat.

• It is malleable.

any two

•It is ductile.

•It can react with water to form hydrogen

13 a) Isotopes are different atoms of an element which have the same number of protons but a different number of neutrons.

b) Relative atomic mass of Si =

28 94.129 2.8 30 3.1

28.09

100

 

 

 

 

14 a) 38

Suggested answers to in-text activities and unit-end exercises

5

Topic 2 Unit 5

 

© Jing Kung. All rights reserved.

New 21st Century Chemistry

b)40

c)Let the relative abundance of gallium-69 and gallium-71 be y % and (100 – y )% respectively.

Relative atomic mass of gallium = weighted average relative isotopic mass of the isotopes

69.8 69 y 71 (100  y) 100

6980 69y 7100 71y y 60

the relative abundance of gallium-69 is 60% and that of gallium-71 is 40%.

15 a) Number of protons = 19 Number of neutrons = 20 Number of electrons = 19

b)

c)Potassium

d)Metal

Suggested answers to in-text activities and unit-end exercises

6

Topic 2 Unit 5

 

© Jing Kung. All rights reserved.