Suggested Answers to In-Text Activities and Unit-End Exercises

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).

Checkpoint (page 12)

Atom / Number of protons / Number of neutrons / Number of electrons
7 3Li / 3 / 4 / 3
27 13Al / 13 / 14 / 13
40 20Ca / 20 / 20 / 20

Atom / Atomic
number / Mass
number / Number of
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 =

2 Let the relative abundance of 10 5B and 11 5B be % and (100 –)% respectively.

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

=20

∴ the relative abundance of 10 5B is 20% and that of 11 5B 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)

Element / Description / Electronic arrangement
A / Its atomic number is 8. / 2,6
B / Its atom has 1 completely filled shell and 7 electrons in the second shell. / 2,7
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)

2 a) False

The most common isotope of hydrogen (1 1H) 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
number / Mass
number / Number of / Electronic 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

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
its atom is 2,8,8,1. / potassium / K

5 a) b) c)

d) e)

6 D

7 B

8 B Number of neutrons = mass number ― atomic number

= 267 ―107

= 160

9 C

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

Relative atomic mass of X = 6.93 =

= 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.

• 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 =

14 a) 38

b) 40

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

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

∴ 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

c) Potassium

d) Metal

Topic 2 Unit 5