12
Topic
Patterns in the Chemical
World
Unit 41 Periodic trends in elements and their
compounds
Unit 42 The transition metals
Key
C o ncepts
Periodic trends in elements and
their compounds
• Periodic variations in bonding and
structures across periods 2 and 3
• Variations in melting point and
electrical conductivity across a period
• Bonding, composition and acid-base
behaviour of oxides of elements across
the third period
Patterns in the
Chemical World
The transition metals
• Coloured compounds of transition
metals
• Variable oxidation states of transition
metals
• Transition metals and their
compounds as catalysts
Topic 12 Patterns in the Chemical World
Unit 41
Periodic trends in elements and their
compounds
Unit 41 Periodic trends in elements and their compounds
41.1 – 41.10
Summary
41.1
The modern periodic table
41.2
Categories of elements in the modern periodic table
41.3
Periodic variations in bonding and structures of elements across
periods 2 and 3
41.4
Variation in atomic radius of elements across a period
41.5
Variation in melting point of elements across the third
period
41.6
Variation in electrical conductivity of elements across the third
period
41.7
Variation of density of elements across a period
41.8
Reaction of elements in Period 3 with water
41.9
Composition of the oxides of Period 3 elements
1 The following table summarizes the variation in nature of bonding, structure and
physical properties of elements across periods 2 and 3.
Group
I
II
III
IV
V
VI
VII
0
Elements in
Period 2
Li
Be
B
C
(diamond)
N
O
F
Ne
Type of
element
metals
Type of
structure
giant metallic
giant covalent
simple molecular
Nature of
bonding
metallic
covalent
weak van der Waals’ forces
between molecules; strong
covalent bonds holding
atoms together within the
molecule (except Ne)
Melting
point (°C)
41.10 Acid-base nature of oxides of Period 3 elements
Electrical
conductivity
180
metalloid
1 280
good
conductors
2 030
3 500
semiconductor
Atomic
radius
Elements in
Period 3
non-metals
–210
–219
–220
–249
Cl
Ar
non-conductors
decrease
Na
Mg
Al
Si
P
S
Type of
element
metals
metalloid
non-metals
Type of
structure
giant metallic
giant covalent
simple molecular
covalent
weak van der Waals’ forces
between molecules; strong
covalent bonds holding
atoms together within the
molecule (except Ar)
Nature of
bonding
Melting
point (°C)
Electrical
conductivity
metallic
98
650
660
good conductors
Atomic
radius
1 410
semiconductor
44
113
non-conductors
decrease
2 The main trends across a period of elements:
• a change from metals through metalloid to non-metals;
• a decrease in atomic radius;
–101
• a maximum in melting point is shown in the middle of the period;
• a change from conductors to non-conductors.
–189
Topic 12 Patterns in the Chemical World
Unit 41 Periodic trends in elements and their compounds
3 The following table summarizes the nature of bonding and the acid-base nature of
oxides of Period 3 elements.
I
II
III
IV
V
VI
VII
e.g.
Element
Na
Mg
Al
Si
P
S
Cl
SiO2
P4O6
P4O10
SO2
SO3
Cl2O
Cl2O7
Moving across Period 3 of the periodic table (from sodium to argon),
protons are added to the nucleus and electrons are added to the
outermost shell of the atoms of the elements.
Oxide(s)
formed
Na2O
Nature of
bonding of
oxide(s)
ionic
Acid-base
nature of
oxide(s)
strongly
basic
Al2O3
mainly ionic
basic
The shielding effect due to the inner shell electrons remains almost the
same.
mainly covalent
amphoteric
Hence the effective nuclear charge felt by the outermost shell electrons
increases across the period.
strongly
acidic
acidic
Outermost shell electrons are pulled closer to the nucleus. So, the atomic
radii of elements decrease.
♦ From lithium to fluorine in the second period of the periodic table, the
carbon in the middle has the highest melting point. The elements do
NOT show a gradual decrease in melting point.
✘
Atomic radius
Exam tips
MgO
♦ DO NOT confuse the concepts of ‘effective nuclear charge’ and ‘screening
effect’.
Group
♦ Students should be able to state and explain the trend of melting points
of non-metals in Period 3.
Phosphorus (white phosphorus) exists as P4 molecules and sulphur exists
as S8 molecules in their solid states.
Melting point (K)
0
2 000
11
12
13
14
15
16
17
18
Atomic number
1 500
♦ Na2O2 reacts with water to give H2O2.
1 000
Cl2O7 reacts with water to give HClO4.
500
0
Na
Mg
Al
Si
P
Element
S
Cl
Ar
Example
Account for each of the following:
a) At room temperature and pressure, Na2O is a solid with a very high melting point
whereas Cl2O is a gas.
(2 marks)
b)Al2O3(s) is soluble in both aqueous acids and aqueous alkalis.
(3 marks)
Answer
a) Na2O is an ionic solid. A lot of heat is needed to overcome the strong attractions
between the cations and anions in the solid.
(1)
Cl2O exists as simple molecules. Weak van der Waals’ forces exist between the
molecules. Little heat is needed to separate the molecules.
(1)
b)Al2O3(s) is amphoteric.
+
Al2O3(s) + 6H (aq)
–
(1)
3+
2Al (aq) + 3H2O(l)
Al2O3(s) + 2OH (aq) + 3H2O(l)
(1)
–
2[Al(OH)4] (aq)
(1)
Topic 12 Patterns in the Chemical World
Remarks*
Remarks
➤Questions often ask about the difference in the reactions of different oxides
with water.
Unit 42 The transition metals
Unit 42
The transition metals
e.g.
Na2O reacts with water to give an alkaline solution;
SiO2 has no reaction with water; and
SO2 reacts with water to give an acidic solution.
➤Questions often ask about the amphoteric nature of Al2O3.
42.1
Introduction to transition metals
42.2
Coloured compounds of transition metals
42.3
Variable oxidation states of transition metals
42.4
Variable oxidation states of vanadium
42.5
Variable oxidation states of manganese
42.6
Variable oxidation states of iron
42.7
Transition metals and their compounds as catalysts
42.8
Uses of transition metals in medicine
Topic 12 Patterns in the Chemical World
10
Unit 42 The transition metals
42.1 – 42.8
Example
Consider the following ionic half-equations in the electrochemical series:
Summary
1 Typical chemical properties of transition metals include:
• formation of coloured compounds;
• existence in more than one oxidation state in their compounds; and
• catalytic properties of the elements and their compounds.
2 The following reductions occur when a solution containing dioxovanadium(V) ions
(VO2+(aq)) is shaken with zinc powder under acidic conditions.
+5
VO2+(aq) name dioxovanadium(V) ions oxovanadium(IV) ions vanadium(III) ions
colour
yellow
blue
green
+4
VO2+(aq) +3
V3+(aq) +2
V2+(aq)
vanadium(II) ions
violet
3 Permanganate ion is a powerful oxidizing agent. In the presence of dilute sulphuric
acid and a reducing agent, permanganate ion is reduced to manganese(II) ion.
+7
MnO4–(aq) + 8H+(aq) + 5e– purple
+2
Mn2+(aq) + 4H2O(l)
colourless
(or pale pink)
higher in
electrochemical series
V3+(aq) + e–
V2+(aq)
VO2+(aq) + 2H+(aq) + e–
+
2
+
VO (aq) + 2H (aq) + e
O2(g) + 4H (aq) + 4e
+
lower in
electrochemical series 3+
Co (aq) + e
–
–
–
V3+(aq) +H2O(l)
VO2+(aq) + H2O(l)
2H2O(l)
2+
Co (aq)
What would happen if an aqueous solution containing the species in each of the
following cases is exposed to air? Explain your answer in each case.
a) V3+(aq) ions
2+
(3 marks)
3+
b)Co (aq) and Co (aq) ions
(3 marks)
Answer
a) V3+(aq) ions will be oxidized to VO2+(aq) ions.
2+
O2(g) is a stronger oxidizing agent than VO (aq) ion.
The following reaction will occur:
4V3+(aq) + O2(g) + 2H2O(l)
4VO2+(aq) + 4H+(aq)
3+
2+
b)Co (aq) ions will be reduced to Co (aq) ions.
Exam tips
11
♦ Question may ask students to compare the chemistry between main
group metals and transition metals.
Similarities
– Both main group metals and transition metals are metals. They are
both lustrous, good conductors of electricity and heat, malleable and
ductile.
– Both main group metals and transition metals form oxides upon
standing in air.
Differences
– Compared with metals in the main groups of the same period, the
transition metals have higher melting points, boiling points, and
density. Moreover, their atomic radii are smaller.
– Main group metals exhibit only one oxidation state while transition
metals exhibit variable oxidation states.
– Main group metals form colourless compounds while transition metals
form coloured compounds.
– Main group metals do not demonstrate catalytic activity while transition
metals do.
3+
Co (aq) ion is a stronger oxidizing agent than O2(g).
The following reaction will occur:
4Co (aq) + 2H2O(l)
3+
2+
+
4Co (aq) + 4H (aq) + O2(g)
Remarks*
Remarks
➤This method of predicting redox reactions that may occur has been discussed
in Topic 5 Redox Reactions, Chemical Cells and Electrolysis.
(1)
(1)
(1)
(1)
(1)
(1)