1 Periodic Variation in Physical Properties of the Elements H to Ar.
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Transcript of 1 Periodic Variation in Physical Properties of the Elements H to Ar.
1
Periodic Variation in Periodic Variation in Physical Properties of Physical Properties of the Elements H to Arthe Elements H to Ar
2 The modern Periodic Table
Elements are arranged in the increasing order of atomic number
3 The modern Periodic Table
Horizontal rows periods same no. of occupied shells7 periods
4 The modern Periodic Table
Vertical columns groups same no. of outermost shell
electrons18
groups
5 The modern Periodic Table
Periodicity : Properties of elements are periodic functions of atomic number
6 The modern Periodic Table
Periodicity : Similar properties of elements recur periodically
7 The modern Periodic Table
Periodicity : Properties of elements vary periodically with atomic number
8
Properties depends on electronic configuration
Four blocks s, p, d, f
9
Outermost orbitals : ns1 ns2
s-block : Groups 1A, 2A
10
Outermost orbitals : ns2 np1 ns2 np6
p-block : Groups 3A 8A(0)
11
s-block & p-block elements are called representative elements
12
Outermost orbitals : ns2 (n-1)d1 ns2 (n-1)d10
n4
d-block : Transition elements (Groups 3B 1B)
13
Outermost orbitals : ns2 (n-2)f1 ns2 (n-2)f14
n6
d-block : Inner transition elements
LanthanidesActinides
14
Q.1
2
15
Q.1
210
16
Q.1
21018
17
Q.1
2101836
18
Q.1
210183654
19
Q.1
21018365486
20
Q.1
21018365486118
Atomic no. = 109
Period 7 = 118-6-3
6d
21
Q.1
21018365486118
Atomic no. = 123
Period 8 = 118+2+3
5g
8
22
Q.1
21018365486118
Atomic no. = 151
Period 8 = 118+2+18+13
6f
8
Period 8 can hold up to 50 elements(119 to 168)
23
24
Periodic variation in physical properties of the elements H to Ar
1. Melting point
2. Atomic radius
3. First ionization enthalpy
4. Electronegativity
25
Melting point Melting point
A measure of the ease of the change from solid phase to liquid phase
Depends on
(a) The strength of the bonds to be broken
(b) The extent of bond breaking
(c) The structure of the crystal lattice
26
Melting point Melting point
Across a period,
1. the type of bonding changes from
Strong metallic
Strong covalent
Weak van der Waals’ forces
2. the structure of elements changes fromClosed-packed metallic
Giant covalent
Simple molecular
periodic variation
27
Structure and Structure and BondingBonding
A summary of the variations in structure and bonding of elements across both Periods 2
28
Structure and Structure and BondingBonding
A summary of the variations in structure and bonding of elements across both Periods 3
29
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
30
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
A.Variations in m.p. across a periodPatterns :
- increases steadily from group 1A to 3A, reaching a maximum in group 4A
drops sharply from group 4A to 5A, and eventually reaching a minimum in group
0
31
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
Interpretation : - m.p. from group 1A to 3A because
(i) the no. of outermost electrons involved in metallic bonds from 1 to 3
strength of bond accordingly
Boron giant covalent
32
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
Interpretation : - m.p. from group 1A to 3A because
(ii) Packing efficiency : - Gp2A/3A(hcp/fcc) > Gp1A(bcc)
33
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
Interpretation : - Gp4A elements(C & Si) giant covalent Covalent bonds are highly directional
Metallic bonds are non-directional
Extent of bond breaking on meltingCovalent >> metallic
34
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
Interpretation : - For metals, the differences between m.p. and b.p. are great
∵ extent of bond breaking : boiling >> melting
Particles are completely separated on boiling For Gp4A elements, the differences between m.p. and b.p. are relatively small
∵ extent of bond breaking : boiling melting
* C sublimes at 1 atm
35
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
Interpretation : - Sharp in m.p. from Gp4A to Gp5A because
Covalent bond(Gp4A) >> van der Waals forces(Gp5A)
36
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
Interpretation : - m.p. of Mg m.p. of Al
∵ only an average of TWO outermost shell electrons per atom of aluminium participate in
the formation of metallic bonds
37
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
Interpretation : - m.p. : N > O > F > Ne (regular)
∵ molecular size : N2 > O2 > F2 > Ne Strength of v.d.w. forces : N2 > O2 > F2 >
Ne
38
Element Li Be B C N O F Ne
Melting point / oC 181 1287 2076 3527* 210 219 220 249
Boiling point / oC 1342 2469 3927 4027* 196 183 188 246
Element Na Mg Al SiP(whit
e)S Cl Ar
Melting point / oC 98 650 660 1414 44 115 101 189
Boiling point / oC 883 1090 2517 2900 277 444 34 186
Interpretation : - m.p. : S > P > Cl > Ar (irregular)
∵ molecular size : S8 > P4 > Cl2 > Ar Strength of v.d.w. forces : S8 > P4 > Cl2 >
Ar
39
Atoms of elements in period 2 tend to form multiple bonds (double or triple) with one another. Examples : O=O (1 + 1), NN (1 + 2)
40
Atoms of elements in period 3 do not form multiple bonds with one another. Instead, they form cyclic structure in which all bonds are bonds.
S
S
S
S
SS
S
S
S8
P
P
PP
P4
bond formation is not favoured due to poor side-way overlap between 3p orbitals
41
Each Si atom forms four single bonds rather than two double bonds with O atoms (∵ poor 3pz-2pz overlap)
Si
O
O O
O
O Si O O C O
C
O
O O
O
Q.2
42
C=O is preferred to C-O because
1. 2pz-2pz overlap > 3pz-2pz overlap
2. Polarization of bond (mesomeric effect) results in stronger double bond
B.E. (kJ mol1) : C=O(749) > 2C-O(358)
Si
O
O O
O
O Si O O C O
C
O
O O
O
43
Simple molecular
Si
O
O O
O
O Si O O C O
C
O
O O
O
Giant covalent
44
B. Variation in m.p. down a group
45
the electrostatic forces of attraction between the positive metal ions and the delocalized electrons down the group
the charge density, of positive ion down the groupsize
charge
For metals in Gp1A/2A/3A,
the strength of metallic bond down the group
m.p. down the group. It is because ionic radius down the group
46
For Gp 4A elements,
m.p. down the groupC(3527
)
Si(141
4)
Ge(937
)
Sn(230
)
Pb(327
)
47
∵ atomic radius down the group
Extent of orbital overlap down the group
Strength of covalent bond down the group
Sn and Pb are metals and thus have exceptionally low m.p. due to less extensive breaking of metallic bonds
C(3527
)
Si(141
4)
Ge(937
)
Sn(230
)
Pb(327
)
For Gp 4A elements,
m.p. down the group
48
∵ Size of molecules down the group
Extent of polarization of electron cloud down the group
Strength of London dispersion forces down the group
For Groups 6A/7A elements
m.p. down the group F(-220)
Cl(-
101)
Br(-
7.2)
I(114)
49
Atomic Atomic radius radius
Refer to notes on ‘Electronic structure of atoms and the periodic table’, pp.25-27
50
Atomic Atomic radius radius Atomic radius when ENC
ENC depends on
1. Nuclear charge
2. Screening effect of electrons (repulsion among
electrons)
51
For the first 2 or 3 elements,
atomic radius more significantly because
nuclear charge sharply
52
Then, atomic radius less sharply because
screening effect is getting more important
53
Refer to notes on ‘Electronic structure of atoms and the periodic table’, pp.27-28
First ionization enthalpy First ionization enthalpy
54Variation in the first ionization enthalpy of
the first 20 elements
First ionization enthalpy First ionization enthalpy
55
Refer to notes on ‘Bonding and structure’, pp.2, 65
Electronegativity Electronegativity
56
Increases when atomic size
Decreases when atomic size
Electronegativity Electronegativity
57
Electronegativity cannot be measured directly
Not a physical properties
Electronegativity Electronegativity
58
Variation in electronegativity values of the first 20 elements
59
Periodic Relationship among the Oxides of the Elements Li
to Cl
60
Li2O BeO B2O3
CO CO2
N2O NON2O3 NO2
N2O4 N2O5
O2, O3 OF2
Na2O Na2O2
MgO Al2O3 SiO2
P4O6 P4O10
SO2 SO3
Cl2OClO2
Cl2O6
Cl2O7
1. Bonding and Stoichiometric Composition
Ionic and basic
Covalent and mainly acidic
Amphoteric
61
2. Reactions with water, acids and alkalisA. Ionic oxide (basic oxide)
Li2O and Na2O react vigorously with water to give alkaline solutions
Li2O(s) + H2O(l) 2Li+(aq) + 2OH(aq)Na2O(s) + H2O(l) 2Na+(aq) + 2OH(aq)
H
O
HO2-(aq) 2 OH-(aq)
stronger base weaker base
-
+
62
2. Reactions with water, acids and alkalisA. Ionic oxide (basic oxide)
MgO reacts slowly with water giving a slightly alkanline solution
MgO(s) + H2O(l) Mg(OH)2(s)
Mg(OH)2(s) Mg2+(aq) + 2OH(aq)High Lattice enthalpies
low solubility and slight dissociation
Equilibrium position lies on the left
63
2. Reactions with water, acids and alkalisA. Ionic oxide (basic oxide)
Na and K form more than one kind of oxides
Na2O sodium oxide
Na2O2 sodium peroxide
KO2 superoxide
64
2. Reactions with water, acids and alkalisA. Ionic oxide (basic oxide)
2Na2O2(s) + 2H2O(l) 4NaOH(aq) + O2(g)4KO2(s) + 2H2O(l) 4KOH(aq) + 3O2(g)
65
Q.3(i)
Na+ and K+ have small charge density (small charge and large size)
They cannot polarize or distort the unstable O2
2, O2
[O – O]2
66
Q.3(i)
Li+ has a high charge density
It is polarizing enough to distort the electron cloud of O2
2, causing it to decompose to give Li2O.
[O – O]2
Li+
Li+
Li+
Li+
2Li2O
67
Q.3(ii)Gp 2 ions has smaller size and greater charge
High charge density and polarizing power
Polarize more the electron cloud of O2
2
Gp 2A metals do not form peroxides
68
B. Ionic oxide with high covalent character (Amphoteric oxides)
BeO(s) + H2O(l) Al2O3(s) + H2O(l)
Do not dissolve nor react due to high lattice enthalpies of oxides
69
B. Ionic oxide with high covalent character (Amphoteric oxides)
BeO(s) + 2H+(aq) Be2+(aq) + H2O(l)
baseBeO(s) + 2OH(aq) + H2O(l) Be(OH)4
2(aq)
acid
Al2O3(s) + 6H+(aq) 2Al3+(aq) + 3H2O(l)
baseAl2O3(s) + 2OH(aq) + 3H2O(l) 2Al(OH)4
(aq)
acid
beryllate
aluminate
70
C. Covalent oxides (acidic oxides)
B2O3(s) + 3H2O(l) 2H3BO3(aq)
1. B2O3
B
O
B
OOB
OH
HO OH
Boric acid or orthoboric acid
71
H3BO3 are held by extensive intermolecular hydrogen bonds A solid at room conditions
72
B
O
H
OO
H
H
B OHO100oC
+ H2O(l)
Thermal dehydration of H3BO31. Intramolecular
HBO2 metaboric acid
73
Thermal dehydration of H3BO32. Intermolecular
160C
H2B4O7 tetraboric acid
74
Q.4
Borax(硼砂 ), Na2B4O7, is used as a preservative and in the making of borax glass.
Draw the structural formula of borax.
O B
O
O
B O B
O
O
B O NaNa
75
2. CO2, CO, SiO2
CO2 reacts with water to give a weak acid which ionizes in two steps to give a weakly acidic solution.
CO2(g) + H2O(l) H2CO3(aq)
H2CO3(aq) + H2O(l) H3O+(aq) + HCO3-(aq)
HCO3-(aq) + H2O(l) H3O+(aq) + CO3
2-(aq)
76
C
O
O
O
H
H
C O
O
O
H
H
+
+
+
-
-
-
H2CO3 carbonic acid
77
CO(g) + H2O(l) no reactionSiO2(s) + H2O(l) no reaction
SiO2 is insoluble in water due to the high lattice energy of the giant covalent structure
acidic
SiO2(l) + 2NaOH(l) Na2SiO3 + H2O
Sodium silicateWater glass
CO2(g) + 2NaOH(aq) Na2CO3 + H2O
Sodium carbonate
78
3. Oxides of nitrogen
N2O5(s) + H2O(l) 2HNO3(aq) N2O4(g) + H2O(l) HNO3(aq) + HNO2(aq)N2O3(g) + H2O(l) 2HNO2(aq)
Acid anhydride
s
Nitric acid
Nitrous acid
Nitric oxide (NO(g)) and nitrous oxide (N2O(g)) are neutral and insoluble in water
NO(g) + H2O(l) no reactionN2O(g) + H2O(l) no reaction
79
Q.5
N2O5(s) + H2O(l) 2HNO3(aq)
N2O4(g) + H2O(l) HNO3(aq) + HNO2(aq)
N2O3(g) + H2O(l) 2HNO2(aq)
+5
+5
+4
+5
+3
+3
+2, +4
O
N
O
N
O+3 +3
Not exist
O
N
N
O
O
+2+4
80
N
O
N
O O
OO
N2O5(l) dinitrogen pentoxide
N2O4(g) dinitrogen tetroxide
N N
OO
OO
O
N
O
NO2(g) nitrogen dioxide
81
N2O3(g) dinitrogen
trioxide
NO(g) nitric oxide
N2O(g) nitrous oxide
O
N
N
O
O
N O
N N O N N O0 +2 -1 +3
82
3. Oxides of phosphorus
P
O O
P
O
PO
O
PO
O
O
O
O
P
O O
P
O
PO
O
PO
P
P P
P
2O23O2
P4P4O6 P4O10
White phosphoru
s
White phosphorus burn spontaneously to relieved the angle strain
60~109 ~109
83
3. Oxides of phosphorus
P4O10 + 6H2O(l) 4H3PO4(aq)
P4O6 + 6H2O(l) 3H3PO4(aq) + PH3(g)
Reaction with water
Q.6
+5
+5
+5
-3+3
84
O
P
HOOH
OH
O
P
OHHO
HO
Thermal dehydration of H3PO4
2H3PO4 H2O + H4P2O7 (pyrophosphoric
acid)
250oC
(i) Intermolecular dehydration
O
P
OHOH
O
P
OHO
HO
250C
+ H2O
85
H3PO4 H2O + HPO3
(metaphosphoric acid)
(ii) Intramolecular dehydration
900oC
Thermal dehydration of H3PO4
O
P
OHHO
HOP OH
O
O
900C+ H2O
86
Q.8
Draw the structure of phosphorous acid, H3PO3
Dibasic acid
O
P
HOH
OH
What is the oxidation number of phosphorus in phosphorous acid?
0
+4
87
O
P
HOH
H
H3PO2
Hypophosphorous acid
Monobasic acid
+3
88
Formation of sodium salts
H3PO4 is tribasic 3 kinds of sodium salts
H3PO4 + NaOH H2O + NaH2PO4
sodium dihydrogenphosphate
NaH2PO4 + NaOH H2O + Na2HPO4
disodium hydrogenphosphate
Na2HPO4 + NaOH H2O + Na3PO4
sodium phosphate
89
Dehydration of sodium salts1. Inter-dehydration
2Na2HPO4 H2O + Na4P2O7
tetrasodium pyrophosphate
2. Intra-dehydration
NaH2PO4 H2O + NaPO3
sodium metaphosphate
90
O
P
HOO-Na+
O-Na+
O
P
OH+Na-O
+Na-O
O
P
O-Na+O-Na+
O
P
O+Na-O
+Na-O
Na4P2O7
O
P+Na-O
OHOH
P+Na-O
O
O
NaPO3
Q.9
91
4. Oxides of oxygen and sulphur
O2 is neutral and only slightly soluble in water
SO2(g) + H2O(l) H2SO3(aq)
H2SO3(aq) + H2O(l) H3O+(aq) + HSO3-(aq)
HSO3-(aq) + H2O(l) H3O+(aq) + SO3
2-(aq)
SO2 reacts with water to give sulphurous acid which ionizes in two steps to give a weakly acidic solution.
92
SO3 reacts with water to give sulphuric acid which ionizes in two steps to give a strongly acidic solution.
SO3(g) + H2O(l) H2SO4(aq)
H2SO4(aq) + H2O(l) H3O+(aq) + HSO4-(aq)
HSO4-(aq) + H2O(l) H3O+(aq) + SO4
2-(aq)
93
O S
O
O
O
H
H
+
+
+
O
S
OOH
OH
94
O
S
O
S
O OHOH
O
S
O O
O
S
OOH
OH
SO3 H2SO4
SO2H2SO3
95
O
F
F
H
O
H
+
+
+
-
-
-
5. Oxides of chlorine and fluorine*
* F2O is oxygen difluoride rather than difluorine monoxide
F2O may react slowly with water, giving oxygen gas and a slightly acidic solution.
F2O(g) + H2O(l) 2HF(aq) + O2(g)
2H–F + O=O
96
OCl
Cl
H
OH
+
+
+
+-
-
Cl2O(g) + H2O(l) 2HOCl(aq)
2H–O–Cl
O
F
F
H
O
H
+
+
+
-
-
-
2H–F + O=O
97
Cl2O(g) + H2O(l) 2HOCl(aq)
2ClO2(g) + H2O(l) HClO3(aq) + HClO2(aq)
Cl2O6(l) + H2O(l) HClO4(aq) + HClO3(aq)
Cl2O7(l) + H2O(l) 2HClO4(aq)
hypochlorous acid chloric(I)
acid
chlorous acid chloric(III) acid
chloric acid chloric(V) acid
perchloric acid chloric(VII) acid
+4 +5 +3
+6 +7 +5
+1 +1
+7+7
98
Cl
O
Cl O
Cl
OCl
O
O
O
Cl
O
O
O
Cl
O
O
O
O Cl
O
O
O
Cl2O ClO2 Cl2O6 Cl2O7
O
Cl
O
3-electron bond
99
Q.10
Cl
O
Cl O
Cl
Oa b
O
Cl
O
b
b > aRepulsion between a double bond and a 5-electron centre
Repulsion between two single bonds
>
The strong repulsion between two lone pairs in Cl2O decreases the bond angle a
100
Q.11
H
O
Cl HO
Cl
O
Cl
O
O OH
O
HOCl HClO2
HClO4HClO3
O
Cl
OOH
101
Q.12 HClO4 > HClO3 > HClO2
> HOCl
ClO ClO2 ClO3
ClO4
Average charge on
each OAttraction for H+
Ease of leaving of H+
Acid strength
-1 -½ - -¼
31
decreases
increases
increases
Explanation : -
HClOx ClOx +
H+
102
Q.12
HClO4 > HClO3 > HClO2 > HOCl
No. of O atoms bonded to Cl atom
Cl atom becomes more positively charged
better electron pair acceptor
stronger Lewis acid
103
The END
104
The atomic numbers of tellurium and iodine are 52 and 53 respectively. Why is tellurium heavier than
iodine?Answer
Atomic number of an element is not related to the mass of an atom
of the element. The atomic number of an element is the number of
protons in an atom of the element. It is unique for each element. The
mass of an atom of the element is mainly determined by the number
of protons and neutrons in the nucleus. Therefore, tellurium is
heavier than iodine though the atomic number of tellurium is smaller
than that of iodine.Back
38.1 The Periodic Table (SB p.3)
105
To which block (s-, p-, d- or f-) in the Periodic Table do rubidium, gold, astatine and uranium belong respectively? AnswerRubidium: s-block
Gold: d-block
Astatine: p-block
Uranium: f-block
Back
38.1 The Periodic Table (SB p.5)
106
Which element would have the highest first ionization enthalpy?
AnswerHelium
Back
38.2 Periodic Variation in Physical Properties of Elements (SB p.6)
107
Which element would have the smallest atomic radius?
AnswerHelium
Back
38.2 Periodic Variation in Physical Properties of Elements (SB p.8)
108
Why is the melting point of chlorine higher than argon?
AnswerChlorine atom has a higher effective nuclear charge than argon atom,
so the atomic radius of chlorine is smaller than that of argon.
Therefore, the van der Waals’ forces between chlorine molecules are
stronger than those between argon molecules. Since a higher amount
of energy is needed to overcome the stronger van der Waals’ forces,
the melting point of chlorine is higher than that of argon.
Back
38.2 Periodic Variation in Physical Properties of Elements (SB p.12)
109
Considering the trend of atomic radius in the Periodic Table, arrange the elements Si, N and P in the order of increasing atomic radius. Explain your answer briefly.
AnswerIn the Periodic Table, N is above P in Group VA. As the atomic radius increases down a group, the atomic radius of N is smaller than that of P.
Si and P belong to the same period. Since the atomic radius decreases across a period, the atomic radius of P is smaller than that of Si.
Therefore, the atomic radius increases in the order: N < P < Si.
Back
38.2 Periodic Variation in Physical Properties of Elements (SB p.13)
110
(a) With the help of the Periodic Table only, arrange the elements selenium, sulphur and argon in the order of increasing first ionization enthalpies. Answer
(a) The first ionization enthalpy increases in the order: Se < S < Ar.
38.2 Periodic Variation in Physical Properties of Elements (SB p.13)
111
(b) Describe and explain the general periodic trend of atomic radius of elements in the Periodic Table. Answer
38.2 Periodic Variation in Physical Properties of Elements (SB p.13)
112
38.2 Periodic Variation in Physical Properties of Elements (SB p.13)
(b) Within a given period, the atomic radii decrease progressively with
increasing atomic numbers. This is because an increase in atomic
number by one means that one more electron and one more proton
are added in the atom. The additional electron would cause an
increase in repulsion between the electrons in the outermost shell and
results in an increase in atomic radius. The additional proton in the
nucleus would cause the electrons to experience greater attractive
forces from the nucleus. Due to the fact that the newly added electron
goes to the outermost shell and is at approximately the same distance
from the nucleus, the repulsion between the electrons is relatively
ineffective to cause an increase in atomic radius. Therefore, the effect
of increasing nuclear charge outweighs the effect of repulsion between
the electrons. That means, there is an increase in effective nuclear
charge. As a result, the atomic radii of elements decrease across a
period.
113
(c) With reference to Fig. 38-9 on p.11 (variation in electronegativity value of the first 20 elements), explain why the alkali metals are almost at the bottom of the troughs, whereas the halogens are at the peaks of the plot. Answer
38.2 Periodic Variation in Physical Properties of Elements (SB p.13)
114
(c) The alkali metals are almost at the bottom of troughs, indicating that
they have low electronegativity values. It is because their nuclear
charge is effectively shielded by the fully-filled inner electron shells
of electrons, and the bonding electrons are attracted less strongly.
On the other hand, the halogens appear at the peaks. This
indicates that they have high electronegativity values. It is because
they have one electron less than the octet electronic configuration.
They tend to attract an electron to complete the octet, and the
bonding electrons are attracted strongly.
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38.2 Periodic Variation in Physical Properties of Elements (SB p.13)
115
(a) To which type of oxide does each of the following oxides belong?
(i) Magnesium oxide
(ii) Nitrogen monoxide
(iii) Silicon dioxide
(iv) Aluminium oxide Answer(a) (i) Ionic oxide
(ii) Covalent oxide
(iii) Covalent oxide
(iv) Ionic oxide with covalent
character
39.1 Bonding of the Oxides of Periods 2 and 3 Elements (SB p.22)
116
(b) Carbon can form two oxides. Name the two oxides and draw their electronic structures.
Answer
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39.1 Bonding of the Oxides of Periods 2 and 3 Elements (SB p.22)
(b) Carbon monoxide (CO):
Carbon dioxide (CO2):
117
(a) Why does silicon(IV) oxide not react with water?
Answer(a) Silicon(IV) oxide does not react with water
because the electronegativity values of silicon
and oxygen are very similar. The Si — O bond
can be considered as nonpolar, so there is no
positive centre for the lone pair electrons of the
water molecule to attack.
39.2 Behaviour of Oxides of Periods 2 and 3 Elements in Water, Dilute Acids and Dilute Alkalis (SB p.27)
118
(b) Complete and balance the following equations:
(i) K2O(s) + H2O(l)
(ii) Na2O2(s) + HCl(aq)
(iii) Al2O3(s) + H2SO4(aq)
(iv) P4O10(s) + NaOH(aq)
(v) SO3(g) + NaOH(aq) Answer
39.2 Behaviour of Oxides of Periods 2 and 3 Elements in Water, Dilute Acids and Dilute Alkalis (SB p.27)
119
(b) (i) K2O(s) + H2O(l) 2KOH(aq)
(ii) Na2O2(s) + 2HCl(aq) 2NaCl(aq) + H2O2(aq)
(iii) Al2O3(s) + 3H2SO4(aq) Al2(SO4)3(aq) + 3H2O(l )
(iv) P4O10(s) + 12NaOH(aq) 4Na3PO4(aq) + 6H2O(l)
(v) SO3(g) + 2NaOH(aq) Na2SO4(aq) + H2O(l)
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39.2 Behaviour of Oxides of Periods 2 and 3 Elements in Water, Dilute Acids and Dilute Alkalis (SB p.27)