Training Presentation

1
Periodic Variation in Physical Properties of the
Elements H to Ar
38.1 The Periodic Table 38.2 Periodic Variation
in Physical Properties of Elements
2
The Periodic Table
3
38.1 The Periodic Table (SB p.2)
The Periodic Table

• With more and more elements being discovered
• ? needed a way to organize them effectively

4
38.1 The Periodic Table (SB p.2)
The Periodic Table

• The modern Periodic Table
• ? the basis of the atomic numbers and
  electronic configurations of element

5
38.1 The Periodic Table (SB p.2)
The modern Periodic Table
6
38.1 The Periodic Table (SB p.3)
The Periodic Table

• The earliest version of the Periodic Table
• ? introduced in 1869
• ? by a Russian chemist called Dimitri
  Mendeleev

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38.1 The Periodic Table (SB p.3)
A portion of one of Dimitri Mendeleevs
handwritten drafts of the Periodic Table
8
38.1 The Periodic Table (SB p.3)
Dimitri Mendeleevs Periodic Table in 1872
9
38.1 The Periodic Table (SB p.3)
The Periodic Table

• Mendeleev created the first Periodic Table based
  on atomic masses
• Many elements had similar properties
• ? occurred periodically
• ? the name Periodic Table was used

10
38.1 The Periodic Table (SB p.3)
The Periodic Table

• The periodic law stated
• ? the chemical and physical properties of the
  elements vary in a periodic way with their
  atomic masses

11
38.1 The Periodic Table (SB p.3)
The Periodic Table

• Example
• Lithium, sodium, potassium, rubidium and caesium
• ? have similar chemical properties

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38.1 The Periodic Table (SB p.3)
The Periodic Table

• Example
• Beryllium, magnesium, calcium, strontium and
  barium
• ? also have similar chemical properties

13
38.1 The Periodic Table (SB p.3)
The Periodic Table

• According to Mendeleevs theory
• ? they could be perfectly arranged by
  increasing atomic masses
• Some elements did not match perfectly

14
38.1 The Periodic Table (SB p.3)
The Periodic Table

• Tellurium is heavier than iodine
• ? but the chemical properties of tellurium
  did not match with those of chlorine and
  bromine
• ? the chemical properties of iodine did not
  match with those of sulphur and selenium

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38.1 The Periodic Table (SB p.3)
The Periodic Table

• Tellurium should be placed before iodine
• ? even though tellurium was heavier than
  iodine

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38.1 The Periodic Table (SB p.3)
The Periodic Table

• The modern Periodic Table
• ? arranged according to atomic numbers
  instead of atomic masses

17
38.1 The Periodic Table (SB p.4)
The Periodic Table

• The modern Periodic Table is divided into
• ? 7 horizontal rows called periods
• ? 18 vertical columns called groups

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38.1 The Periodic Table (SB p.4)
The Periodic Table

• Elements with atoms having the same number of
  electron shells
• ? put in the same period
• Elements having the same number of outermost
  shell electrons
• ? put in the same group

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38.1 The Periodic Table (SB p.4)
The Periodic Table

• Elements can be classified as
• ? s-block elements
• ? p-block elements
• ? d-block elements
• ? f-block elements

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38.1 The Periodic Table (SB p.4)
1. s -Block Elements

• Group IA and Group IIA elements constitute the
  s-block
• They are elements with outermost shell electrons
  occupying the s orbital

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38.1 The Periodic Table (SB p.4)
1. s -Block Elements

• Group IA elements have only one outermost shell
  electron occupying the s orbital
• Examples
• Lithium, sodium, potassium, rubidium, caesium
  and francium

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38.1 The Periodic Table (SB p.4)
1. s -Block Elements

• They are highly reactive metals
• They are known as the alkali metals

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38.1 The Periodic Table (SB p.4)
1. s -Block Elements

• Group IIA elements have two outermost shell
  electrons in the s orbital
• Example
• Beryllium, magnesium, calcium, strontium, barium
  and radium

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38.1 The Periodic Table (SB p.4)
1. s -Block Elements

• They are also chemically reactive
• They are known as the alkaline earth metals

25
38.1 The Periodic Table (SB p.4)
2. p -Block Elements

• Elements having electronic configurations from
  ns2np1 to ns2np6
• They include Group IIIA, IVA, VA, VIA, VIIA and 0

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38.1 The Periodic Table (SB p.4)
2. p -Block Elements

• Group VIIA elements are all non-metals
• They are known as the halogens

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38.1 The Periodic Table (SB p.4)
2. p -Block Elements

• Group 0 elements are called noble gases
• They have a fully-filled outermost electron shell
• ? gives rise to extra stability
• ? the very stable electronic configuration

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38.1 The Periodic Table (SB p.4)
2. p -Block Elements

• s-Block and p-block elements together are also
  known as representative elements

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38.1 The Periodic Table (SB p.4)
3. d -Block Elements

• Elements with electronic configurations from
  (n 1)d1ns2 (Group IIIB) to (n 1)d10ns2
  (Group IIB)
• They are also called transition elements

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38.1 The Periodic Table (SB p.4)
4. f -Block Elements

• Two series of f-block elements in which the 4f
  and 5f orbitals being filled up with 1 to 14
  electrons respectively
• They are the lanthanide series and the actinide
  series
• They are sometimes called inner-transition
  elements

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38.1 The Periodic Table (SB p.5)
Elements can be classified as s-block elements,
p-block elements, d-block elements and f-block
elements in the Periodic Table
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38.1 The Periodic Table (SB p.5)
33
38.2 Periodic Variation in Physical Properties
of Elements (SB p.6)
First ionization enthalpy
The first ionization enthalpy of an atom is the
energy required to remove one mole of electrons
from one mole of its gaseous atoms to form one
mole of gaseous ions with one positive charge.
34
38.2 Periodic Variation in Physical Properties
of Elements (SB p.6)
First ionization enthalpy

• Energy is required
• ? overcome the attractive forces between the
  nucleus and the electron to be removed
• ? the ionization enthalpy always has a
  positive value

35
38.2 Periodic Variation in Physical Properties
of Elements (SB p.6)
First ionization enthalpy

• The ionization enthalpy of an element
• ? reflects the relative force of attraction
  between the nucleus and the electron being
  removed

36
38.2 Periodic Variation in Physical Properties
of Elements (SB p.6)
First ionization enthalpy

• Four main factors affecting the magnitude of the
  ionization enthalpy of an atom
• 1. the electronic configuration of an atom
• 2. the nuclear charge
• 3. the screening effect and
• 4. the atomic radius

37
38.2 Periodic Variation in Physical Properties
of Elements (SB p.6)
The first ionization enthalpies of the first 20
elements
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38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
Variation in the first ionization enthalpy of the
first 20 elements
39
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
1. General increase in the first ionization
enthalpy across both Periods 2 and 3

• The consequence of the increase in nuclear charge
  with atomic numbers

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38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
1. General increase in the first ionization
enthalpy across both Periods 2 and 3

• At the same time
• ? additional electrons are entering the same
  electron shell
• ? they have poor screening effect

41
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
1. General increase in the first ionization
enthalpy across both Periods 2 and 3

• In other words
• ? an increase in effective nuclear charge
  across the periods

42
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
1. General increase in the first ionization
enthalpy across both Periods 2 and 3

• Going across a period
• ? the electrons are drawn closer to the
  nucleus
• ? more energy is required to remove an
  electron from the atom
• ? the first ionization enthalpy generally
  increases across both Periods 2 and 3

43
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3

• In Period 2
• ? the first ionization enthalpy of boron is
  lower than that of beryllium

44
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3

• In Period 3
• ? the first ionization enthalpy of aluminium
  is lower than that of magnesium

45
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3

• Boron and aluminium have ns2np1 electronic
  configurations
• ? easier to remove the outermost p electron

46
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3
? the electron is shielded from the attraction of
the nucleus by the completely filled s orbitals
(ns2) ? The first ionization enthalpies of Group
III elements are not very high
47
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3

• Beryllium and magnesium have a relatively stable
  electronic configuration
• ? the s orbital is completely filled
• ? a relatively large amount of energy is
  needed to ionize their atoms

48
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3

• In Period 2
• ? the first ionization enthalpy of oxygen is
  lower than that of nitrogen

49
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3

• In Period 3
• ? the first ionization enthalpy of sulphur is
  lower than that of phosphorus

50
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3

• The atoms of oxygen and sulphur have one electron
  more than the half-filled p sub-shell
• ? when the electronic configuration of
  half-filled p sub-shell (np3) is attained
• ? extra stability is gained

51
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3

• A relatively small amount of energy is required
  to remove the first electron from the atoms of
  oxygen and sulphur

52
38.2 Periodic Variation in Physical Properties
of Elements (SB p.7)
2. Irregularities with general increase in the
first ionization enthalpy across both Periods 2
and 3

• The electronic configurations of nitrogen and
  phosphorus are ns2np3 (i.e. half-filled p
  sub-shell)
• ? a relatively stable electronic configuration
• ? more energy is required to remove an
  electron from their atoms

53
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
3. A sharp drop in the first ionization enthalpy
from one period to the next

• The element at the end of each period (i.e. the
  noble gas)
• ? has a completely filled octet (except helium
  which has a duplet)
• ? this electronic configuration is very stable

54
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
3. A sharp drop in the first ionization enthalpy
from one period to the next
? A large amount of energy is required to
remove an electron from their atoms
55
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
3. A sharp drop in the first ionization enthalpy
from one period to the next

• The element at the beginning of the next period
  (i.e. the Group I element)
• ? has an electron entering a new electron
  shell
• ? further away from the nucleus

56
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
3. A sharp drop in the first ionization enthalpy
from one period to the next
? The attractive force between the nucleus and
the electron is relatively weak
57
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
3. A sharp drop in the first ionization enthalpy
from one period to the next

• This s electron is shielded from the attraction
  of the nucleus effectively by the inner electron
  shells
• ? once this electron is removed, a stable
  electronic configuration is attained

58
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
3. A sharp drop in the first ionization enthalpy
from one period to the next
? The first ionization enthalpies of Group I
elements are relatively low
59
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
4. The first ionization enthalpy decreases down
any group in the Periodic Table

• When going down a group
• ? increase in atomic radius
• ? the outermost shell electrons will
  experience less attraction from the nucleus

60
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
4. The first ionization enthalpy decreases down
any group in the Periodic Table

• There is an increase in the nuclear charge down a
  group
• ? the outermost shell electrons would
  experience less attraction from the
  positively charged nucleus
• ? the first ionization enthalpy decreases down
  a group

61
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
Atomic radius
Atomic radius is used to describe the size of an
atom.
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38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
Atomic radius

• For non-metals
• ? the atomic radii commonly used are the
  covalent radii

63
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
Atomic radius

• For metals
• ? the metallic radii are used

64
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
Atomic radius
Covalent radius is defined as half the
internuclear distance between two covalently
bonded atoms in a molecule of the element.
65
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
Atomic radius
Metallic radius is defined as half the
internuclear distance between two atoms in a
metallic crystal.
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38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
Atomic radius

• The atomic radius of an atom is governed by two
  factors
• 1. Attraction between the nucleus and the
  electrons
• 2. Screening of the outermost shell electrons
  from the nucleus by inner electron shells

67
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
1. Attraction between the nucleus and the
electrons

• The greater the number of protons in the nucleus
• ? the higher the nuclear charge

68

38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
1. Attraction between the nucleus and the
electrons

• This results in greater attraction between the
  nucleus and the electrons
• ? the electrons are drawn closer to the
  nucleus
• ? the atomic radius becomes smaller

69
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
2. Screening of the outermost shell electrons
from the nucleus by inner electron shells

• As electrons are negatively charged
• ? repulsion between the outermost shell
  electrons and the electrons on the inner
  shells of an atom
• ? the outermost shell electrons are screened
  from the attraction of the nucleus

70
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
2. Screening of the outermost shell electrons
from the nucleus by inner electron shells

• The greater the number of electron shells in the
  atom
• ? the greater the screening effect

71
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
2. Screening of the outermost shell electrons
from the nucleus by inner electron shells

• The outermost shell electrons are less strongly
  held by the nucleus
• ? the atomic radius becomes larger

72
38.2 Periodic Variation in Physical Properties
of Elements (SB p.9)
The atomic radii of the first 20 elements
73
38.2 Periodic Variation in Physical Properties
of Elements (SB p.9)
Variation in atomic radius of the first 20
elements
74
38.2 Periodic Variation in Physical Properties
of Elements (SB p.9)
Atomic radius

• Within a given period
• ? the atomic radii decrease progressively with
  increasing atomic numbers
• ? an increase in atomic number by one means
  that one more electron and one more proton are
  added in the atom

75
38.2 Periodic Variation in Physical Properties
of Elements (SB p.9)
Atomic radius

• The additional electron
• ? cause an increase in repulsion between the
  electrons in the outermost shell
• ? results in an increase in atomic radius

76
38.2 Periodic Variation in Physical Properties
of Elements (SB p.9)
Atomic radius

• The additional proton in the nucleus
• ? cause the electrons to experience greater
  attractive forces from the nucleus

77
38.2 Periodic Variation in Physical Properties
of Elements (SB p.9)
Atomic radius

• The newly added electron
• ? goes to the outermost shell
• ? is at approximately the same distance from
  the nucleus
• ? the repulsion between the electrons is
  relatively ineffective to cause an increase in
  atomic radius

78
38.2 Periodic Variation in Physical Properties
of Elements (SB p.9)
Atomic radius

• The effect of increasing nuclear charge outweighs
  the effect of repulsion between the electrons
• ? an increase in effective nuclear charge
• ? the atomic radii of elements decrease across
  a period

79
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Atomic radius

• If we look closer
• ? sharp decrease in atomic radius from the
  first element to the third element of each
  period
• ? followed by a gradual decrease along
  subsequent elements

80
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Atomic radius

• At the beginning of each period
• ? increasing effective nuclear charge with
  atomic numbers predominates
• ? greater contraction of the electron cloud

81
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Atomic radius

• When more electrons are added to the same
  electron shell
• ? the effect of repulsion between electrons
  becomes more significant
• ? the effective nuclear charge increases
  only slowly towards the end of the period
• ? the decrease in atomic radius is thus smaller

82
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Atomic radius

• Going down a group in the Periodic Table
• ? the atoms have more electron shells occupied
• ? the outermost electron shells become further
  away from the nucleus

83
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Atomic radius

• The outermost shell electrons
• ? more effectively shielded by the inner
  electron shells from the nuclear charge
• ? decrease in the attractive force between
  the nucleus and the outermost shell electrons
• ? the atomic radii of elements increase down a
  group

84
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Electronegativity
Electronegativity is the relative tendency of an
atom to attract bonding electrons towards itself
in a covalent bond.
85
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Electronegativity

• Pauling assigned electronegativity values to the
  elements on an arbitrary scale from 0 to 4

86
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Electronegativity

• The higher the electronegativity value of an atom
• ? the higher the ability of the atom to
  attract bonding electrons towards itself in
  a covalent bond

87
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Electronegativity

• Fluorine
• ? the most electronegative element
• ? assigned an electronegativity value of 4.0
  in Paulings scale

88
38.2 Periodic Variation in Physical Properties
of Elements (SB p.10)
Electronegativity values of the first 20 elements
89
38.2 Periodic Variation in Physical Properties
of Elements (SB p.11)
Variation in electronegativity values of the
first 20 elements
90
38.2 Periodic Variation in Physical Properties
of Elements (SB p.11)
Electronegativity

• Going across Periods 2 and 3 in the Periodic
  Table
• ? electronegativity of the elements increases
  from left to right
• ? the decrease in atomic size

91
38.2 Periodic Variation in Physical Properties
of Elements (SB p.11)
Electronegativity

• As the effect of increasing nuclear charge
  outweighs the screening effect of the electrons
  in the same electron shell
• ? the bonding electrons are attracted more
  strongly

92
38.2 Periodic Variation in Physical Properties
of Elements (SB p.11)
Electronegativity

• Moving down a group in the Periodic Table
• ? electronegativity of the elements decreases
• ? the increase in atomic size

93
38.2 Periodic Variation in Physical Properties
of Elements (SB p.11)
Electronegativity

• With the increase in number of electron shells
  and greater screening effect
• ? the bonding electrons are attracted less
  strongly

94
38.2 Periodic Variation in Physical Properties
of Elements (SB p.11)
Melting point
The melting point of a substance is the
temperature at which the substance changes from
its solid phase to liquid phase.
95
38.2 Periodic Variation in Physical Properties
of Elements (SB p.11)
Melting point

• A solid does not melt
• ? unless there is sufficient energy to
  overcome the forces holding the particles
  together in the solid state

96
38.2 Periodic Variation in Physical Properties
of Elements (SB p.11)
Melting point

• The amount of energy depends on
• 1. the magnitude of the attractive forces
  between the particles
• 2. how the particles are arranged in the solid

97
38.2 Periodic Variation in Physical Properties
of Elements (SB p.11)
The melting points of the first 20 elements
98
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
Variation in melting point of the first 20
elements
99
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
1. A steady increase in melting point from
lithium to boron and from sodium to aluminium

• Both lithium and beryllium have a giant metallic
  structure
• The metallic bond strength increases with the
  number of outermost shell electrons

100
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
1. A steady increase in melting point from
lithium to boron and from sodium to aluminium

• As lithium has only one outermost shell
  electron while beryllium has two
• ? the metallic bond strength in beryllium is
  stronger than that in lithium
• ? beryllium has a higher melting point than
  lithium

101
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
1. A steady increase in melting point from
lithium to boron and from sodium to aluminium

• Boron has a giant covalent structure
• The bonding that holds the boron atoms together
  is stronger than those of lithium and beryllium
• ? the melting point of boron is higher than
  those of lithium and beryllium

102
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
1. A steady increase in melting point from
lithium to boron and from sodium to aluminium

• For Period 3 elements
• ? sodium, magnesium and aluminium all have a
  giant metallic structure

103
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
1. A steady increase in melting point from
lithium to boron and from sodium to aluminium

• There is an increase in number of electrons
  involved in the metallic bond
• ? the strength of metallic bond increases from
  sodium to aluminium
• ? the melting point increases from sodium to
  aluminium

104
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
2. Carbon and silicon correspond to the maxima in
Periods 2 and 3 respectively

• Both carbon and silicon have a giant covalent
  structure
• ? the atoms are held together by strong
  covalent bonds

105
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
2. Carbon and silicon correspond to the maxima in
Periods 2 and 3 respectively

• A large amount of energy is needed to overcome
  the strong covalent bonds
• ? the melting points of carbon and silicon are
  extremely high

106
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
3. The melting points of the elements from
nitrogen to neon and from phosphorus to argon are
relatively low

• They all exist as discrete molecules
• ? held together by weak van der Waals forces

107
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
3. The melting points of the elements from
nitrogen to neon and from phosphorus to argon are
relatively low

• Only a little amount of energy is needed to
  overcome the weak van der Waals forces
• ? their melting points are relatively low

108
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
Melting point

• In Period 3
• ? sulphur exists as S8 molecules in its
  molecular crystal
• ? phosphorus exists as P4 molecules in its
  solid molecular crystal

109
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
Melting point

• S8 molecule
• ? a higher molecular mass
• ? a greater surface area for contact with
  neighbouring molecules

110
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
Melting point

• The van der Waals forces between S8 molecules
  are stronger than those between P4 molecules
• ? the melting point of sulphur is higher than
  that of phosphorus

111
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
Melting point

• Sulphur
• ? higher melting point than chlorine

112
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
Melting point

• Chlorine
• ? only exists as diatomic molecules
• ? the van der Waals forces between S8
  molecules are stronger than those between Cl2
  molecules

113
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
114
38.2 Periodic Variation in Physical Properties
of Elements (SB p.14)
Structure and Bonding
A summary of the variations in structure and
bonding of elements across both Periods 2
115
38.2 Periodic Variation in Physical Properties
of Elements (SB p.14)
Structure and Bonding
A summary of the variations in structure and
bonding of elements across both Periods 3
116
38.2 Periodic Variation in Physical Properties
of Elements (SB p.14)
Structure and Bonding

• In each of the periods, the structures of the
  elements changes from
• ? giant metallic structures
• ? followed by giant covalent structures
• ? finally to simple molecular structures

117
38.2 Periodic Variation in Physical Properties
of Elements (SB p.14)
Structure and Bonding

• Going across the periods from left to right
• ? the bonding of the elements also varies in
  a repeating pattern
• ? from metallic bonding to covalent bonding

118
The END
119
38.1 The Periodic Table (SB p.3)
Let's Think 1
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
120
38.1 The Periodic Table (SB p.5)
Check Point 38-1
To which block (s-, p-, d- or f-) in the Periodic
Table do rubidium, gold, astatine and uranium
belong respectively?
Answer
Rubidium s-block Gold d-block Astatine
p-block Uranium f-block
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121
38.2 Periodic Variation in Physical Properties
of Elements (SB p.6)
Let's Think 2
Which element would have the highest first
ionization enthalpy?
Answer
Helium
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122
38.2 Periodic Variation in Physical Properties
of Elements (SB p.8)
Let's Think 3
Which element would have the smallest atomic
radius?
Answer
Helium
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123
38.2 Periodic Variation in Physical Properties
of Elements (SB p.12)
Let's Think 4
Why is the melting point of chlorine higher than
argon?
Answer
Chlorine 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.
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124
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
Example 38-1
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.
Answer
In 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 lt P lt Si.
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125
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
Check Point 38-2
(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 lt S lt Ar.
126
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
Check Point 38-2
(b) Describe and explain the general periodic
trend of atomic radius of elements in the
Periodic Table.
Answer
127
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.
128
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
Check Point 38-2
(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
129
38.2 Periodic Variation in Physical Properties
of Elements (SB p.13)
(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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