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39.1
39.2
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39.1 The Periodic Table (SB p.2)
Modern Periodic Table
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The elements are arranged in the order of atomic number
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39.1 The Periodic Table (SB p.3)
•
Elements were first arranged in order of increasing atomic masses by Dimitri Mendeleev (1834 - 1907)
•
The elements were observed to repeat their properties periodically
(a)
Periodic Table (early forms)
(b)
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(a) A portion of one of Dimitri Mendeleev’s handwritten drafts of the
Periodic Table (b) Dimitri Mendeleev’s Periodic Table in 1872
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39.1 The Periodic Table (SB p.3)
Modern periodic table:
Rows
 periods
•
Classified into 4 areas :
Columns
 groups
4 p -block d -block transition elements f -block inner transition elements
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39.1 The Periodic Table (SB p.4) s-block elements:
Group IA: alkali metals
1 e
– in outermost shell
(e.g. Li, Na, K)
Group IIA: alkaline earth metals
2 e
– in the outermost shell
(e.g. Be, Mg, Ca)
p-block elements:
Groups IIIA, IVA, VA, VIA, VIIA, 0
Group VIIA : halogens
Group 0 : noble gases
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Representative elements
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39.1 The Periodic Table (SB p.4)
d-block elements:
Electronic configuration : [ ](n – 1) d 1 n s 2 to [ ](n – 1) d 10 n s 2
(Group IIIB) (Group IIB)
Transition elements
f-block elements:
Lanthanide series and actinide series :
4 f and 5 f orbitals are filled up with 1 to 14 e inner-transition elements
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39.1 The Periodic Table (SB p.5)
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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39.2 Periodic Variation in Physical Properties of Elements (SB p.5)
The following periodic variations of the elements will be discussed:
Atomic Radius
First Ionization Enthalpy
Electronegativity
Structure and Bonding
Melting Point
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39.2 Periodic Variation in Physical Properties of Elements (SB p.5)
Atomic Radius How can scientists measure the sizes of atoms?
(1) For non-metals
, ‘atomic radius’ refers to the ‘covalent
radius’ :
Covalent radius is defined as half the internuclear distance between two covalently bonded atoms in a molecule of the element.
(2) For metals
, ‘atomic radius’ refers to the ‘metallic radius’
:
Metallic radius is defined as half the internuclear distance between atoms in a metallic crystal .
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39.2 Periodic Variation in Physical Properties of Elements (SB p.5)
The atomic radius is governed by two factors :
(1) Attraction of the nucleus for the electrons
More proton
 greater attraction to e
–  small atomic radius
(2) Screening of the outer electrons from the nucleus by the inner electron shells
Repulsion between e
– outermost shell e
–
 outermost shell e
– screen by inner e
–  less strongly held
 larger atomic radius
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39.2 Periodic Variation in Physical Properties of Elements (SB p.6)
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The atomic radii of the first 20 elements
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39.2 Periodic Variation in Physical Properties of Elements (SB p.6)
Variation in atomic radius of the first 20 elements
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Across the period, the atomic radii decrease progressively
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39.2 Periodic Variation in Physical Properties of Elements (SB p.7)
First Ionization Enthalpy
X(g)

X + (g) + e
–
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The first ionization enthalpies of the first 20 elements
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39.2 Periodic Variation in Physical Properties of Elements (SB p.7)
Four factors affecting the magnitude of ionization enthalpy:
•
Electronic configuration
• Nuclear charge
• Screening effect
• Atomic radius
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39.2 Periodic Variation in Physical Properties of Elements (SB p.8)
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Variation in the first ionization enthalpy of the first 20 elements
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39.2 Periodic Variation in Physical Properties of Elements (SB p.8)
1.
General increase across periods 2 and 3
∵ increase in nuclear charge with increasing atomic numbers
2.
Irregularities with the general increase
Peaks in the general increase due to the extra stability provided by full-filled s sub-shell and half-filled p subshell
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39.2 Periodic Variation in Physical Properties of Elements (SB p.9)
3.
Sharp drop from one period to the next
∵ The electronic configuration of the element at the end of period is very stable (completely filled octet)
4.
Decreases on going down any group in the periodic table
Increase in the number of inner electron shells provides better shielding for the outermost shell electrons
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39.2 Periodic Variation in Physical Properties of Elements (SB p.9)
Electronegativity
•
Electronegativity is the measure of the relative tendency of an atom to attract bond pair(s) of electrons towards itself in a covalent bond
•
Electronegativity values on an arbitrary scale from 0 to 4
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39.2 Periodic Variation in Physical Properties of Elements (SB p.10)
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Electronegativity values of the first 20 elements
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39.2 Periodic Variation in Physical Properties of Elements (SB p.10)
Variation in electronegativity value of the first 20 elements • Across the period, electronegativity increases from left to right
•
Down the group, electronegativity decreases
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39.2 Periodic Variation in Physical Properties of Elements (SB p.11)
Structure and Bonding
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39.2 Periodic Variation in Physical Properties of Elements (SB p.11)
The structure and bonding vary from left to right
•
Structure
Giant metallic structure
 giant covalent structure
 simple molecular structure
 atomic structure
•
Bonding
Metallic bonding
 covalent bonding
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39.2 Periodic Variation in Physical Properties of Elements (SB p.11)
Melting point
• Melting point of a substance is the temperature at which a transition from its solid phase to liquid phase
• Temperature depends on the magnitude of bonding
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39.2 Periodic Variation in Physical Properties of Elements (SB p.11)
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The melting points of the first 20 elements
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39.2 Periodic Variation in Physical Properties of Elements (SB p.12)
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Variation in melting point of the first 20 elements
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39.2 Periodic Variation in Physical Properties of Elements (SB p.12)
1.
Steady increase in melting point from Li to B and Na to
Al
∵ metallic bond strength increase as no. of outermost shell electrons increase . B has a giant covalent structure , thus the melting point is much higher
2.
Carbon and silicon correspond to the maxima in Periods
2 and 3
∵ both have giant covalent structures . Atoms are held together by strong covalent bonds
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39.2 Periodic Variation in Physical Properties of Elements (SB p.13)
3.
The melting points of elements from N to Ne and P to
Ar are relatively low
∵ they exist as discrete molecules which held by weak van der Waals’ forces
4.
Sharp drop in melting point from C to N and Si to P
∵ dramatic change in molecular structure (giant covalent structure to discrete molecules held by weak van der Waals’ forces )
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39.2 Periodic Variation in Physical Properties of Elements (SB p.13)
• S has a higher melting point than P
∵ S exists as S
8 molecules in its molecular crystal whereas P exists as P
4 molecules in its molecular crystal
• As S
8 has a higher molecular mass and larger surface area for contact with other molecules , the melting point of S is higher than that of P
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39.2 Periodic Variation in Physical Properties of Elements (SB p.14)
By considering the trend of atomic radius in the Periodic
Table. Arrange the elements Si, N and P in order of increasing atomic radii. Explain your answer briefly.
In the Periodic Table, N is above P in Group VA. As the atomic radii increase down a group, the atomic radius of N is smaller than that of P.
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Si and P belong to the same period. Since the atomic radii decrease across a period, the atomic radius of P is smaller than that of Si.
Answer
Hence, the atomic radii increase in the order: N < P < Si
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39.1 The Periodic Table (SB p.14)
(a) Describe and explain the general periodic trend of atomic radius.
(a) Going across a period, the atomic radii progressively
Answer increase in atomic number by one means one more electron and one more proton in the atom. Since the additional electron goes to the same shell and is at approximately the same distance from the nucleus, it is only weakly shielded by other electrons in that shell from the attraction of the nucleus. On the other hand, as there is an additional proton added to the nucleus, the electrons will experience a greater attractive force from the nucleus. Hence, going across a period, there is an increase in effective nuclear charge and a reduction in atomic radius is resulted.
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39.1 The Periodic Table (SB p.14)
(b) With the help of the Periodic Table only, arrange the elements Se, S and Ar in an order of increasing first ionization enthalpies.
Answer
(b) The first ionization enthalpy increases in the order:
Se < S < Ar
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39.1 The Periodic Table (SB p.14)
(c) With reference to Fig. 39-9 on p.10 (variation in electronegativity value of the first 20 elements), explain why the alkali metals are almost at the bottom of the
(c) The fact that the alkali metals are almost at the bottom of troughs, whereas the halogens appear at the peaks.
troughs indicates that they have low electronegativity values. It is because they have their nuclear charge effectively shielded by the fully-filled inner shell of electrons and their electron-attracting ability is low.
Answer
On the other hand, the halogens appear at the peaks, i.e. 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 hence their electron-attracting ability is high.
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