Unit 1 Module 3 The Periodic Table - Pearson Schools

The trend in boiling points across each period is shown in Table 1.
Group 1 to Group 4 Group 4 to Group 5 Group 5 to Group 0
Li → C C → N N → Ne
Na → Si Si → P P → Ar
general increase
in boiling points
sharp decrease
in boiling point
Table 1 Boiling points across Periods 2 and 3
comparatively low
boiling points
There is a distinct change between Group 4 and Group 5 in both the physical structure
of the elements and the forces holding the structures together. This change is:
• from giant structures to simple molecular structures
• from strong forces to weak forces.
The trend in melting point is similar to the trend in boiling point. There is a sharp
decrease between Group 4 and Group 5 marking the change from giant to simple
molecular structure.
The structure and bonding across Periods 2 and 3 is shown in detail in Table 2. The
molecules making up the simple molecular structures are shown by their molecular
formulae.
Examiner tip
Module 3
The Periodic Table
Periodicity: boiling points
In ex, fi
fi.
Y particles
forces
p.
I ,
boiling p .
I k,
boiling p .
A Table,
p drop p
Group 4 and Group 5. T
mark
k
p .
Period 2 Li Be B C N 2 O 2 F 2 Ne
Period 3 Na Mg Al Si P 4 S 8 Cl 2 Ar
structure giant metallic giant
covalent
forces
strong forces between positive
ions and negative delocalised
electrons
strong
forces
between
atoms
bonding metallic bonding covalent
bonding
Table 2 Structure and bonding types across Periods 2 and 3
simple molecular structures
weak forces between molecules
van der Waals’ forces
The melting and boiling points of the metals Na, Mg and Al in Period 3 increase across
the period.
Figure 3 illustrates how the attractive forces between the metal ions and delocalised
electrons increase from Na → Al.
Na Mg Al
+ − + − +
2+
− − 2+ − 2+ −
− −
−
+ + +
− −
− − −3+ 3+ 3+
−
− 2+ −
− − − − −−
2+ 2+
+
− − + +
− − −
−
−
− 3+ 3+ −
− − −
− 3+
− −
−
2+ 2+ − 2+ − 3+ 3+ − − 3+ −
−
+
−
+
− − − − − − −
− − − −
+
2+ − 2+ 2+
3+ 3+ 3+
− −− −− − − − −
Ionic charge increases
Ionic size decreases
Number of outer-shell electrons increases
Attraction increases: melting and boiling point increases
Figure 3 Increase in boiling point from Na to Al
Questions
1 Why is the boiling point of
carbon much higher than that of
nitrogen?
2 Why is the melting point of
aluminium higher than that of
magnesium?
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935 chemistry.U1 M3.indd 87 13/11/07 11:50:05 am