SUMMARY: MELTING POINT
*Defined: The melting point is the __________________ that gives the particles enough ___________ energy
so that they can overcome the _________________ forces acting in the ___________ of the ___________
crystal.
 Melting points are a measure of the ___________________ forces that hold the _______________ together.
The stronger the attractive forces, the _________________ the melting point.
 Melting does not decompose a MOLECULAR compound into its ________________ – it’s only a
change of _________!!
For example:
A. Melting Points of Group 1  The ALKALI METALS
EXPLANATION OF TRENDS IN MELTING POINTS OF METALS (limit to s and p block elements)
*Most metals have quite high melting points. Some exceptions include: Mercury (liquid and room temperature) and the
alkali metals (all melt below 181°C).
Consider the following:
Alkali Metal
Li
Na
K
Rb
Cs
Charge
+1
+1
+1
+1
+1
Ionic Radius (pm)
76
102
138
152
167
Melting Point (°C)
180.5
97.8
63.5
39.3
28.5
*Describe and Explain the Trend in Melting Point Down Group The melting points decrease down the
group because as the ionic radius get larger, the forces of attraction between them ___ (i.e. the strength of the
metallic bond ___).
Consider the following:
Metal
Potassium
Calcium
Delocalized Electrons
per Atom
1
2
Charge of
Cation
+1
+2
Ionic Radius (pm)
Melting Point (°C)
138
100
63.5
842
*Describe and Explain the Trend in Melting Point of the two Metals: The melting point of calcium is
significantly higher than potassium. The electrostatic attraction between the cations and the delocalized
electrons (i.e. the strength of the metallic bond) is greater in calcium due to the:
1. greater number of delocalized electrons
2. higher charge; and,
3. smaller size of the cation.
1
B. Melting Points of Group VII – The HALOGENS
Diatomic
Element
F2
State at 25°C
Mass(diatomic
molecules, X2)
38.00
Number of
electrons
Melting Point
(K)
54
Cl2
70.90
172
Br2
159.8
266
I2
253.8
387
*Describe the trend.  The melting points ____ down this group (i.e. the halogen group).
*Explanation:
The solid halogens have _________-polar diatomic _____________ at their lattice points. The attractive forces
holding the lattice together therefore are London Dispersion Forces. The strength of the LDF force increases
as the number of ________________ increases. (i.e. the strength of the LDF increases down the group,
resulting in an increase in melting point.)
C. Melting Points Across Period 3
Na
Mg
Al
_______________ Bonding
Increasing  due to greater
__________ charge and
increased number of
_______________ electrons
371 K
922 K
936 K
Si
_______
Covalent
Very
strong
covalent
bonds
between
__ atoms
*1683 K
P4
S8
Cl2
______________ Covalent
Ar
_______
S8 is highest due to greater
London forces, then P4, then
Cl2
Weak
London
forces
317 K
392 K
172 K
84 K
2