Types of Solids
Molecular solids result due to weak intermolecular forces that attract the individual
molecules to each other. These solids require relatively little energy to become
liquids (or gases) and have the lowest melting points of all solids.
Research the properties of the carbon tetrahalides (CX4). Provide the state, electronic
shape, and melting point for each.
Table 1 Trends in the properties of the carbon tetrahalides
Formula
State at SATP
Electronic Shape
mp (oC)
CF4
Gas
Tetrahedral
-183.6 °C
CCl4
Liquid
Tetrahedral
-22.92 °C
CBr4
Solid
Tetrahedral
91 °C
CI4
solid
Tetrahedral
171 °C
1. Explain the trend in melting points for the carbon tetrahalides.
The melting point of carbon tetrahalides rises as one moves down the group. They are a kind of
nonpolar molecular compound. As a result, the only intermolecular forces present are London
dispersion forces. The number of London dispersion forces increases as the number of electrons
increases.
Ionic solids arise from the crystal lattice arrangement of positive and negative ions that exert electrostatic attractions
toward oppositely charged ions – ionic bonds.
Ionic compounds tend to have much higher melting points due to the strength of the ionic bonds, but this can vary.
Table 2 The chlorides of alkaline earth metals
Ionic compound
mp (oC)
magnesium chloride
714 °C
calcium chloride
772 °C
strontium chloride
874 °C
barium chloride
962 °C
Research the melting points for the compounds formed by alkaline earth metals with chloride. What is the trend that
arises for the melting point of chloride compounds in this group?
The melting point of alkaline earth metal compounds rises as one moves down the group. When bonded,
electronegativity decreases down a group, resulting in a higher electronegativity difference. A greater difference in
electronegativity indicates a more stable connection.
2. Predict the melting point for radium chloride.
1000 °C
Metallic solids are held together by an arrangement of metal atoms sharing the electrons of their valence level. The
positive nuclei of each metal atom are attracted to the loosely held valence electrons through metallic bonding. View
images at http://www.ausetute.com.au/metallic.html.
Depending on the attraction of the nuclei to the valence
electrons, the strength of metallic bonds will vary, resulting in a
wide array of melting points for these solids.
Research the alkali metals to observe one of the trends noted in
the periodic table.
Table 3 Melting points of alkali metals
Alkali metal
lithium
sodium
potassium
rubidium
cesium
First ionization energy
(kJ/mol)
520
496
419
403
376
mp
(oC)
180
98
63
39
28
3. Explain the trend in the melting point for the
alkali metals.
Because the ionisation energy decreases,
the group decreases. Because the valence
electrons are further away from the nucleus,
the attractive force weakens, resulting in a
weaker bond.
4. The only metal to exist in the liquid state, mercury has one of the broadest temperature ranges as a liquid of any
metal. Yet, mercury is unreactive. Research why mercury forms weak metallic bonds.
Mercury strongly holds its valence electrons because all valence orbitals are filled ([Xe]4f145d106s2).
This indicates that electrons cannot freely flow, implying that there is a weak "electron sea."
Covalent network solids are the final type of solid using strong covalent bonds
to hold the atoms together in large structures (sometimes referred to as
macromolecules). The strength of these bonds creates the most stable solids
with the highest melting points.
The two allotropes of carbon, diamond and graphite, fit into this category.
Research the properties of these allotropes.
Table 4 The allotropes of carbon
Allotrope
VSEPR shape around
carbon
Hybridization of
carbon
Hardness
(Moh’s scale)
diamond
tetrahedral
sp^3
10
graphite
trigonal planar
sp^2
1-2
5. Explain the difference(s) in the intramolecular force within diamond and graphite.
In graphite, the fourth carbon bond is longer and weaker, allowing the layers
to slide. Diamond's tetrahedral structure makes it very strong.
6. Quartz also falls into this category of solids. Research the structure (formula, hybridization, bonding) and
properties (hardness, mp, solubility) of this covalent network.
SiO2, trigonal and hexagonal crystal structure, Moh's scale = 7, melting point = 1670°C, shiny, insoluble