Molecule of methane
Silicon dioxide (SiO2) is an example of a giant molecular structure. The bonding between the atoms goes on and on in three dimensions. The covalent bonds that exist between the atoms are very strong, and as a result these substances are hard, with high melting and boiling points. Giant covalent molecules generally do not conduct electricity, with the exception of graphite, and are insoluble in water. In a silicon dioxide molecule, each silicon atom is bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms.
Molecules of SiO2
When an atom loses its outer electrons, and another atom gains them, ions are formed. In giant ionic compounds very strong electrostatic forces exist between the ions. A lot of energy is needed to break these forces, and thus giant ionic compounds have high melting points. When solid, giant ionic compounds do not conduct electricity as the ions are held in fixed positions and are unable to move. They only conduct electricity when molten or dissolved in water, and ions are free to move around. Ionic compounds are soluble in water. Sodium chloride is an example of a giant ionic lattice. Because of its high melting point, it is dissolved in water in order to conduct electricity.
NaCL forms a 3D ionic lattice
NaCl is ionised in water
Atoms of iron form metallic bonds with each other. The valence electrons are free to move around from atom to atom, and atoms are said to be in a “sea” of electrons. Because of this structure, metals conduct heat and electricity very well, are ductile and malleable. Iron is not a very good conductor of electricity, compared to other metals like copper. It is also not very ductile in its pure form.
Metallic Bonding
2.
Some substances have unusual properties for their bonding type or structure. Some examples of these substances are water, polyethene, sodium, graphite and aluminium oxide.
Water molecules are polar as they contain hydrogen bonds. Hydrogen bonds are the strongest of the intermolecular forces, though they are not as strong as normal covalent or ionic bonds. They are formed when an atom of hydrogen bonds with atoms of oxygen, nitrogen or fluorine. These elements are strongly electronegative and so attract the electron from hydrogen toward them, leaving the hydrogen nucleus exposed. This creates a positive charge from the proton and the molecule becomes polarised. The ability of ions and other molecules to dissolve in water is due to polarity. Water is said to have a permanent dipole (separation of charge in a molecule).
Hydrogen bond between molecules of water
Van der Waal’s Forces are the attractive forces between atoms or molecules. They are relatively weak forces, except in the case of very large molecules. Polyethene is an example of a large molecule. The Van der Waal forces between the molecules are very strong because they are very large. Molecules of polyethene contain many electrons, which make the forces stronger. Polyethene is hard, stiff and strong, and has a low melting point.
Sodium behaves uncharacteristically compared to other metals. It has a relatively low melting and boiling point, and is said to be a soft metal. This is because sodium is a Group I element, and has only one valence electron. Fewer electrons within the substance mean that the electrostatic forces are weaker, and are easier to break apart.
Graphite is a giant covalent molecule. Each carbon atom in a molecule is bonded to only three other carbon atoms, leaving a fourth free or “delocalised” electron. This is what enables graphite to conduct electricity. The interparticle forces between the carbon atoms are made from the covalent bonds and, since there is one less bond formed due to the free electrons, the forces between them are even stronger. The intermolecular forces in graphite are Van der Waal’s forces. This means that they are relatively weak. Graphite has a soft and slippery feel, and is used as lubricant, because the molecules slide over each other so easily. Due to the covalent bonds throughout the structure, graphite has a high melting point and boiling point, and like most other giant covalent structures, it is insoluble in water.
Structure of graphite
Aluminium is a metal in Group III which forms an ionic bond with oxygen to make aluminium oxide. However, aluminium oxide has some covalent characteristics. Aluminium oxide has a high melting and boiling point, and is insoluble in water. It is also an amorphous substance, meaning that it can react with both acids and bases.
