|How to find formula of ionic compound - magnesium chloride|
|Structure of magensium chloride - dot- and- cross diagram showing all electrons|
|Structure of magnesium chloride - dot- and- cross diagram showing electrons in outermost shells only|
1.3 Properties of ionic compounds
This is due to the strong electrostatic forces of attraction between oppositely charged ions. A large amount of energy is required to overcome these forces during melting or boiling. Thus, ionic compounds have high melting and boiling point.
In the solid state, the ions are held together by strong electrostatic attraction and are not free to move about to conduct electricity. When molten or in aqueous solution, the lattice structure of the ionic compound is broken down, and the ions are free to move about to conduct electricity.
(As ionic compounds are able to form ion- dipole interactions with water. --- Note this is not in the syllabus.)
2. Covalent Molecules
A covalent bond is formed by the sharing of electrons between non- metals to achieve state electronic configuration.
There are two types of covalent moelcules. Simple molecules, and giant molecules. In this section, we will be looking at simple molecules.
2.2 Dot- and- cross diagram of simple molecules
Example: Draw the dot- and- cross diagram of water.
Step 1: Determine the type of bonding.
As water and oxygen are both non- metal, the type of bonding present is covalent bonding.
Step 2: Determine the electronic configuration of the atoms.
H has a proton number of 1, thus the electronic configuration is (1). To achieve stable duplet configuration, each H atom requires one more electron. Hence, each H will share 1 electron.
O has a proton number of 8, thus the electronic configuration is (2,6). To achieve stable octet configuration, each O atom requires 2 more electrons. hence, each O will share 2 electrons.
Step 3: Draw the dot- and- cross diagram!
Dot- and- cross diagram showing all electrons:
Dot- and- cross diagrams showing only outermost electrons:
1.3 Properties of simple covalent molecules
This is due to the weak intermolecular forces of attraction. Small amount of energy is required to overcome these forces of attraction. Hence, simple molecules have low melting and boiling point.
[Note: melting and boiling of simple molecules DO NOT involve breaking covalent bonds. The molecules are only separated further apart.]
This is because they do not have free electrons to conduct electricity.
The metallic bond consists of a lattice of positive ions in sea of electrons.
Metals can conduct electricity (in all states) due to the presence of mobile electrons.
Metals have high melting and boiling point due to the strong electrostatic forces of attraction between the positive ions and the sea of electrons. Large amount of energy is required to overcome this strong forces of attraction. Hence, metals have high melting and boiling point.
4. Macromolecules or Giant Molecules
These are covalent molecules made up of large number of atoms covalently bonded together. Examples of giant molecules are diamond, graphite, silicon, silicon dioxide and poly(ethene).
In diamond, each carbon atom is covalently bonded to four other carbon atoms, forming a tetrahedral structure. This is then repeated infinitely, forming the giant molecule of diamond. This infinite structure makes diamond hard, and is used as drill bits, and cutting tools.
Diamond cannot conduct electricity as all the valence electrons are used for the covalent bonding, and there is no mobile electron to conduct electricity.
Diamond has a high melting and boiling point, as melting and boiling involves breaking the extensive covalent bonds in diamond. This requires a large amount of energy, resulting in diamond having a high melting and boiling point.
In graphite, one carbon atom is covalently bonded to three other carbon atoms, forming a hexagonal structure.
Graphite can conduct electricity, as only three out of the four valence electrons in each atom is used for the covalent bonding. The presence of mobile electrons allow graphite to conduct electricity.
Graphite is slippery, as layers of atoms are held together by weak intermolecular forces (van der Waals), and can slide past each other easily. Hence, graphite is used as lubricant.
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