Chemical Bonding

1. Ionic compounds
1.1 General
Metals and non- metals form ionic compounds.
Ions are formed when electrons are lost or gained, so that the ion will achieve stable electronic configuration (i.e. 8 valence electrons in the outermost shell, or we call this stable octet configuration. Note that if the outermost shell is the first shell, then 2 electrons in the outermost shell is the stable electronic configuration. We call this stable duplet configuration).
An ionic bond is the electrostatic force of attraction between a positively charged ion (cation) and a negatively charged ion (anion).

1.2 Dot- and- cross diagram of ionic compounds
Next, let's look at how to draw the dot- and- cross diagram for ionic compounds.

Example: Draw the dot- and- cross diagram of magnesium chloride.
Step 1: Determine the type of bonding.
As magnesium is a metal, and chlorine is a non- metal, magnesium chloride is an ionic compound.

Step 2: Determine the charge of the metal and non-metal ion.
Since magnesium is in group II, the charge is +2. Magnesium ion has a formula of Mg2+.
Since chlorine is in group VII, the charge is -1. Chloride ion has a formula of Cl-.

Step 3; Determine the formula of the ionic compound, using the cross method.

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How to find formula of ionic compound - magnesium chloride


Step 4: Determine the proton number of the elements.
magnesium has a proton number of 12, and so the electronic configuration of the atom is (2,8,2).
chlorine has a proton number of 17, and so the electronic configuration of the atom is (2,8,7).

Step 5: Determine the electronic configuration of the ions.
Metal will lose the entire last shell of electrons to form ions. Hence, magnesium ion has an electronic configuration of (2,8).
Non- metal will gain electrons until the last shell has 8 electron. Hence, chloride ion has an electronic configuration of (2,8,8).

Step 6: Draw the dot- and- cross diagram!

Dot- and- cross diagram showing all electrons:
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Structure of magensium chloride - dot- and- cross diagram showing all electrons


Dot- and- cross diagrams showing only outermost electrons:
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Structure of magnesium chloride - dot- and- cross diagram showing electrons in outermost shells only







1.3 Properties of ionic compounds
  • Ionic compounds have high melting and boiling point.
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.
  • Ionic compounds conduct electricity when molten or in aqueous solution, but not in the solid state.
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.
  • Ionic compounds are generally soluble in water.
(As ionic compounds are able to form ion- dipole interactions with water. --- Note this is not in the syllabus.)


2. Covalent Molecules
2.1 General
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:
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Structure of water - dot- and- cross diagram of water, showing all electrons.


 Dot- and- cross diagrams showing only outermost electrons:
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structure of water - dot and cross diagram showing outermost electrons only

 1.3 Properties of simple covalent molecules

  • Simple molecules have low melting and boiling point
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.]

  • Simple molecules do not conduct electricity at all states (solid, liquid and gas).
This is because they do not have free electrons to conduct electricity.
  • Simple molecules are generally insoluble in water, but soluble in organic solvents.
3. Metals
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).

4.1. Diamond
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.

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Structure of diamond



4.2. Graphite
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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Structure of graphite



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