Chapter 04: Drawing Dot and Cross Diagram
Secondary Chemistry Revision: Drawing of Dot and Cross Diagram
Tips & Tricks to Draw Dot and Cross Diagram:
- Covalent bonding is the sharing of electrons between non-metal atoms. In the overlap of the valence shells, there must at least be a dot from one non-metal atom and a cross from another non-metal atom.
- Both non-metal atoms must achieve stable noble gas electronic configuration (duplet or octet - full valence shell)
- Hydrogen and Group VII atoms: 1 bond.
- Group VI atoms (oxygen, sulfur etc): 2 bonds
- Group V atoms (nitrogen, phosphorus): 3 bonds
- Group IV atoms (carbon): 4 bonds.
Take note that students are to draw the full electronic configuration of the atoms unless specified in the question to only draw valence electrons.
Some Examples for Dot and Cross Diagrams
| Proton number |
Electronic configuration |
Dot-and-cross diagram | |
|---|---|---|---|
| Hydrogen atom |
1 |
1 Therefore, H |
Hydrogen atom |
| Chlorine atom |
17 | 2.8.7
Therefore, the |
Chlorine atom |
| Oxygen atom |
8 | 2.6
Therefore, an |
Oxygen atom |
| Dichlorine Monoxide (C2O) |
Chlorine and oxygen are non-meta atoms, thus the compound formed is a covalent compound. Oxygen is the lone atom, so it is placed in the centre. Oxygen is in group VI; therefore, it can form 2 bonds. Chlorine, a group VII atom, can form 1 bond. |  |
|
| Proton number |
Electronic configuration |
Dot-and-cross diagram | |
|---|---|---|---|
| Sodium atom | 11 | 2.8.1 | Sodium atom
|
|
Sodium chloride (Ionic compound) |
- | A sodium atom will transfer its valence electron to a chlorine atom, forming sodium cation and chloride anion. Na+: 2.8 CI-: 2.8.8 |
Sodium chloride -> ionic compound -> transfer of e- from the metal atom to non-metal atom to form cations and anions.  |
Common error #1 that students do is to miss out on the charges on the ions. Since ions are formed due to the transfer of electrons from metal atoms to non-metal atoms, there should be both positive and negative charges in the diagram.
Another common error #2 is forgetting that the electron transfer to the non-metal atom has to follow the legend for the metal atom.
For example, in the diagram above, the dot electron represents the valence electron from the sodium atom that got transferred to the chlorine atom so that both atoms can obtain stable noble gas electronic configuration to form sodium ion and chloride anion.
Thus, the non-metal atom must have two types of electrons (dot AND cross) in its valence shell of electrons.
Popular Exam Question #1: Hydrogen Chloride (Both Ionic & Covalent)
Hydrogen chloride is a covalent molecule in a gaseous state (a). However, it can dissociate in water to form H+ and Cl- ions (b - Ionic).
| Proton number |
Electronic configuration |
Dot-and-cross diagram | |
|---|---|---|---|
| Hydrogen chloride - (a) Simple covalent molecule
(In gaseous state) |
- | Both hydrogen and chlorine atoms should attain a full valence shell of electrons - stable noble gas electronic configuration (duplet of octet). | Hydrogen chloride -> Covalent compound -> sharing of electrons
|
| Proton number |
Electronic configuration |
Dot-and-cross diagram | |
|---|---|---|---|
| Hydrogen chloride - (b) Ionic Compound
(In aqueous state) |
- | Hydrogen will transfer its valence electron to chlorine atom to form hydrogen ion and chloride ion. |
Hydrogen chloride
|
This is the reason why HCl or other hydrogen halides can conduct electricity when dissolved in water. The ions formed are free to move and can act as mobile charge carriers.