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
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).
|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
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.