9701 · 3.7
Dot-and-cross diagrams — FAQ
Frequently asked questions for 9701 Dot-and-cross diagrams. Direct answers first, then deeper explanation — then practise with marking.
Do I have to use dots and crosses? Can I use other symbols?
While dots and crosses are the standard convention, you can use any two different symbols (e.g., small circles and triangles). The key is to be consistent and clearly distinguish between the electrons from different atoms. Using only one symbol is incorrect.
How do I know how many electrons an atom has in its outer shell?
For main group elements, the number of outer shell (valence) electrons is the same as the element's group number in the Periodic Table. For example, Carbon is in Group 14, so it has 4 valence electrons. Chlorine is in Group 17, so it has 7.
In a compound like MgCl₂, why do I need to draw two chloride ions?
The overall charge of an ionic compound must be neutral. A magnesium atom loses two electrons to form a Mg²⁺ ion. Each chlorine atom gains one electron to form a Cl⁻ ion. Therefore, two Cl⁻ ions are needed to balance the single Mg²⁺ ion (a 2+ charge is balanced by two 1- charges).
What's the difference between a dative bond and a normal covalent bond in a finished diagram?
Once formed and drawn, there is no physical difference; they are both a shared pair of electrons. The only distinction in a dot-and-cross diagram is the origin of the electrons. In a dative bond, both symbols (e.g., two dots) will show they came from the same atom, whereas a normal covalent bond will have one of each symbol (one dot, one cross).