Dot-and-cross diagrams — common mistakes

In a displayed formula, a dative bond is sometimes shown with an arrow pointing from the donor to the acceptor atom (e.g., N → H). However, in a dot-and-cross diagram, once formed, it is indistinguishable from a normal covalent bond. The key is to use dots and crosses to show that both electrons in that specific bond originated from the same atom.

Remember that only elements in Period 3 or below (e.g., P, S, Cl) can expand their octet. Period 2 elements (e.g., C, N, O, F) can NEVER have more than 8 electrons in their outer shell because they do not have accessible d-orbitals. Be very careful not to draw expanded octets for Period 2 elements.

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.

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.

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).

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).