3.7.1—Dot-and-cross diagrams to illustrate ionic,

A dot-and-cross diagram shows only the outer-shell electrons of the atoms or ions. Use dots and crosses to keep the source atoms distinguishable; for an ion, enclose the electron arrangement in brackets and write the charge at the top right.

Choose the bonding model before drawing: show transfer of valence electrons for an ionic compound, shared pairs for a covalent compound, and a shared pair whose two electrons come from one donor for a coordinate bond. In a displayed formula, the coordinate-bond arrow points away from the donor lone pair.

After drawing, count the shown outer electrons, check each ion's charge and confirm that the shared or transferred electrons match the bonding model. The normal noble-gas/octet pattern is a useful check, but it is not an unconditional rule.

Keep the source-supported exception branches visible: some species have an expanded octet, some have an incomplete octet/electron-deficient centre, and some contain an odd number of valence electrons. Do not add inner-shell electrons or force these species into an eight-electron arrangement.