The Problem With Drawing Cations
Most people mess up the transition from neutral atom to ion. They either leave the dots in place or randomly erase them and then wonder why the diagram doesn't make sense. I ran into this constantly when I was grading first-year chem labs. Students would draw sodium with eight dots and a plus sign, which is physically impossible. You can't have eight valence electrons and be a cation at the same time. The actual process is straightforward once you accept that forming a cation means losing your valence electrons entirely. Not partially. Not "adjusting slightly." The atom sheds its outer shell electrons and what remains is the inner shell, which may or may not have a full octet depending on which element you are dealing with.
Draw The Lewis Dot Diagram For A Cation
Start by determining how many valence electrons the neutral atom has. For main group elements this is just the group number divided by ten or your basic periodic table shortcut. Sodium is group one so it has one valence electron. Magnesium is group two so it has two. Aluminum is group thirteen so it has three. That part is easy. Then remove those electrons. Each positive charge on the cation corresponds to one electron you take away. Na+ loses one. Mg2+ loses two. Al3+ loses three. The result is you writing the elemental symbol with whatever electrons remain in the new outermost occupied shell, surrounded by dots, and a charge superscript on the upper right. Here is where it gets tricky and where I usually see people stall out. After losing electrons, the remaining valence shell might not look like a complete octet. Take aluminum. Neutral aluminum has three valence electrons. Remove all three to get Al3+. The remaining outer shell is the n=2 level, which has eight electrons in the 2s and 2p orbitals combined. But you aren't showing those inner electrons as dots in the Lewis structure. So you end up writing Al3+ with zero dots around it. It looks wrong. It looks empty. It is actually correct.
I remember spending twenty minutes with a grad student who refused to draw Al3+ with no dots. She kept adding four dots because she thought every Lewis structure needed at least a partial octet. We went back and forth until I literally pulled up a textbook and pointed to the section. The convention is clear. For cations that lose their entire valence shell, you draw zero dots. Period. Another edge case that trips people up is transition metals. Take Fe2+ versus Fe3+. The Lewis dot approach barely works for these because d-electrons complicate the whole picture. I don't recommend trying to draw a Lewis structure for a transition metal cation unless your professor explicitly asks you to. Even then, just put the symbol, the charge, and maybe note that d-electron configurations are outside the standard model. The whole exercise breaks down here. For main group cations like Li+, Ca2+, and K+, the diagram is always just the element symbol with a positive charge and no dots. The octet is satisfied by the inner shell. There is nothing more to draw. The simplicity is what causes confusion.
Get the Full Details
A few things to keep in mind before you finish. First, never leave dots behind after removing electrons. Some students try to redistribute what they removed into a new pattern around the symbol. That is not how it works. The electrons leave and they do not come back. Second, the charge must always be included. Writing just "Na" with no dots and no charge is incomplete. It is just an atom, not a cation. Third, for polyatomic cations like NH4+ or H3O+, you treat them differently because you are removing electrons from a bonded structure, not from a single atom. That is a separate exercise entirely. If your element is in period 3 or below and you are dealing with a heavy metal cation like Pb2+ or Sn2+, the inert pair effect becomes relevant. These elements tend to keep their s-electrons and lose only their p-electrons. The Lewis representation gets messy because you are now deciding which electrons to show as remaining and which to treat as lost. I usually just stop and tell students to represent the charge and move on unless they are in an advanced inorganic course. The Lewis model was never really designed for this level of complexity anyway.
