Lewis Symbol For As 3

Draw­ing the Lewis Diagram for As³

Arsenic sits in group 15, which means a neutral atom starts with five valence electrons. Remove three of them and you are left with two. That is the whole calculation. The Lewis diagram for As³ is simply the element symbol with two dots and a 3+ superscript. I spent years correcting student work on this exact ion, and the pattern is always the same. People either leave all five dots because they forget to subtract, or they draw eight dots because they confuse the ion with the sulfide isoelectronic configuration and assume a full octet forms automatically. It does not. The 3+ charge comes from losing the three p-electrons, leaving behind the two s-electrons. Those are the dots you draw. Here is what it looks like on paper:

[As with two dots beside it, superscript 3+] In text form that is usually written as :As³ or [As]³ with two dots representing the remaining valence pair. Most people just draw the two dots on one side of the As symbol. The trick that trips people up every time is that As³ is isoelectronic with Ge² and Ga, but each of those ions has a completely different number of total electrons. The Lewis dot picture only tracks valence electrons, so the symbol looks deceptively simple even though the underlying electron configurations diverge once you go past the fourth shell. I once had a colleague insist that As³ should have eight dots because the nearest noble gas is krypton. That is wrong. The ion does not gain electrons to complete an octet. It loses them. Writing [Ar] 3d¹ 4s² as the configuration makes it obvious: only the 4s² electrons remain as valence.

Another thing that is worth noting upfront is that the Lewis model itself breaks down here. This ion is a heavy post-transition metal cation with a pseudo-noble-gas core. In real compounds like AsCl or the arsenite ion, the chemistry is driven by d-orbital participation and stereochemically active lone pairs, none of which shows up in a two-dot diagram. If you need accuracy for bonding predictions or spectroscopy, use a molecular orbital treatment or at minimum a VSEPR analysis with the full electron count. The Lewis dot is fine for a general chemistry quiz. It is not fine for anything that requires knowing geometry or reactivity. Step by step, the method is straightforward: Find the group number. Group 15 gives you five valence electrons for the neutral atom.

Account for the charge. A 3+ charge means three electrons are removed. Five minus three equals two. Place the two remaining dots around the symbol As. Add the 3+ charge outside the brackets if you choose to bracket the symbol. I ran into a specific edge case last year when a student asked why the answer key showed zero dots instead of two. The key was treating As³ as if it had emptied its valence shell completely, which would only be true for a group 13 element losing three electrons. Arsenic is not group 13. The two remaining s-electrons do not disappear. I had to walk the student through the difference between a true empty-shell cation like Al³ and a post-transition cation where the s² pair is stubbornly retained. Once that distinction clicked, the problem resolved in about ten minutes.

If you want a quick reference chart or a downloadable worksheet for practicing these ions, most general chemistry textbooks and OpenStax resources include arsenic polyatomic ions and cations in their Lewis structure problem sets. You can find them by searching "Lewis structure practice As³" on sites like chem.libretexts.org or the Purdue OWL chemistry pages.