A Lewis Structure is a diagram that shows how valence electrons within a compound are distributed among its atoms. Those electrons that are shared by two atoms are referred to as a covalent bond and represented by a dash.  Those electons that are located on a single atom are referred to as lone pairs and represented by two dots.
 

What are Lewis Structures?
This question is about recognizing the correct and incorrect Lewis Structures for three given compounds. Having a solid understanding of what Lewis Structures will help you with the process of drawing a Lewis Structure. Study the graphic below in order to conceptualize what Lewis Structures are.


Identifying Incorrect Lewis Structures
This question provides chemical formulae for three compounds and asks you to identify which are incorrect. You need to select all the incorrect structures.  You will need to give attention to ...
 

1. The number of electrons drawn in the structure.
   The number of electrons in a Lewis Structure should be equal to the number of valence electrons in the given compound. The first activity of this Concept Builder was about Counting Valence Electrons. You now need to practice that skill here to determine the number of valence electrons for the given formula. Then count the number of electrons shown in the diagram (recall a dot is 1 electron and a dash is 2 electrons). If there is a mismatch between the number of electrons shown in the diagram and the number of valence electrons, then the diagram is incorrect.
   
   View: Help Page for Counting Valence Electrons. Be sure to give special attention to the section about ion charges.
   
    
2. The number of electrons surrounding each atom.
   Atoms form bonds with one another in order to become more stable. This usually means that they acquire an octet of electrons. For instance, atoms of the elements carbon (C), nitrogen (N), oxygen (O), and fluorine (F) will always acquire an octet of electrons around them. That is, atoms of these elements should always be surrounded by 8 electrons - no more and no less.  If you observe it being otherwise in any of the diagrams, then you can select the diagram as being incorrect.
   
   Hydrogen (H) has its own means of being stable; it becomes stable with two electrons around it. Any diagram that shows more than two electrons around hydrogen is incorrect.  Boron (B) is also a special case. It is a non-metal that has three valence electrons. It is seldom able to acquire an octet since it is unable to find enough willing atoms to share five electrons. As such, expect boron to have six electrons around it.  While it can be expected that most elements will follow the Octet Rule, there are other Octet Rule breakers. Read the section below to learn more.

Be Careful About Octet Rule Breakers
The Octet Rule is usually followed. However, there are occasions in which the central atom will sometimes acquire more than 8 electrons around it. This is most likely to happen in compounds in which ...
 

1. ... the central atom is surrounded by more than 4 other atoms.
   In cases like PCl₅ or SF₆ in which the central P or central S atom is surrounded by more than 5 atoms, there is no possibility for that central atom to have only 8 electrons. When there are five or six bonded atoms to the central atom, that central atom will have to have 10 or 12 electrons surrounding it. With there being two electrons per bond, there is no option but to break the Octet Rule. 
2. ... the central atom is a noble gas like xenon (Xe).
   Atoms of noble gases already have an octet of electrons. As they bond with other atoms, they will acquire more. So the xenon atom in XeF₂ will be guaranteed to break the Octet Rule. 
3. ... there are negatively-charged polyatomic ions containing mostly halogens.
   There are rarer instances in which the central atom is a halogen and the surrounding atoms are halogens  (like chlorine or bromine or iodine). Halogens begin with seven valence electrons. This makes them one electron short of a full octet.  Whenever there are several halogens packed into a molecule or polyatomic ion or a molecule with a halogen as the central atom, it isn't entirely unusual that the central atom breaks the octet rule.

Octet rule breaking is rare but it does occur (especially in Activity 3 of this Concept Builder). When drawing Lewis structures, one should always make an effort to satisfy the octet for all atoms. Break the Octet Rule only after you recognize that the supply of valence electrons is so large that the octet rule must be broken. Never break it for atoms of C, N, O, or F. Whenever there is a breaking of the Octet Rule, it is the central atom that breaks it. The exterior atoms (those bonded to the central atom) will never break the Octet Rule.