Identify the polar molecules from the following list. CO, NH3, XeF2, COS, CCl4

Question

OPTIONS:

 

CO, NH3, COS
NH3, XeF2, COS
CO, NH3

OR
CO, NH3, XeF2, COS, CCl4

Answer 1

First determine the molecular geometry of each of these examples.  Start with a Lewis structure and use valence shell electron repulsion theory to determine geometry.  Molecules that are symmetrical are non-polar, those that are not symmetrical are polar.  

 

Lewis structures: 

1. determine the number of valence electrons for each atom

2. write down the central atom and put a single bond between each central atom and the surrounding atoms (skeleton structure)

3. fill in the octets around the outer atoms (excluding H which can ONLY have TWO electrons therefore one bond)

4a. put any remaining valence electrons around the central atom to create an octet* 

4b. if there are not enough remaining valence electrons to create an octet put a double bond between the central atom and the most electronegative outer atom (electronegativity trend on periodic table)**

4c. add additional double bonds if there are more outer atoms and the central atom does not have an octet yet if there are not other outer atoms make a triple bond

5. if you can make more than one structure based on the above choose the option that has the lowest values for formal charges

*there are exceptions to the octet rule

     -sometimes less than eight electrons can occur these are called free radicals

     -for elements with an atomic number greater than 12, there can be greater than an octet (use the d block for               bonding)

**If there is more than one option for where to put a double bond you will need to draw resonance structures.

 

To determine molecular geometry look up a table of for VSEPR.  The counts for electron pairs will be based on the central atom.  Bonding pair = a bond between central atoms and outer atoms, double bonds count as one.  Lone pair = two electrons represented by dots around the central atom.  

Example CCl₄: 4 VE from C + 7 VE * 4 Cl = 32 VE total on compound

     Cl

      l

Cl--C--Cl

      l

     Cl

Which accounts for eight electrons, once you complete the octet around each Cl (outer atoms) you will have accounted for all 32 VE and the central atom has an octet so we are done!

From VSEPR there are 4 bonding pairs and 0 lone pair electrons; this is tetrahedral geometry which is perfectly symmetrical and therefore non-polar.  

 

XeF₂ is an exception to the octet rule since Z>12

COS will have C as the central atom.  S and O do not have equal electronegativities therefore instead of a fully linear geometry this molecule will be slightly bent

You should be able to determine Lewis structures and geometries for all of these examples at this point.

 

You already know you can exclude the answer with CCl₄ because it is non-polar.

CO has a triple bond and is linear BUT C and O have differing electronegativity so it is slightly polar.

NH₃ has a trigonal pyramidal structure and is by definition not symmetrical and therefore polar.

XeF₂ has a linear and therefore non-polar structure.  

Therefore, the answer should be the first one unless your key does not use nuances of electronegativity differences.