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Midterm 2011 question 6B

Posted: Sat Oct 31, 2015 4:46 pm
by Edris Saadat 1A
I am struggling with how exactly one should determine whether one compound is more ionic than the other. The question asks which compound is more ionic, then lists off compounds such as Chlorine Monoxide or Sodium Oxide, Antimony Trichloride or Indium Trichloride, Lithium Hydride or Hydrochloric acid, and finally Magnesium Chloride and Phosphorous Trichloride. Would I need to draw the Lewis structures of each compound or just look at the periodic table and see which compound is a greater difference in electronegativity?

Re: Midterm 2011 question 6B

Posted: Sat Oct 31, 2015 6:16 pm
by SubparChemist
Yes, you'd need to base your decision off of trends in the periodic table such as you pointed out, ionization energies as well as factors such as atomic radius.

Re: Midterm 2011 question 6B

Posted: Tue Nov 03, 2015 3:09 pm
by Aya Ghoneum 1H
In order to rank compounds in terms of their relative ionic character, one needs to first understand the spectrum of covalent and ionic comounds. Covalent compounds share electrons, and the closer together two atoms’ electronegativities are, the better they will share their electrons. Similarly, the closer in size two atoms’ radii are will also allow better sharing of electrons between them. As atoms begin to have a larger electronegativity difference or have a large difference in atomic radii, they become more and more ionic in character. (An electronegativity difference larger than 1.8 actually is classified as an ionic bond). Therefore, the “most” ionic compounds will have a larger electronegativity differences and/or have larger differences in atomic radii sizes.
This is why Subparchemist said that in order to answer this problem, it is important to look at the periodic table (to compare their relative atomic radii), and also compare atoms’ relative ionization energies (electronegativity is a better standard for this purpose).
In the original problem in Midterm 2011 Question 6B, one must decide which of two compounds have the most ionic character: a) Cl2O or Na2O b) InCl3 or SbCl3 c) LiH or HCl d) MgCl2 or PCl3.

a) Cl2O or Na2O: The bond between Na and O is a metal and non-metal bond, which is by definition an ionic bond, whereas the bond between Cl and O is by definition a covalent bond since it is a non-metal/non-metal bond. This means Na2O is definitely more ionic than Cl2O. Also, if you had an electronegativity chart, you would see that the electronegativity difference for Na2O is 2.51 and for Cl2O is 0.28, again confirming that Na2O is more ionic.
b) InCl3 or SbCl3: Indium (In) and Antimony (Sb) are right next to each other in period 5 of the periodic table, however In is further to the left of the periodic table, meaning it is larger than Sb and has more ionic character than Sb. For these reasons, InCl3 would be more ionic.
c) LiH or HCl: Like in part (a), LiH is a bond between a metal and a non-metal, which is a classic ionic bond, whereas HCl is a non-metal/non-metal bond, a classic covalent bond. Therefore, LiH is more ionic.
d) MgCl2 or PCl3: Same concepts as part (a) and (c), therefore MgCl2 is more ionic.