## Why is I3- linear? Why is O3 polar?

(Polar molecules, Non-polar molecules, etc.)

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### Why is I3- linear? Why is O3 polar?

Question: The book says that $\dpi{100} \fn_jvn I^{3-}$ has a linear shape. Doesn't the central iodine atom have 3 lone pairs, therefore making it bent? I calculated that there are 22e- for this ion, which led me to draw a structure where there are 3 lone pairs on the central I-atom. I thought that whenever there are any lone pairs on the central atom and the overall compound/ion has two bonding pairs, then the shape is bent.
Also, why is ozone polar? Is it because it has a bent shape, with two of the oxygens having a greater downward pull on the electrons than the other oxygen? O3 should be non-polar since all of the atoms are the same right? All three atoms are oxygen molecules with the same value of electronegativity..

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### Re: Why is I3- linear? Why is O3 polar?

Answer: Yes, $\dpi{100} \fn_jvn I^{3-}$ has has three lone pairs in the equatorial position. But since they are equally spaced they repel the axial I equally, resulting in a linear shape.What matters is the number of regions of electron density, rather than just saying lone pairs = bent. I3- has 2 bonds and 3 lone pairs for 5 regions of electron density. Placing that into a trigonal bipyramidal structure, the lone pairs go equitorial and the two other I atoms go axial. With the three atoms in a line, the molecule is linear. More generally, an atom with 5 regions of electron density and 3 lone pairs (which make a trigonal plane) will be more or less linear, and an atom with 6 regions of electron density and 4 lone pairs (which end up in a square plane) will be linear.
In the case of ozone, there is more to the polarity of a molecule than just the difference in electronegativity for each atom. The central atom is in a different electronic environment, bound to two other oxygens, rather than just one. Both oxygens it is bound to are pulling electrons away, rather than just one other oxygen. This gives it an electron density different from the other two atoms. This is somewhat illustrated when we calculate the formal charge on each atom, as the central oxygen is found to have a different charge from the other two. The bent shape of ozone also helps us understand the polar nature of ozone, as the charges cannot cancel in that arrangement.

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