Question

Although ${\mathrm{I}}_3^{-}$ is known, ${\mathrm{F}}_3^{-}$ is not. Using Lewis structures, explain why ${\mathrm{F}}_3^{-}$ does not form.

Short answer

In conclusion, ${\mathrm{F}}_3^{-}$ does not form because its structure cannot accommodate the extra electron and would have non-zero formal charges, making it unstable. In contrast, the ${\mathrm{I}}_3^{-}$ ion forms a stable structure with minimized formal charges, allowing it to exist. The Lewis structures of the two ions illustrate these differences, with ${\mathrm{F}}_3^{-}$ having no valid structure to maintain stability, whereas ${\mathrm{I}}_3^{-}$ has a stable linear arrangement.

Step-by-step solution

Draw the Lewis structures

First, we need to draw the Lewis structure for ${\mathrm{I}}_3^{-}$ and ${\mathrm{F}}_3^{-}$. To do this, we follow these steps: 1. Determine the central atom. 2. Calculate the total number of valence electrons. 3. Arrange the atoms and distribute the electrons. 4. Check for formal charge minimization.

Determine the central atom

The central atom for both ${\mathrm{I}}_3^{-}$ and ${\mathrm{F}}_3^{-}$ is the atom that appears the most in the compound. In ${\mathrm{I}}_3^{-}$, it's Iodine (I), while in ${\mathrm{F}}_3^{-}$, it's Fluorine (F).

Calculate the total valence electrons

To calculate the total number of valence electrons in both ions, we refer to the periodic table and count the valence electrons of the atoms. In ${\mathrm{I}}_3^{-}$: - Iodine has 7 valence electrons, and there are 3 Iodine atoms. - The ion has one additional electron due to its -1 charge. Total valence electrons: (3 × 7) + 1 = 22 electrons In ${\mathrm{F}}_3^{-}$: - Fluorine has 7 valence electrons, and there are 3 Fluorine atoms. - The ion has one additional electron due to its -1 charge. Total valence electrons: (3 × 7) + 1 = 22 electrons

Arrange the atoms and distribute the electrons

Now, arrange the atoms and distribute the electrons by using the electrons to form bonds between the central atom and the surrounding atoms. For ${\mathrm{I}}_3^{-}$: A possible structure could have all three Iodine atoms linearly connected. One pair of electrons is used for each bond, and the remaining 16 electrons are distributed as lone pairs on each atom. Lewis structure of ${\mathrm{I}}_3^{-}$: $$\mathrm{I}-\mathrm{I}-\mathrm{I}$$ For ${\mathrm{F}}_3^{-}$: If we were to connect the three Fluorine atoms linearly, similar to the ${\mathrm{I}}_3^{-}$ ion, we would use up 6 electrons and have 16 remaining. However, each Fluorine atom already has 7 valence electrons, and connecting them with single bonds would result in an octet configuration for each Fluorine, which doesn't allow for existence of the extra electron. This hints that the ${\mathrm{F}}_3^{-}$ ion cannot be stable.

Check for formal charge minimization

In order to further show the instability of ${\mathrm{F}}_3^{-}$, we could also calculate the formal charges on each atom to confirm that they are not minimized. For the ${\mathrm{I}}_3^{-}$ Lewis structure, each iodine has a formal charge of 0, meaning the structure is stable. In the hypothetical ${\mathrm{F}}_3^{-}$ structure, there would be a non-zero formal charge on at least one of the fluorine atoms, indicating unstability and making it improbable to exist. In conclusion, by comparing the Lewis structures, we can see that ${\mathrm{F}}_3^{-}$ doesn't form because its structure would not accommodate the extra electron and would have non-zero formal charges, making it unstable. On the other hand, ${\mathrm{I}}_3^{-}$ forms a stable structure with minimized formal charges, and that's why it exists.

Key concepts

Valence Electrons

Valence electrons play a foundational role in the structure of atoms and molecules. They are the electrons located in the outermost shell of an atom and are, therefore, the most accessible for forming chemical bonds. The number of valence electrons corresponds directly to an element’s group number on the periodic table for groups 1, 2, and 13-18.

For instance, in the exercise outlining the differences between ${\mathrm{I}}_3^{-}$ and ${\mathrm{F}}_3^{-}$ ions, we note that both iodine and fluorine reside in group 17, thus each has seven valence electrons. Understanding the valence electrons count is crucial before proceeding to draw Lewis structures since it determines how atoms bond and whether they achieve a stable electron configuration.

Formal Charge

The formal charge is a concept used to estimate the distribution of electric charge in a molecule. It is calculated for each atom in a molecule, assuming that electrons in chemical bonds are shared equally between atoms, irrespective of their electronegativity. The formal charge formula is: $$\text{Formal charge}=(\text{Number of valence electrons})-(\text{Number of lone pair electrons})-\frac{1}{2}(\text{Number of bonding electrons})$$

Determining the formal charge helps in predicting the most stable Lewis structure, as it's generally more favorable for atoms to have a formal charge of zero. In our provided exercise, ${\mathrm{I}}_3^{-}$ has iodine atoms with a formal charge of zero, indicating a stable structure. Contrastingly, any possible structure for ${\mathrm{F}}_3^{-}$ would result in non-zero formal charges, signifying instability and providing a rationale for why ${\mathrm{F}}_3^{-}$ does not exist.

Stability of Ions

The stability of ions hinges on several factors, including achieving a noble gas configuration, the charge distribution, and the overall energy of the ion. A stable ion typically has an octet of electrons in its outer shell, minimizing potential formal charges.

Within the context of the exercise, ${\mathrm{I}}_3^{-}$ is stable because it can distribute its electrons in a way that each iodine atom reaches an octet, mimicking the electron configuration of noble gases, which is a condition of stability. However, for ${\mathrm{F}}_3^{-}$ to exist, it would have to accommodate the extra electron, which is not possible while maintaining the octet rule and a favorable charge distribution. Consequently, because ${\mathrm{F}}_3^{-}$ would violate these principles, it is deemed unstable and not observed in nature.