What is the Lewis structure, electron geometry, and molecular geometry of periodate (IO4-)? a) Is periodate polar or nonpolar? b) Why does it have 3 double bonds and a single bond (7 bonds in total) instead of having 4 bonds in total? c) When determining the geometry of the molecule, do you have to take into account the double/triple bonds?
The correct answer and explanation is:
Correct Answer:
- Lewis Structure of Periodate (IO₄⁻): Iodine (I) is the central atom with four oxygen (O) atoms bonded to it. One of the I–O bonds is a single bond with a formal negative charge on the oxygen, while the other three are double bonds. The total number of bonds is seven (3 double bonds + 1 single bond).
- Electron Geometry: Tetrahedral
- Molecular Geometry: Tetrahedral
- a) Periodate is polar.
- b) It has seven bonds due to formal charge minimization and expanded octet capability of iodine.
- c) Double and triple bonds count as one region of electron density when determining geometry.
Explanation:
The periodate ion (IO₄⁻) consists of one iodine atom surrounded by four oxygen atoms. Iodine has seven valence electrons, and each oxygen contributes six. The ion carries an overall −1 charge, so one extra electron is added. This gives a total of 7 (I) + 4×6 (O) + 1 (charge) = 32 valence electrons.
When drawing the Lewis structure, the iodine atom forms one single bond and three double bonds with the oxygen atoms. This distribution helps minimize formal charges. The single-bonded oxygen carries the negative charge. Iodine exceeds the octet rule because it is in Period 5 and has d-orbitals, which allow an expanded valence shell.
a) Periodate is polar because of the asymmetrical distribution of electrons. The differing bond types and the presence of a formal negative charge on one oxygen atom create a net dipole moment.
b) Periodate has three double bonds and one single bond because this configuration minimizes formal charges across the molecule. Each oxygen likes to have a formal charge close to zero. The structure also takes advantage of iodine’s ability to expand its octet and accommodate more than eight electrons.
c) When determining molecular geometry, only the number of electron domains (regions of electron density) around the central atom is considered. Whether a bond is single, double, or triple, it counts as one region. So, IO₄⁻ has four bonding domains, which gives a tetrahedral geometry for both electron and molecular shape.