The Structure and Bonding of IO3- An example of the use of Lewis Structures and VSEPR Theory Lecturer: Dr. Andreas Lemmerer

What type of compound is IO3- ? IUPAC Nomenclature What type of compound is IO3- ? According to IUPAC Nomenclature, it is a Polyatomic Ion. Polyatomic ions contain more than one kind of atom. In particular, it is a oxyanion that contains a nonmetal (I) and oxygen. Oxyanions (polyatomic anions containing oxygen) are named with -ite or –ate as a base ending, depending on the number of oxygen atoms bonded to the nonmetal. IO3- has one atom of iodine and three atoms of oxygen. Since iodine is a halogen and hence in the VIIA Group, the oxyanion with three O atoms is given the -ate ending and hence it’s name is IODATE. cf. Group VA and VIA oxyanions with the -ate ending have four oxygen atoms (phosphate (PO43-) , and group IVA oxyanions have three oxygen atoms (carbonate CO32-)

Note that O is in the second period and I is in the fifth period. Structure and Bonding of IO3- To determine the structure and the bonding of the iodate anion, we will make use of the OCTET RULE: The Octet Rule: Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons. Or: In compound formation an atom gains or loses electrons, or shares pairs of electrons, until its valence shell has eight electrons. Atoms in the second period are limited to eight valence electrons, and there are exceptions to the octet rule for atoms in the higher periods, i.e. atoms from the 3rd period onwards can accommodate more than an octet. Note that O is in the second period and I is in the fifth period.

Lewis Structures - Lewis structures are representations of molecules showing all electrons, bonding and nonbonding. - Electron pairs shared between atoms are shown as a line “-” and valence electrons around the atom as dots “•”. - There is a sequence of five steps required to get to the correct Lewis Structure of a compound.

Writing Lewis Structure for IO3- Find the sum of valence electrons of all atoms in the polyatomic ion or molecule. If it is an anion, add one electron for each negative charge. If it is a cation, subtract one electron for each positive charge. IO3- 7 + 3(6) + 1 = 26

Writing Lewis Structures The central atom is the least electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds. Keep track of the electrons: 26  6 = 20

Writing Lewis Structures Fill the octets of the outer atoms. Keep track of the electrons: 26  6 = 20  18 = 2

Writing Lewis Structures Fill the octet of the central atom. Keep track of the electrons: 26  6 = 20  18 = 2  2 = 0

Writing Lewis Structures Then assign formal charges. For each atom, count the electrons in lone pairs and half the electrons it shares with other atoms. Subtract that from the number of valence electrons for that atom: The difference is its formal charge. The best Lewis structure… …is the one with the fewest charges. …puts a negative charge on the most electronegative atom.

Arrange the pairs of electrons until the formal charges are at a minimum, taking into account the net charge on the compound.

Resonance of IO3- One Lewis structure cannot accurately depict an anion such as iodate. We use multiple structures, resonance structures, to describe the molecule. The electrons are not localized, but rather are delocalized. Experimental Observation: All three I-O bonds are approximately equivalent, about 1.8 Å. I-O is about 2.2 Å and I=O 1.7 Å.

The Shape of the Iodate Anion What Determines the Shape of a Molecule? Simply put, electron pairs, whether they are bonding or nonbonding, repel each other. By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule. non-bonding electrons bonding electrons

Electron Domains We can refer to the electron pairs (bonding/non-bonding) as electron domains. In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. This molecule has four electron domains, i.e. 4 areas of electron-density around I.

Valence Shell Electron Pair Repulsion VSEPR Theory is based on the main idea that “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”

Bonding vs Non-bonding domains Non-bonding e-pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule. The final geometry or shape of the molecule depends on the relative number of bonding and non-bonding domains.

Tetrahedral Electron Domain There are three molecular geometries according to VSEPR Theory: Tetrahedral, if all are bonding pairs Trigonal pyramidal if one is a nonbonding pair Bent if there are two nonbonding pairs

Finally, the Shape: •• Trigonal Pyramidal:

Tetrahedral Electron Domain There are three molecular geometries: Tetrahedral, if all are bonding pairs Trigonal pyramidal if one is a nonbonding pair Bent if there are two nonbonding pairs

Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry: Axial Equatorial

Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.

Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain: Trigonal bipyramidal Seesaw T-shaped Linear

Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: Octahedral Square pyramidal Square planar