Hybridization of Triiodide Ion

Elements that are unstable combine usually with other elements either by losing or gaining or sharing electrons. This is said to be chemical bonding. Usually, elements combine with other elements only with the help of valence electrons. Valence electrons are those that are present in the outermost shell of an atom. These are far from the nucleus and can easily involve in the bonding with other elements. 

Hybridization is the phenomenon that involves intermixing of atomic orbitals of the central atom that differ in  energy and their re-distribution to an equal number of hybrid orbitals of equal shape and energy. Hybridization occurs in the central atom only. In a covalent molecule, the element with a high valency is considered the central atom.

Purpose of Hybridization

Hybridization occurs in covalent molecules and between the atomic orbitals of the central atom. Hybridization does not occur in all covalent molecules and occurs in heteroatomic molecules like methane, ammonia, etc. Hybridization occurs in the central atom, where the electrons in the valence shell exist in orbitals of different energy and shape.

Importance of Hybridization

Hybridization occurs in the central atom to get the orbitals of equal energy, and the overlapping of orbitals with equal energy is effective. Hybridization helps in knowing the types of bonds formed between the two elements. Bonds formed between the covalently bonded atoms can be of two types: sigma bond and pi bond.

Sigma bond: The sigma bond is a covalent bond formed by the sharing of electrons present in the orbitals. The type of overlapping observed between the orbitals is head-on overlapping or inter-axial overlapping. This type of overlapping leads to a strong overlapping of orbitals and hence a stronger bond, which is a stable bond with minimum energy.

Pi bond: Pi bond is a covalent bond formed by the sharing of electrons present in the orbitals. The type of overlapping observed between the orbitals is sideways overlapping or parallel overlapping. Sideways overlapping leads to a weaker bond with high energy and less stability.

Sigma bond is a stronger bond and can exist independently, but the pi bond is weaker and cannot exist independently.

Hybridisation in Triiodide Ion

Triiodide ion is the iodine compound with the formula (I3-). The name of the compound ends with the -ide term, which means it is a binary compound and an anion. This ion is linear and symmetrical. In this compound, iodine is the central atom and has seven electrons in its outermost shell.

I – atomic number- 53

Valence electronic configuration – [Kr] 4d10 5s2 5p5

Iodine belongs to the halogen family and has one electron less than the octet configuration. Iodine uses all the seven electrons present in 5s, 5p and 4d orbitals to form the hybrid orbitals. I3- is a linear ion formed by combining I2 molecule with me- ion in which I2 molecule acts as an acceptor and I- acts as an electron donor. The hybridization that I3- undergoes sp3 hybridization. 

The formula to calculate the hybridization of the element:

Number of hybridisation = valence electrons + monovalent atoms + (negative charge) – (positive charge)/2

= 7+ 2 + 1- 0/2 =10/2 = 5

If the hybridization number is equal to 5, then the hybridization is sp3d.

According to VSEPR ( valence shell electron pair repulsion) theory, the number for hybridization can be calculated by the sum of bond pairs and lone pairs. In I3-, the number of bond pairs around I is two and the number of lone pairs is 3. So, the sum of lone pairs and bond pairs is 2 + 3 = 5, which is sp3d hybridisation. For sp3d hybridization, the expected geometry is trigonal bipyramidal geometry when 5 bond pairs are present. In triiodide ion, there are lone pairs as well as bond pairs. According to VSEPR theory, the electron pairs present in the valence shell undergo repulsions. Repulsions can be between bond pairs, lone pairs, and lone pair-bond pairs. Bond-bond pair repulsions are less strong and can show an increase in bond angle as the bonds move far away. Lone pair-lone pair repulsions are strong and cause a decrease in the bond angle. Lone pair-bond pair repulsions are strong but less than lone pair – lone pair repulsions. The order of repulsions is

Bond pair- bond pair repulsions< lone pair-bond pair repulsions< lone pair- lone pair  repulsions

In triiodide ion, there are lone pair- lone pair repulsions, lone pair-bond pair repulsions, and bond pair-bond pair repulsions. So, the molecule deviates from the original geometry, shape, and bond angle. The expected geometry is trigonal bipyramidal, and the shape attained is linear. The expected bond angle for sp3d hybridization is 120 and 90, but due to the linear shape, its bond angle is 180.

Conclusion

Ionic bonding is the bonding that takes place when elements combine by losing electrons or gaining electrons, i.e. through the transfer of electrons. This type of bonding occurs between elements with different electronegativity or between a metal and a nonmetal. Elements with almost the same electronegativity or with less electronegativity difference form covalent bonding by sharing electrons.