The triiodide ion, \(\text{I}_3^-\), is a polyatomic anion composed of three iodine atoms carrying a net negative charge. Understanding its three-dimensional structure is fundamental to predicting its chemical behavior and reactivity. To determine the shape of this ion, chemists rely on the Valence Shell Electron Pair Repulsion (VSEPR) theory. VSEPR provides a model for predicting the geometry of molecules and ions based on the repulsion between electron pairs around a central atom. This method helps identify the electron geometry by visualizing how electron domains arrange themselves to minimize repulsion in three-dimensional space.
Calculating the Electron Domains for the Central Iodine Atom
The first step in applying VSEPR theory to the triiodide ion is to determine the total number of valence electrons available. Each iodine atom contributes seven valence electrons, totaling twenty-one electrons from the three atoms. Since the ion carries a negative one charge, an additional electron must be included in the count, resulting in a total of twenty-two valence electrons for the \(\text{I}_3^-\) ion.
The central iodine atom forms single bonds with the two terminal iodine atoms, using four of the twenty-two available electrons. After satisfying the octets of the terminal atoms, the remaining electrons are placed on the central atom. This placement results in the central iodine atom possessing three non-bonding electron pairs, commonly known as lone pairs.
The arrangement of electrons around the central atom defines its electron domains, which include both bonding pairs and lone pairs. An electron domain is any region of electron density, such as a single bond or a lone pair. For the central iodine atom in \(\text{I}_3^-\), there are two bonding domains and three lone pair domains.
The sum of these electron groups gives the total number of electron domains, or the steric number, which is five. This number directly dictates the ion’s overall electron geometry.
Determining the Electron Geometry
The electron geometry of \(\text{I}_3^-\) is determined by the five electron domains calculated for the central atom. Electron geometry refers to the three-dimensional arrangement of all electron domains—both bonding pairs and lone pairs—around the central atom. The arrangement is governed by the principle that these groups of electrons push away from each other to achieve maximum separation in space.
For any central atom surrounded by five electron domains, the electron geometry is always Trigonal Bipyramidal. This shape is defined by a central atom with five positions pointing toward the corners of a geometric figure known as a triangular bipyramid.
The trigonal bipyramidal structure features two distinct types of positions for the electron domains. Three positions lie in a plane around the central atom, called the equatorial positions, separated by \(120^\circ\) angles. The remaining two positions are located above and below this plane, known as the axial positions, which are separated from the equatorial positions by \(90^\circ\) angles.
The three lone pairs on the central iodine atom must be placed within this geometry according to VSEPR rules. To minimize repulsive forces, these non-bonding electrons preferentially occupy the equatorial positions where the bond angles are wider (\(120^\circ\)). Placing the lone pairs here minimizes the total number of \(90^\circ\) repulsions, which are the most destabilizing interactions.
Understanding the Molecular Geometry
While the electron geometry of \(\text{I}_3^-\) is trigonal bipyramidal, the final observed shape of the ion is described by its molecular geometry. Molecular geometry differs from electron geometry because it describes the spatial arrangement of only the atoms, ignoring the location of the lone pairs.
Since the three lone pairs occupy the equatorial plane of the trigonal bipyramid, the two remaining bonding domains must occupy the axial positions. These axial positions are located directly opposite each other, one above and one below the central plane.
This specific arrangement is designated in VSEPR notation as \(\text{AX}_2\text{E}_3\), where ‘A’ is the central atom, ‘\(\text{X}_2\)‘ represents the two bonded atoms, and ‘\(\text{E}_3\)‘ represents the three lone pairs. The lone pairs force the two terminal iodine atoms into the remaining available axial positions.
The two terminal iodine atoms are therefore bonded to the central atom in a straight line, resulting in a Linear molecular geometry. This linear shape gives the triiodide ion a bond angle of exactly \(180^\circ\) between the three iodine nuclei. This represents a classic example where the underlying electron arrangement differs significantly from the final atomic shape.