3:24 SOS Question 3 of 10 Submit Draw the Lewis structure of
and then determine its electron domain and molecular geometries. Click to draw a new structure A) trigonal planar / square planar B) trigonal bipyramidal / linear C) trigonal bipyramidal / planar Tap here or pull up for additional resources
The Correct Answer and Explanation is:
) trigonal bipyramidal / linear
To determine the electron domain and molecular geometries for the tribromide ion (Br₃⁻), we must first construct its Lewis structure and then apply the Valence Shell Electron Pair Repulsion (VSEPR) theory.
The initial step is to calculate the total number of valence electrons available for bonding. Bromine (Br) is a halogen in Group 17 of the periodic table, so each bromine atom contributes 7 valence electrons. Since there are three bromine atoms, this gives us 21 electrons. The ion has a negative one charge, which means we must add one extra electron. This brings the total count to 22 valence electrons.
Next, we draw the skeletal structure. For a molecule with three identical atoms, one atom will be central, bonded to the other two. The structure is Br-Br-Br. These two single bonds account for 4 of the 22 valence electrons. The remaining 18 electrons are distributed as lone pairs. We first complete the octets of the two terminal bromine atoms, giving each atom 6 electrons (3 lone pairs). This uses 12 more electrons. The final 6 electrons are placed on the central bromine atom as 3 lone pairs.
Now we check the central atom. It is bonded to two atoms and has three lone pairs. This means the central bromine atom has an expanded octet with 10 valence electrons (2 bonding pairs and 3 lone pairs). This is acceptable because bromine is in the fourth period and can accommodate more than eight electrons in its valence shell.
Using VSEPR theory, we find the electron domain geometry by counting the total number of electron domains (bonding pairs and lone pairs) around the central atom. There are 2 bonding domains and 3 lone pair domains, for a total of 5 electron domains. A molecule with 5 electron domains has a trigonal bipyramidal electron domain geometry.
The molecular geometry, which describes the arrangement of only the atoms, is determined from this electron geometry. In a trigonal bipyramidal arrangement, lone pairs occupy the equatorial positions to minimize electron repulsion. With all three lone pairs in the equatorial plane, the two terminal bromine atoms are positioned in the axial locations, one directly above and one directly below the central atom. This arrangement results in the three bromine atoms lying in a straight line, giving the ion a linear molecular geometry with a 180° bond angle.
