Passage I

A teacher asked each of 4 students to describe the molecular shape (the geometrical arrangement of the atoms in a molecule) of each of 4 molecules: arsenic trifluoride (), arsenic trichloride(), arsenic tribromide (), and arsenic triiodide().

Student 1

An molecule is T-shaped, with the As atom in the center. All the atoms in lie in the same plane, and there are 2 unique angles—90° and 180°—between adjacent As-F bonds(see Figure 1).

The As atom has a lone pair (an outer pair of electrons not involved in chemical bonding) that lies in the same plane as the As and F atoms. These electrons strongly repel the 3 As-F bonds, resulting in the 2 unique bond angles of 90° and 180°.

, , and are also T-shaped.

Student 2

The shape of an molecule is trigonal planar, with the As atom in the center. All the atoms in lie in the same plane, and there is only 1 unique angle—120°—between adjacent As-F bonds(see Figure 2).

The As atom does not have a lone pair. The 3 As-F bonds repel each other equally, resulting in the 1 unique bond angle of 120°.

,, and are also trigonal planar.

Student 3

The shape of an , molecule is trigonal pyramidal, with the As atom in the center. All the atoms indo not lie in the same plane, and there is only 1 unique angle—109°—between adjacent As-F bonds (see Figure 3).

The As atom has a lone pair. The lone pair and the 3 As-F In bonds repel each other equally, resulting in the 1 unique bond angle of 109°.

,, and are also trigonal pyramidal.

Student 4

Student 3 is correct that, due to the lone pair, ,,, and molecules are all trigonal pyramidal.,,, and each have only 1 unique bond angle, but that bond angle is different for each of the 4 molecules. The bond angle depends on the size of the atom that is bound to the As atom: the larger the atom that is bound to the As atom, the larger the bond angle. The atoms bound to the As atom, listed by size from smallest to largest, are F, Cl, Br, and I.

After the 4 descriptions were offered, the teacher used a computer program that (1) determined that in each of the 4 molecules, there is only 1 unique bond angle and (2) calculated the bond angle for each molecule(see Table 1).

Table adapted from lan J. McNaught, "Testing and Extending VSEPR with WebMO and MOPAC or GAMESS. @2011 by Division of Chemical Education, Inc, American Chemical Society.