Question

Draw the Lewis structure of AsO43– showing all lone pairs. Identify the molecular geometry of AsO43–. What is the hybridization of the central As atom? What are the approximate bond angles in AsO43–? An AsO43– ion is polar or non polar?

Answer 1

The molecular formula is AsO4 with a -3 charge. Arsenic on the periodic table is in Group 5, therefore it has 5 valence electrons. Oxygen is in group 6, therefore each oxygen has 6 valence electrons. Since there are 4 oxygen atoms in the molecule, we multiply 6 valence electrons by 4 oxygen atoms, which results in 24 valence electrons total for oxygen. Since the molecule has a -3 charge, this means the molecule has 3 EXTRA electrons found in this particular molecule. Next, we add all the valence electrons to get the total for the molecule: 5+24+3 = 32 total valence electrons.

These 32 valence electrons will need to be placed around the atoms to show the bonds formed in the molecule. Hopefully you have learned about the "octet rule" in your class. The octet rule states that each atom in the molecule needs to have 4 electron PAIRS (8 individual valence electrons) in order to form an octet. Therefore, we place Arsenic in the center of the molecule, and place an Oxygen atom above, below, left, and right of the arsenic atom to form the Lewis structure. Begin by placing 2 of the valence electrons around each atom, and repeat until all 32 available valence electrons in the molecule are used up (keep in mind not to violate the "octet rule"). You have now formed the Lewis structure for AsO4 3-. By looking at the Lewis structure we can now determine the number of electron pairs around the molecule.