Question

Question 18 Not yet answered Marked out of 3 P Flag question The electronic structure of which of the following species cannot be adequately described by a single Lewis formula? Select one: O a. POF3 O b. CS2 c. H2NNH2 O d. HNNH e. NO3-

Answer 1

The electronic structure of a) POF3 of the following species cannot be adequately described by a single Lewis formula.

The total number of valence electrons in POF3 is = 5 + 6 + (3 $\times$ 7) = 32. (No of valence electrons is 5, 6 and 7 in P, O and F respectively).

Now, let us look at POF3, whose Lewis structure must accommodate a total of 32 valence electrons. We know the electronic geometry of POF3 is tetrahedral. In order to have the formal charges equal to zero and obtain a tetrahedral structure, we need to have a double bond between the Phosphorous and Oxygen atom and 3 single bonds between 3 fluorine atom and the central phosphorus(P) atom (as shown below).

:|: n༌་༌༌་་པ =6 ：1;

Now, in this case, each fluorine atom and the terminal oxygen atom has an octet, but the central phosphorus atom has a total of 10 electrons surrounding it rather than 8. For writing Lewis electron structures, in which we place an electron pair between each pair of bonded atoms, which requires that an atom have more than 8 electrons whenever has 5 bonds.

Thus the central phosphorus atom violates the octet rule.