Question

Lewis Structures; Resonance Structures (Sections)

(a) Draw the dominant Lewis structure for the phosphorus trifluoride molecule, PF3, (b) Determine the oxidation numbers of the P and F atoms, (c) Determine the formal charges of the P and F atoms.

Answer 1

(a)

The Lewis structure can be drawn based on the valence electrons of each element in Phosphorus trifluoride. Phosphorous has 5 and fluorine has 7 valence electrons. The total valence electrons for phosphorus trifluoride molecule are thus 26. Phosphorus is the central atom as it is the least electronegative element of the two.

Now, each of the three fluorine atoms needs one electron each to complete their octet. Phosphorus needs 3 electrons to do the same.

So, phosphorus makes 3 single bonds with the three fluorine atoms.

The Lewis structure can be drawn given as follows:

Lewis Structure of

There are 3 lone pairs of electrons on each of the fluorine atoms and one lone pair of electron on phosphorus atom.

(b)

Oxidation number is the number that atoms would posses if the compound would be made of only ions.

To calculate oxidation number the bonds are considered ionic and hence the shared electrons are counted as being part of the most electronegative atom.

In bond between phosphorous and fluorine which is covalent, fluorine gets assigned the electron due to its high electronegativity and so instead of 7, it has eight valence electrons, one more than neutral atom. Thus, the oxidation number of fluorine is (1 for additional electron and negative sign for gaining the electron).

Since there are 3 atoms of fluorine, so there is total 3 negative charges.

The molecule as a whole needs to be neutral, so there should be 3 positive charge on phosphorus atom.

Therefore, the oxidation number on phosphorous is +3 and on each of the fluorine atoms is .

(c)

Formal charge can be calculated by using the formula given as follows:

Phosphorus has 5 valence electrons, 2 non-bonding electrons and 6 bonding electrons.

So, calculate the formal charge on it as follows:

Each of the 3 fluorine atoms has 7 valence electrons, 2 bonding electrons and 6 non-bonding electrons.

So, calculate the formal charge on it as follows:

Therefore, on both phosphorus and fluorine atoms there are no formal charge.