Question

Consider the following molecules

PF5, BF3, IF7, NO2, H3PO4, SF4, SO3 CC-662 The total number of molecules which have expanded octet is

• A. 5
• B. 3
• C. 4
• D. 6

Answer 1

Answer: (A)

5

Answer 2

To determine which molecules have an expanded octet, we need to examine the central atom in each molecule and count the total number of electrons around it (bonding electrons + lone pair electrons). An expanded octet means the central atom has more than eight valence electrons. This is possible for elements in the third period and beyond, as they have available d-orbitals to accommodate extra electrons.

Let's analyze each molecule:

1. PF5 (Phosphorus Pentafluoride):

   • The central atom is Phosphorus (P), which is in Group 15 and Period 3. P has 5 valence electrons.
   • P forms 5 single bonds with 5 Fluorine (F) atoms.
   • Number of electrons around P = 5 (bonds) × 2 (electrons/bond) = 10 electrons.
   • Since 10 > 8, PF5 has an expanded octet.

2. BF3 (Boron Trifluoride):

   • The central atom is Boron (B), which is in Group 13 and Period 2. B has 3 valence electrons.
   • B forms 3 single bonds with 3 Fluorine (F) atoms.
   • Number of electrons around B = 3 (bonds) × 2 (electrons/bond) = 6 electrons.
   • Since 6 < 8, BF3 has an incomplete octet (electron deficient). It does not have an expanded octet.

3. IF7 (Iodine Heptafluoride):

   • The central atom is Iodine (I), which is in Group 17 and Period 5. I has 7 valence electrons.
   • I forms 7 single bonds with 7 Fluorine (F) atoms.
   • Number of electrons around I = 7 (bonds) × 2 (electrons/bond) = 14 electrons.
   • Since 14 > 8, IF7 has an expanded octet.

4. NO2 (Nitrogen Dioxide):

   • The central atom is Nitrogen (N), which is in Group 15 and Period 2. N has 5 valence electrons.
   • NO2 is an odd-electron molecule (total valence electrons = 5 + 2*6 = 17).
   • In its resonance structures, the central N atom typically has 7 electrons (e.g., one double bond, one single bond, and one unpaired electron).
   • Since 7 < 8, NO2 does not have an expanded octet.

5. H3PO4 (Phosphoric Acid):

   • The central atom is Phosphorus (P), which is in Group 15 and Period 3. P has 5 valence electrons.
   • The common Lewis structure for H3PO4 shows P double-bonded to one Oxygen (O) and single-bonded to three -OH groups.
   • Number of electrons around P = 4 (from P=O double bond) + 3 (from P-OH single bonds) × 2 (electrons/bond) = 4 + 6 = 10 electrons.
   • Since 10 > 8, H3PO4 has an expanded octet.

6. SF4 (Sulfur Tetrafluoride):

   • The central atom is Sulfur (S), which is in Group 16 and Period 3. S has 6 valence electrons.
   • S forms 4 single bonds with 4 Fluorine (F) atoms, using 4 electrons.
   • Remaining valence electrons on S = 6 - 4 = 2 electrons, which form 1 lone pair.
   • Number of electrons around S = 4 (bonds) × 2 (electrons/bond) + 2 (lone pair electrons) = 8 + 2 = 10 electrons.
   • Since 10 > 8, SF4 has an expanded octet.

7. SO3 (Sulfur Trioxide):

   • The central atom is Sulfur (S), which is in Group 16 and Period 3. S has 6 valence electrons.
   • The most stable Lewis structure for SO3 has S forming three double bonds with three Oxygen (O) atoms to minimize formal charges.
   • Number of electrons around S = 3 (double bonds) × 4 (electrons/double bond) = 12 electrons.
   • Since 12 > 8, SO3 has an expanded octet.

Molecules with expanded octet are: PF5, IF7, H3PO4, SF4, SO3. The total number of molecules with an expanded octet is 5.

The final answer is $\boxed{5}$