Question

Count the total number of unshared electrons in $I_3^- = x$

Count the total number of $p\pi - d\pi$ bonds in $PO_4^{3-} = y$

Count the total number of lone pairs in $XeF_4 = z$

Hence, find the value of $x + y + z$

Answer 1

32

Answer 2

1. For I₃⁻:

   • Total valence electrons = 3(7) + 1 = 22.
   • Two bonds (each 2 e⁻) use 4 electrons; remaining electrons = 22 − 4 = 18.
   • These 18 electrons are present as lone (unshared) electrons on the three iodine atoms.
   • Hence, x = 18 electrons.

2. For PO₄³⁻:

   • In PO₄³⁻, the best resonance hybrid shows four equivalent P–O bonds.
   • Modern understanding shows that effective pπ–dπ bonding is negligible.
   • Thus, y = 0 (no effective pπ–dπ bonds).

3. For XeF₄:

   • XeF₄ has a square planar structure.
   • Xenon: 8 valence electrons; 4 are used in bonding so remaining 4 electrons make 2 lone pairs.
   • Each Fluorine: 7 valence electrons; 1 used in bonding so remaining 6 electrons make 3 lone pairs.
   • With 4 fluorines, lone pairs on F = 4 × 3 = 12.
   • Total lone pairs in the molecule = 2 (on Xe) + 12 (on F) = 14 pairs.
   • Hence, z = 14.

Finally, $x + y + z = 18 + 0 + 14 = 32.$