SF₄ vs XeF₄ — The difference 90% of students get wrong

Mistakes/Tricks

1. TITLE

SF₄ vs XeF₄ — The difference 90% of students get wrong

2. THE MISTAKE STATEMENT

THE COMMON MISTAKE:
"Most students think: Since both molecules have the general formula $AB_4$ (one central atom attached to four fluorine atoms), they must have the same shape—usually assumed to be Tetrahedral or Square Planar."

Students confuse $SF_4$ with $XeF_4$ because:
They count the bonded atoms (4) but forget to count the valence electrons of the central atom to determine the number of lone pairs. Without the lone pairs, the hybridization calculation is wrong.

3. WHY THIS MISTAKE HAPPENS

• Similar Formula: Both look like $XY_4$, so the brain defaults to the most common $XY_4$ shape (Tetrahedral, like $CH_4$).
• Valence Confusion: Students forget that Sulphur (Group 16) has 6 valence electrons, while Xenon (Group 18) has 8.
• Ignoring Repulsion Rules: Even if they find the lone pairs, students often place them randomly. They forget that in $sp^3d$ (5 domains), lone pairs claim the equatorial positions to minimize repulsion, while in $sp^3d^2$ (6 domains), they take axial positions to be as far apart as possible.

4. THE CORRECT UNDERSTANDING

THE TRUTH:
You must count Bond Pairs (bp) AND Lone Pairs (lp) to find the structure.

[A] $SF_4$ (Sulphur Tetrafluoride)
- Central Atom: Sulphur (6 valence $e^-$).
- Math: 6 (S) + 4 (bonds) = 10 electrons $\rightarrow$ 5 pairs.
- Hybridization: $sp^3d$ (Trigonal Bipyramidal geometry).
- Setup: 4 Bond Pairs + 1 Lone Pair.
- Result: The lone pair sits in the equatorial position (more space). The shape becomes See-saw.

[B] $XeF_4$ (Xenon Tetrafluoride)
- Central Atom: Xenon (8 valence $e^-$).
- Math: 8 (Xe) + 4 (bonds) = 12 electrons $\rightarrow$ 6 pairs.
- Hybridization: $sp^3d^2$ (Octahedral geometry).
- Setup: 4 Bond Pairs + 2 Lone Pairs.
- Result: The two lone pairs sit opposite each other (axial) to cancel repulsion. The atoms form a flat square. The shape is Square Planar.

Key difference: $SF_4$ has 1 lone pair (See-saw), while $XeF_4$ has 2 lone pairs (Square Planar).

5. COMPARISON TABLE

6. MCQ (Tests the exact confusion point)

Q: A molecule has the formula $AB_4$. The central atom 'A' has 2 lone pairs of electrons. What is the shape of this molecule?

A) Tetrahedral
B) See-saw
C) Square Planar
D) Trigonal Bipyramidal

7. ANSWER + WHY THE TRAP WORKS

Correct: (C) Square Planar

Why the trap works:
If you chose (A) Tetrahedral, you ignored the lone pairs entirely (thinking of $CH_4$).
If you chose (B) See-saw, you confused the 2-lone-pair case ($XeF_4$) with the 1-lone-pair case ($SF_4$).
If you chose (D), you named the electron geometry of a 5-pair system ($sp^3d$) instead of the molecular shape of the 6-pair system ($sp^3d^2$).

Explanation: 4 bonds + 2 lone pairs = 6 electron domains ($sp^3d^2$). The lone pairs arrange themselves $180^{\circ}$ apart (above and below the plane), leaving the 4 bonds in a flat square plane.

8. THE TRICK TO NEVER FORGET

MEMORY TRICK:

1. S for S: Sulphur in SF₄ makes a See-Saw.
2. Xe is Xtra: Xenon has Xtra electrons (2 lone pairs), so it lies flat like a sheet of paper (Square Planar).

Visual Mnemonics:
* $SF_4$: Imagine a playground see-saw. It needs 1 fulcrum (the 1 lone pair).
* $XeF_4$: Imagine a square pizza box. It's flat.

9. RAPID FIRE — Test Yourself

Quick check — Which one?

1. Central atom has 8 valence electrons, bonded to 4 atoms → [A] or [B]?
2. Shape is distorted tetrahedral (folded square) → [A] or [B]?
3. Hybridization is $sp^3d^2$ → [A] or [B]?

Answers:
1. [B] ($XeF_4$ - Noble gas)
2. [A] ($SF_4$ - See-saw is also called distorted tetrahedron)
3. [B] ($XeF_4$)

10. NCERT REFERENCE

[A] $SF_4$ (See-saw) is defined in: NCERT Class 11, Part 1, Unit 4, Page 116 (Table 4.8).
[B] $XeF_4$ (Square Planar) is defined in: NCERT Class 11, Part 1, Unit 4, Page 115 (Table 4.7).

11. CTA

More comparisons from this chapter → Chemical Bonding & Molecular Structure Guide