Drawing Molecular Structures: XeOF₄ and H₂SO₄

To draw the structure of a molecule, we use the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory states that electron pairs around a central atom arrange themselves to be as far apart as possible to minimize repulsion. The arrangement of these electron pairs (bonding and lone pairs) determines the molecular geometry.

a. Structure of XeOF₄ (Xenon Oxytetrafluoride)

Step 1: Determine the central atom.
The central atom is Xenon (Xe).

Step 2: Count the total number of valence electrons.
$Xe : 8 O : 6 F : 7 \times 4 = 28$
Total valence electrons = 8 (Xe) + 6 (O) + 28 (4F) = 42 electrons.

Step 3: Draw the skeletal structure and form bonds.
Xe is bonded to one O atom and four F atoms. Each single bond uses 2 electrons.
Electrons used in bonding: 5 bonds × 2 electrons = 10 electrons.
Remaining electrons: 42 - 10 = 32 electrons. These are placed as lone pairs on the terminal atoms (O and F). Oxygen and Fluorine atoms are highly electronegative and will complete their octets. Each F atom needs 3 lone pairs (6 electrons), and the O atom needs 2 lone pairs (4 electrons).
Electrons used in lone pairs on terminal atoms: (4 F atoms × 6 e⁻) + (1 O atom × 4 e⁻) = 24 + 4 = 28 electrons.
We have used 10 (bonds) + 28 (terminal lone pairs) = 38 electrons. But we only have 42. The difference is 4 electrons, which must be placed as 2 lone pairs on the central Xe atom.

Step 4: Determine the electron geometry and molecular geometry.
Around Xe, we have:
- 5 bonding pairs (1 Xe=O double bond and 4 Xe-F single bonds)
- 1 lone pair (from the 2 lone pairs on Xe, they are often considered as one "lone pair domain" in simplified VSEPR models for hypervalent molecules, but the standard prediction is 6 electron domains).
Total electron domains = 6 (5 atoms + 1 lone pair).
The electron domain geometry for 6 domains is Octahedral.
The molecular geometry, with one position occupied by a lone pair, is Square Pyramidal.
The four F atoms form a square base, the O atom is at one apex, and the lone pair occupies the other apex.

Final Structure: A square pyramidal molecule. The Xenon atom is at the center of a square base formed by the four Fluorine atoms. The Oxygen atom is directly above the Xenon atom, and a lone pair of electrons is directly below it.

b. Structure of H₂SO₄ (Sulfuric Acid)

Step 1: Determine the central atom.
The central atom is Sulfur (S).

Step 2: Count the total number of valence electrons.
$S : 6 O : 6 \times 4 = 24 H : 1 \times 2 = 2$
Total valence electrons = 6 (S) + 24 (4O) + 2 (2H) = 32 electrons.

Step 3: Draw the skeletal structure and form bonds.
S is bonded to two O atoms (which will be bonded to H atoms) and two O atoms. We form bonds first.
Electrons used in bonding: 4 S-O bonds × 2 electrons = 8 electrons. The two H atoms are attached to two of the oxygen atoms, adding 2 more bonds (2 × 2e⁻ = 4e⁻). Total electrons used in bonding so far: 12 electrons.
Remaining electrons: 32 - 12 = 20 electrons. These are placed as lone pairs to complete the octets of all atoms. Each oxygen atom needs 2 lone pairs (4 electrons) to complete its octet.
Electrons used in lone pairs: 4 O atoms × 4 e⁻ = 16 electrons.
We have used 12 (bonds) + 16 (lone pairs) = 28 electrons. We are short 4 electrons from our total of 32. This indicates that we need to form two double bonds. Typically, sulfuric acid is drawn with S=O double bonds to two oxygen atoms.

Revised Step 3: S is bonded to two OH groups (single bonds) and has two double bonds to two other oxygen atoms.
Electrons used:
- 2 S-O (single) bonds: 2 × 2e⁻ = 4e⁻
- 2 S=O (double) bonds: 2 × 4e⁻ = 8e⁻
- 2 O-H bonds: 2 × 2e⁻ = 4e⁻
Total bonding electrons = 4 + 8 + 4 = 16e⁻.
Remaining electrons = 32 - 16 = 16e⁻.
These are placed as lone pairs on the oxygen atoms. The two oxygen atoms with double bonds (S=O) each need 2 lone pairs (4e⁻ each, total 8e⁻). The two oxygen atoms in the OH groups each need 2 lone pairs (4e⁻ each, total 8e⁻). 8e⁻ + 8e⁻ = 16e⁻. Perfect.

Step 4: Determine the electron geometry and molecular geometry.
Around the central S atom, we have 4 electron domains (4 atoms bonded, no lone pairs on S).
The electron domain geometry for 4 domains is Tetrahedral.
The molecular geometry is also Tetrahedral.

Final Structure: A tetrahedral molecule. The Sulfur atom is at the center. Two oxygen atoms are attached with single bonds, and each of these is also bonded to a Hydrogen atom (making OH groups). The other two oxygen atoms are attached to Sulfur with double bonds.

Related Topics & Formulae

Key Theory: VSEPR Theory
The Valence Shell Electron Pair Repulsion theory predicts the 3D shape of molecules based on the repulsion between electron domains (bonding pairs and lone pairs) around a central atom. The order of repulsion strength is: Lone pair-Lone pair > Lone pair-Bonding pair > Bonding pair-Bonding pair.

Common Molecular Geometries:
- 2 domains: Linear ( $180 °$ )
- 3 domains: Trigonal Planar ( $120 °$ )
- 4 domains: Tetrahedral ( $109.5 °$ )
- 5 domains: Trigonal Bipyramidal ( $90 °$ , $120 °$ )
- 6 domains: Octahedral ( $90 °$ )

Important Formula:
Total Valence Electrons = Sum of valence electrons from all atoms.
For ions, add electrons for negative charges or subtract electrons for positive charges.