Chemical Formulas - Quick Reference

Molecular Formula

• Definition: uses chemical symbols with subscripts to indicate the number of atoms of each type in a molecule; a subscript is included only if more than one atom of that type is present.
• Purpose: abbreviates the types and counts of atoms in a compound.

Structural Formula

• Definition: shows the same atom types and counts as the molecular formula but also reveals how atoms are connected.
• In methane: one C and four H atoms with lines representing bonds between them.

Ball-and-Stick Model

• Purpose: shows geometric arrangement of atoms.
• Note: atom sizes are not to scale.

Space-Filling Model

• Purpose: shows relative sizes of atoms in the molecule.

Diatomic and Sulfur Molecules

• Common diatomic molecules: $H<em>2, O</em>2, N<em>2, F</em>2, Cl<em>2, Br</em>2, I_2$.
• Sulfur: most common form is S8 (a molecule of eight sulfur atoms).
• Sulfur representations can be shown as structural, ball-and-stick, or space-filling models.

Important Distinctions: H atoms and diatomic molecules

• $H_2$ = molecular formula for diatomic hydrogen (two chemically bonded H atoms).
• $2H$ = two separate hydrogen atoms (not a bonded unit).
• $2H_2$ = two molecules of diatomic hydrogen.
• Symbols $H, 2H, H<em>2, 2H</em>2$ denote distinctly different species.

Empirical vs Molecular Formulas

• Empirical formula: simplest whole-number ratio of the types of atoms in a compound.
• Example: titanium dioxide has empirical formula $TiO_2$ (Ti:O in a 1:2 ratio).
• Molecular formula: shows actual numbers of each type of atom in a molecule; can be a whole-number multiple of the empirical formula.
• Benzene:
  • Empirical formula: $CH_1$? (historically described as CH, since C and H are present in equal relative numbers in the empirical simplification used in the text). The text explicitly states the empirical formula is CH.
  • Molecular formula: $C<em>6H</em>6$.

Benzene and Acetic Acid Examples

• Benzene representations: structural formula, ball-and-stick model, space-filling model (same atom counts as $C<em>6H</em>6$).
• Acetic acid:
  • Molecular formula: $C<em>2H</em>4O_2$.
  • Empirical formula: $CH_2O$ (ratio 2:4:2).
  • Ratio reduction: dividing by the lowest whole number (2) gives $1:2:1$, so empirical formula is $CH_2O$.

Relationship and Practical Notes

• A molecular formula is always a whole-number multiple of its empirical formula.
• If the empirical formula and molecular mass are known, the molecular formula can be determined.
• Experimental mass data for benzene confirms its molecular formula $C<em>6H</em>6$ with empirical formula $CH<em>2O$? (Note: benzene’s classic empirical formula is CH, but the text uses the concrete example of benzene as $C</em>6H_6$ with empirical formula CH; the key point is the molecular formula is a multiple of the empirical formula.)

Quick Reference Points

• Representations summarize different aspects: composition (molecular formula), connectivity (structural formula), and geometry (ball-and-stick, space-filling).
• Common diatomics: $H<em>2, O</em>2, N<em>2, F</em>2, Cl<em>2, Br</em>2, I_2$.
• Sulfur: $S_8$.
• Distinctions among notations are crucial for proper interpretation of molecular composition and bonding.