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: H2, O2, N2, F2, Cl2, Br2, 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, H2, 2H2 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: C6H6.

Benzene and Acetic Acid Examples

  • Benzene representations: structural formula, ball-and-stick model, space-filling model (same atom counts as C6H6).
  • Acetic acid:
    • Molecular formula: C2H4O_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 C6H6 with empirical formula CH2O? (Note: benzene’s classic empirical formula is CH, but the text uses the concrete example of benzene as C6H_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: H2, O2, N2, F2, Cl2, Br2, I_2.
  • Sulfur: S_8.
  • Distinctions among notations are crucial for proper interpretation of molecular composition and bonding.