Chemical formulas

• chemical formula
  • non-structural formulas
    • molecular formula of a compound gives the actual number of atoms of each element in a molecule of the compound.
    • empirical formula of a compound gives the simplest whole-number ratio of the atoms of each element in the compound.
    • formula unit
  • structural formulas (uses lines to represent covalent bonds and shows how atoms in a molecule connect or bond to each other)
    • Lewis structure
    • skeletal formula (or line-angle formula, bond-line formula or shorthand formula)
• molecular model
  • ball-and-stick model
  • space-filling model

EXAMPLE

Name of Compound	Empirical Formula	Molecular Formula
Benzene	$\mathrm{CH}$	$\mathrm{C}{}_6\mathrm{H}{}_6$
Acetylene	$\mathrm{CH}$	$\mathrm{C}{}_2\mathrm{H}{}_2$
Glucose	$\mathrm{CH}{}_2\mathrm{O}$	$\mathrm{C}{}_6\mathrm{H}{}_{12}\mathrm{O}{}_6$
Ammonia	$\mathrm{NH}{}_3$	$\mathrm{NH}{}_3$
Water	$\mathrm{H}{}_2\mathrm{O}$	$\mathrm{H}{}_2\mathrm{O}$

Empirical Formula

Determining Empirical Formula of a Compound from Mass Percentages

Suppose you have a compound with the following composition by mass: 40.0% Carbon (C), 6.7% Hydrogen (H), and 53.3% Oxygen (O). Determine the empirical formula of the compound. Answer:

1. Assume you have 100 g of the compound, so you have:

• $40.0\mathsf{g}$ of Carbon
• $6.7\mathsf{g}$ of Hydrogen
• $53.3\mathsf{g}$ of Oxygen

2. Calculate the number of moles of each element:

• $n(\mathrm{C})=\frac{40.0\mathsf{g}}{12.011g/mol}=3.33\mathsf{mol}$
• $n(\mathrm{H})=\frac{6.7\mathsf{g}}{1.008g/mol}=6.65\mathsf{mol}$
• $n(\mathrm{O})=\frac{53.3\mathsf{g}}{16.00g/mol}=3.33\mathsf{mol}$

3. Divide the number of moles of each element by the smallest number of moles to get the simplest whole-number ratio:

• $\mathrm{C}:\mathrm{H}:\mathrm{O}=3.33:6.65:3.33=1:2:1$

4. The empirical formula of the compound is $\mathrm{CH}{}_2\mathrm{O}$.

Lewis structure

• count and add total valence electrons

• central atom:

  • the least electronegative atom is usually central
  • hydrogen is almost never a central atom
  • halogens are usually terminal
  • carbon is very commonly the central atom in organic molecules

• each bond corresponds to 2 electrons

• complete octets of terminal atoms distribute remaining electrons as lone pairs on outer atoms until each has an octet (or 2 for H)

• incomplete octet

• resonance (or mesomerism)

  • resonance hybrid
  • resonance form (or contributing structure)

• odd electron

• expanded octet

VSEPR theory

• Valence shell electron pair repulsion (VSEPR) theory
• molecular geometry
• bond angles

	0	1	2
2	linear (180°)
3	trigonal planar (120°)	angular (or bent) (<120°)
4	tetrahedral (109.5°)	trigonal pyramid (~107°)	angular (or bent) (~104.5°)