AP Chemistry Unit 1 Cheatsheet: Atomic Structure and Properties

Core equation: n = m/M

• n = moles
• m = mass (g)
• M = molar mass (g/mol)

Key insight: Cannot count particles directly in lab. Avogadro's number (N_A = 6.022 × 10²³ mol⁻¹) bridges moles to particle count.

Atomic mass unit (amu) connection: Average mass of one particle in amu = molar mass in g/mol numerically. Example: C-12 has mass 12 amu; molar mass = 12 g/mol.

Solving strategy: Use dimensional analysis with conversion factors: moles ↔ grams ↔ particles.

Periodic table organization: Based on electron configuration patterns and filled/partially filled shells.

Key trends (left to right across period, top to bottom down group):

Why? Effective nuclear charge (Z_eff) = actual protons − shielding by inner electrons. More Z_eff = stronger attraction to nucleus.

Pure substance: Single type of particle (atom, molecule, or formula unit). Fixed composition.

Mixture: Two or more types of particles. Variable composition.

Elemental analysis use: Determines relative number of atoms and purity of a substance.

Example: A sample labeled "NaCl" might be 95% NaCl + 5% H₂O (mixture). Elemental analysis reveals actual composition.