Atomic Structure and the Periodic Table
Atomic structure
Atoms contain protons (positive, atomic number Z), neutrons (neutral, mass number A − Z), and electrons. Isotopes have the same Z but different A; atomic mass is the weighted average of isotope masses. Cations form by losing electrons; anions by gaining electrons. The mole connects mass, particles, and molar mass.
Electron configuration
Electrons fill orbitals by increasing energy: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p. Each orbital holds two electrons. Use the Aufbau principle, Pauli exclusion, and Hund's rule. Noble-gas shorthand is common. Exceptions include Cr ([Ar]4s1 3d5) and Cu ([Ar]4s1 3d10). Valence electrons determine periodic trends.
Periodic trends
Across a period, effective nuclear charge rises, so atomic radius decreases and ionization energy/electronegativity generally increase. Down a group, added shells increase radius and decrease ionization energy. Metals lose electrons; nonmetals gain or share.
Worked example
Write the electron configuration of Fe (Z = 26): 1s2 2s2 2p6 3s2 3p6 4s2 3d6, or [Ar]4s2 3d6. For Fe2+, remove the 4s electrons first: [Ar]3d6. Fe3+ is [Ar]3d5, a stable half-filled set.
Traps
Removing 3d electrons before 4s when forming transition-metal cations; confusing mass number with atomic mass; assuming all isotopes have equal abundance; and reversing periodic trends down a group.