| 1.7 Periodic Trends |
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The periodic table's organization reflects recurring patterns in ground-state electron configurations. Elements in the same group share valence electron configurations, producing similar chemical properties, while movement across a period corresponds to the sequential filling of subshells within the same principal energy level. Completely filled and half-filled subshells provide enhanced stability, which explains the chemical inertness of noble gases and certain anomalies in periodic trends. The block structure of the periodic table (s, p, d, f) maps directly to which subshell is being filled, providing an immediate connection between an element's position and its electron configuration.
Four key atomic properties
- Ionization energy — the energy required to remove an electron from a gaseous atom or ion
- Atomic and ionic radii — the sizes of atoms and of the ions they form
- Electron affinity — the energy change when an electron is added to a gaseous atom
- Electronegativity — the ability of an atom to attract the shared electrons in a bond
These properties follow systematic trends that are explained by Coulomb's law, the shell model, and the concepts of shielding and effective nuclear charge ($Z_{\mathrm{eff}}$).
Across a period (left to right)
- Effective nuclear charge ($Z_{\mathrm{eff}}$) increases, because nuclear charge increases while core-electron shielding stays roughly constant
- Ionization energy and electronegativity increase
- Atomic radius decreases
- Electron affinity generally becomes more negative (more energy released)
Down a group
- Each new principal energy level adds distance from the nucleus and increases shielding by inner electrons
- Ionization energy and electronegativity decrease
- Atomic radius increases
- Electron affinity generally becomes less negative
Predictable exceptions — such as the ionization-energy drops between Groups 2 and 13 and between Groups 15 and 16 — arise from subshell energy differences and from the repulsion that occurs when electrons are paired in the same orbital. These trends enable estimation of unknown property values by interpolating from neighboring elements, a skill frequently assessed on the AP exam.