Periodic trends

Periodic trends are recurring changes in elemental properties across rows and down columns of the periodic table, driven chiefly by electron configuration and effective nuclear charge.

Effective nuclear charge: The net positive charge experienced by an electron after shielding by other electrons is considered. Its increase across a period draws electrons closer and underlies several major trends.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring chemical properties. Its periods and groups are the directions along which periodic trends are measured.
Chemical bond: An attractive interaction that holds atoms or ions together in a molecule or solid. Electronegativity differences help predict bond polarity and the distribution of electrons.
Diagonal relationship: A similarity in properties between certain diagonally adjacent elements, especially in the second and third periods. It complicates simple row-and-column predictions through paired similarities such as lithium and magnesium.
Electron shielding: The reduction in nuclear attraction on an electron caused by repulsion from other electrons. Greater shielding down a group offsets the added nuclear charge and helps enlarge atoms.
Atomic number: The number of protons in an atomic nucleus, which determines an element’s identity. Increasing atomic number across a period adds nuclear charge one proton at a time.
Chemical reactivity: The tendency of a substance to undergo chemical change through reactions with other substances. Trends in electron loss or gain help compare reactivity within element families.
Lanthanide contraction: The gradual decrease in atomic and ionic radii across the lanthanide series as nuclear charge increases. It produces unusually small later elements and alters expected size comparisons across periods.
Electron configuration: The arrangement of electrons among an atom’s shells, subshells, and orbitals. Shell filling creates the repeating structure behind periodic changes in properties.
Valence electron: An electron in an atom’s outer shell that can participate in chemical bonding. Valence-electron counts repeat by group and help explain recurring chemical behavior.

Linked from 29 pages