- Mass number
- Atomic number
- Density
- Volume
The modern periodic table arranges elements in order of increasing atomic number (Z)—the number of protons in the nucleus. This organizing principle, established by Henry Moseley’s 1913 X-ray experiments, corrected earlier tables (like Mendeleev’s 1869 version) arranged by atomic mass, which caused occasional inconsistencies. Atomic number determines an element’s identity and electron configuration, which in turn governs chemical properties. The periodic law states that element properties are periodic functions of atomic number—meaning elements with similar electron configurations (hence similar valence electron arrangements) appear in the same vertical columns (groups), exhibiting recurring chemical behavior. Horizontal rows (periods) represent filling of electron shells: period 1 fills 1s, period 2 fills 2s and 2p, etc. This arrangement reveals trends: atomic radius generally decreases left to right (increasing nuclear charge pulls electrons inward) and increases top to bottom (additional shells). Ionization energy, electronegativity, and electron affinity also show predictable periodic trends. Understanding this organization is fundamental to predicting chemical reactivity, bonding patterns, and compound formation across all elements.
